NEW INTEGRATED KNOWLEDGE BASED NIKER APPROACHES TO THE PROTECTION OF CULTURAL Grant Agreement n° HERITAGE FROM EARTHQUAKE-INDUCED RISK 244123
Deliverable 3.1
Inventory of earthquake-induced failure mechanisms related to construction types, structural elements, and materials
Due date: April 2010 Submission date: September 2010 Issued by: POLIMI
WORKPACKAGE 3: Damage based selection of technologies
Leader: POLIMI
PROJECT N°: 244123 ACRONYM: NIKER TITLE: New integrated knowledge based approaches to the protection of cultural heritage from earthquake-induced risk COORDINATOR: Università di Padova (Italy) START DATE: 01 January 2010 START DATE: 01 January 2010 INSTRUMENT: Collaborative Project Small or medium scale focused research project THEME: Environment (including Climate Change)
Dissemination level: PU Rev: FIN NEW INTEGRATED KNOWLEDGE BASED NIKER APPROACHES TO THE PROTECTION OF CULTURAL Grant Agreement n° HERITAGE FROM EARTHQUAKE-INDUCED RISK 244123
INDEX
INDEX ...... 1 1 2 2 INTRODUCTION ...... 3 2.1 Description and objectives of the workpackage ...... 3 2.2 Objectives and structure of the deliverable ...... 3 3 SEISMIC DAMAGES ...... 4 3.1 General consideration ...... 7 3.2 The seismic behaviour of existing masonry buildings: morphology of damages and failure mechanisms ...... 9 3.2.1 In-plane behaviour ...... 10 3.2.2 Out-of-plane behaviour ...... 12 3.3 Masonry typologies and masonry quality ...... 17 3.4 Building typologies ...... 20 4 COMPONENT TYPOLOGIES AND DAMAGE MECHANISMS ...... 27 4.1 Masonry walls and pillars ...... 27 4.1.1 Masonry typologies ...... 27 4.1.2 Damage mechanisms ...... 57 4.2 Arches / Vaults / Domes ...... 74 4.2.1 Introduction ...... 74 4.2.2 Typologies ...... 77 4.3 Floors ...... 91 4.3.1 Wooden floor typologies ...... 92 4.3.2 Other typologies ...... 102 4.4 Roofs ...... 103 4.5 Sub assemblages (wall-wall, wall-floor, etc.) / connections ...... 107 5 BUILDING TYPOLOGIES AND DAMAGE MECHANISM ...... 114 5.1 Isolated buildings ...... 114 5.1.1 Earth constructions ...... 114 5.2 Row buildings ...... 117 5.3 Complex buildings ...... 124 5.4 Palaces ...... 125 5.5 Churches ...... 125 5.6 Slender buildings (towers, minarets) ...... 131 6 COLLAPSE MECHANISMS OF REPAIRED BUILDINGS ...... 134 6.1 Traditional repair techniques ...... 134
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6.1.1 Earth constructions ...... 139 6.2 Modern repair techniques ...... 141 6.2.1 Earthen buildings ...... 144 7 REFERENCES ...... 148 ANNEX 1 – SEISMIC DAMAGE ABACUS ANNEX 2 – Partners contribution
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1 INTRODUCTION
1.1 DESCRIPTION AND OBJECTIVES OF THE WORKPACKAGE WP3, in general, is aimed to the collection of information for increasing the existing state of knowledge linking earthquake induced failure mechanisms, construction types and materials, interventions, assessment techniques. According to the project document, other aims of the WP are the follow: Development of concepts for materials and intervention techniques based on the structured database; definition of the main design parameters and requirements for materials and intervention techniques; Definition of the main on-site control techniques and strategies; Development of advanced materials and improved techniques for intervention and to produce/assemble those required for testing and case studies; Development of laboratory procedures and choice of parameters for the final validation of the durability, compatibility, and effectiveness of new techniques and materials; Parameterization of all the above mentioned information to set the basis of optimized design and required laboratory testing in following WPs.
1.2 OBJECTIVES AND STRUCTURE OF THE DELIVERABLE The aim of the Deliverable D3.1 is to produce an inventory of earthquake-induced failure mechanisms related to construction types, structural elements, and materials. Information was collected by the partner expertise and in literature in order to amplify the case histories. The analysis of the information was carried out the base of the state of the art, clustering masonry, component and building typologies, in order to recognise characteristic behaviours and recursive damages. The deliverable starts from a general description of the seismic behaviour of masonry buildings, stressing differences between theoretical approaches and real behaviour surveyed after the recent earthquakes. The analysis explores open topics within the international debate like the problem of the agglomerate of buildings and of the real behaviour of modern strengthening/repair techniques, mainly r.c. addition. Concerning this topic the most recent applications of new techniques are not considered, being not already present meaningful case histories in literature. Annex 1 details all the damage mechanisms, related to masonry typologies, components, and building typologies. Annex 2 includes all the partners contributions, summarized in the deliverable.
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2 SEISMIC DAMAGES
Through the different ages of architectural history, earthquakes have always represented one of the main causes of damage and losses of cultural heritage, both monumental buildings and historic centres. Post earthquake damage observation is a remarkable source of information on the recurrent damage patterns. The damages observed in various countries due to recent earthquakes (e.g. Umbria-Marche, Italy (1997-98), Açores, Portugal (1998), Andravida, Greece (2008), Abruzzo, Italy (2009), etc. …), as well as the results of both experimental and analytical research carried out in the ‘90s, have proven that there is an urgent need for improved knowledge of the seismic behaviour of old masonry buildings and, as a consequence, for enhanced reliability of the retrofitting techniques. Particularly the seismic sequence in Italy in 1997 produced a deep re-thinking about the theoretical basis of seismic vulnerability of historic masonry building, which led to the introduction of a new seismic code. The fact that several buildings were repaired and/or strengthened prior to the earthquakes, allowed the assessment of the applied intervention techniques. In addition to the typical damages observed in numerous historic structures, the effects of the seismic events showed that, in several cases, the adopted structural models (that presumably described the structural system of the historic construction) were not adequate and the retrofitting techniques had not provided the expected effects. Actually, the earthquakes have revealed the cases of incompatibility between the existing structure and the way the intervention technique was applied. This incompatibility is attributed to insufficient knowledge of either the bearing system or the properties of the in-situ materials. The lack of sufficient documentation has led to the selection of inadequate repair/strengthening techniques, (Figure 2.1).
(a) (b)
Figure 2.1 - Example of (a) Out-of-plane collapse of a wall with r.c. tie beams, (b) roof hammering the masonry walls, (Binda, 2006a).
Most of the failures were due to lack of knowledge of the materials and building construction details which caused a wrong choice of the repair technique. Furthermore, in several cases, poor application of the selected techniques was observed, due to the lack of knowledge and of skill. Although these observations call for regulatory documents that would provide the Engineer with adequate guidance, the complexity of the subject (a multi-parameter problem with social, historic, aesthetic, technical and economic aspects). Nevertheless, the complexity of the structural typologies of masonry buildings does not allow for the definition of general rules and operative modalities, as it was tempted in the past. The observation of failures of repaired buildings due to incompatibility between the original structure and the repair, showed the necessity of developing new structural models for the old masonry buildings and code requirements for the intervention, (Borri, 1991a), (D’Ayala, 1999a),
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(Magenes, 2000), (Modena, 2004a). The code requirements were oriented previously to a concept of seismic adequacy of the structures, (LLPP, 1996). The new Italian seismic code moved from theories of "adequacy" to "improvement", (PCM, 2003), (PCM, 2005), (LLPP, 2008, 2009), which meant more compatible and respectful interventions on the historic building patrimony, (Corsanego, 1992), (Corsanego, 1993), (Gavarini, 1994). Following the previous issue, extensive damage surveys were carried out on damaged centres, in order to define the real structural behaviour of the historic buildings. Based on this criterion new methodologies were proposed as an approach to the seismic behaviour of these buildings. Several studies based on in-situ observations after seismic events allowed to arrange abacuses of the typical damages occurring to different typologies (buildings, churches, etc…), which led to the consequent systematization of mechanical models able to describe their specific behaviour by kinematics models, both for in-plane and out-of-plane mechanisms. The Civil Protection Department and the Ministry of Cultural Properties in Italy have published damage survey templates with detailed drawings illustrating the most important crack pattern after the earthquake in churches, (Figure 2.2), and palaces, (PCM, 2001). Other templates, with a more simplified scheme, were also prepared, (GNDT, 1999), (Aedes, 2000), (PCM, 2000) for the post seismic damage survey and used in post-earthquake emergency time for the prompt safety evaluation of dwellings. The occurrence of a peculiar mechanism depends on the level and type of connection of the façade to the side walls.
Figure 2.2 - Survey templates with drawings illustrating the most important crack pattern after earthquakes in churches, (PCM, 2001).
On the base of the previous activities, the Italian seismic code, (PCM, 2003), (PCM, 2005), (LLPP, 2008, 2009), requires since 2003 accurate surveys and deep attention to existing buildings, introducing the following news not previous examined by any code, (Borri, 2009): the deep attention dedicated to the question of masonry and its quality; the classification of the intervention in “seismic upgrading”, “seismic improvement” and “local interventions”, depending on basic criteria of extension of the intervention, transformation of the original behaviour of the construction and safety levels to be performed; the case of aggregated buildings is discussed; this is a typical configuration in the Italian historic centres;
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the case of historical and architectonical heritage is discussed; this is another typical problem of retrofitting of buildings; safety factors depend on knowledge of the construction; three levels of knowledge has been defined; in such a way there is a direct correspondence between safety and knowledge; a range of possible values of the principal mechanical parameters are defined for masonry; the most common techniques of intervention are shortly explained.
The Circular n.617/2009, (LLPP, 2009), deals with one of the most important problems of the Italian buildings in seismic zones: the behaviour of aggregated constructions under seismic actions, (Borri, 2009). It is a really frequent eventuality, especially in the historic centres deriving from a building process which lasts for centuries. The aggregation could be defined as a construction delimited by open spaces. It is formed by Structural Units (S.U.) which could be defined as portions of the aggregation which have a unitary behaviour from a static and seismic point of view. S.U. are defined thanks to structural criteria (i.e. a rigid floor defines a single S.U. or two parts with a different kind of masonry are two S.U.) and thanks to historical criteria, according to the age of construction of the several parts. The analysis of a structural unit belonging to an aggregation is different from the case of an isolated building, due to the interaction effect introduced by adjacent buildings on the analysed structural unit. The Italian body of laws and codes on constructions faces the problem of the artistic buildings in the “Guidelines for valuation and decrease of seismic hazard of cultural heritage referring to technical construction code”, (Borri, 2009). The purpose of the Guidelines is to match seismic-safety requirements with preservation requirements when the building is a unique and unrepeatable artistic construction. In fact, in such an eventuality, it is necessary to graduate the safety level of the intervention to be the highest possible without modify (or, at worst, damage) the nature and the characteristics of the protected building, (Borri, 2009). The Italian Guidelines for cultural heritage are based on this concept (BBCC, 2006), (Moro, 2007). The Guidelines, in fact, direct the attention to seismic improvement rather than seismic update interventions because seismic improvement does not imply any fixed safety threshold achievement. The only law prescription is to demonstrate that, in consequence of the intervention, the safety level increases. In Guidelines much attention is pointed on relief and knowledge of the building, safety calculations to be performed, an adequate structural model (i.e. cinematic model) and monitoring the building after the realization of the intervention, (Borri, 2009). Seismic design actions depend on the importance of the artistic building and its utilization class (occasional presence, frequent presence, very frequent presence). In order to prevent the seismic collapse of objects and decorations in the building, an appropriate “artistic limit state” (SLA) has been introduced in the guidelines, (Borri, 2009). Recently, in Greece a regulatory document was drafted within EPPO (Earthquake Planning and Protection Organization) jointly with the Ministry of Culture. The document, (EPPO, 2010), is going to be made available to the Public for comments, before being re-drafted and approved by the Greek Authorities. This regulatory document that covers the entire process of intervening on historic structures and monuments (namely, in-situ and in Laboratory investigations, assessment, study of pathology-qualitative and quantitative, parameter analyses, intervention techniques, etc.) has allowed for several gaps in our knowledge to be identified. In the post-earthquake emergency period, the degree of damage needs to be assessed to establish if the building is fit for habitation, to identify public safety concerns, to avoid further damage to the architectural heritage due to aftershocks, and to protect or shelter the movable artistic objects in a safe place. These inspections are also necessary to determine a first, approximate economic estimate of the damage, in order to allocate the resources for their rehabilitation. The need to act quickly, because of the risks connected to the inspection of a damaged structure and because reports need to be made as quickly as possible, often means the adoption of forms
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that are not rich in details, the use of which is limited to the emergency phase. However, it is important that the first assessment of damage represents a structural interpretation of the damage, in order to understand the vulnerability of the building and to serve as a starting point for the rehabilitation. So as it is important to define the need for emergency provisional structures in the meantime. In fact, it is worth noting that the survey of damage in emergency represents the state of the fabric immediately after the traumatic event, but this useful information risks being cancelled by further collapses, partial demolition (always to be avoided where possible), and provisional interventions. The empirical observation of the damages caused to buildings by earthquakes of medium or high intensity highlighted that buildings subject to the same seismic excitation show radically different behaviour, related to their typology, construction rules, materials used and maintenance level. In case of complex buildings, that are the result of subsequent changes, (Figure 2.3), the analyses of the historic centres need a proper structural modelling: this is necessary in order to appraise the specific vulnerability of complex buildings, due to their typical historical evolution (constructive sequence, damages, previous interventions, etc.). The research previously developed simplified procedures and models for the limit analysis of existing masonry structures in seismic area, (Borri, 2004a, c), (Modena, 2004b), (Tassios, 2009). Such procedures allow the evaluation of the safety level of existing buildings in old centres according to the real conditions detectable in-situ, often not satisfying the main hypothesis at the base of calculation methods designed and aimed at modern masonry structures (insufficient connections among components, presence of poor and non-homogeneous materials, lack of bond in the thickness of masonry wall, etc...).
Figure 2.3 - Building evolution of the 4th level of the Suore della Visitazione Convent at Taggia, (Binda, 2003d).
2.1 GENERAL CONSIDERATION Masonry building represents a box-type structural system composed of vertical structural elements - walls - and horizontal structural elements - floors and roofs. Vertical loads are transferred from the floors, acting as horizontal flexural members, to the bearing walls, and from the bearing walls, acting as vertical compression members, to the foundation system. As known, when a building is subjected to an earthquake motion, the inertia force, proportional to the masses of the structural system has to be taken into account. Those action effects depend on various parameters, such as the mass and the stiffness of the structure and their distribution, the magnitude of the imposed actions, the number of cycles of the earthquake motion, the characteristics of the foundation soil, etc… Since the ground motion is in general three-directional both vertical and horizontal inertia forces will be acting on the structure, inducing displacements, changing in-time (both in magnitude and in sign), resulting in the three-dimensional vibrations of the building. Horizontal inertia actions are transferred from the floor structures, which should act as rigid horizontal diaphragms, into the bearing walls, causing shearing and bending effects, and from the bearing walls into the foundation system. Additionally, due to the distributed mass of wall elements, distributed inertia forces are induced resulting in out-of-plane bending of walls.
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Experimental testing and observation of damage modality of real structures have shown that masonry walls are less resistant to actions perpendicular to their medium plane (out-of-plane actions) than to actions parallel to this plane (in-plane actions). In the first case, the stiffness of the wall is far less than in the other. For a good load bearing behaviour, all walls of a masonry building should resist actions parallel to them, avoiding inflection and overturning. This philosophy considers the behaviour of the building as a box. The walls should be connected, by stiff constraints to the floor, because the floor should be able to distribute the seismic actions between the walls as a function of their stiffness. It is generally recognized that a satisfactory seismic behaviour is attained only if out-of-plane collapse is prevented and in-plane strength and deformation capacity of walls can be fully exploited. The seismic vulnerability of masonry buildings depends on several parameters, such as in-plane and/or in-height irregularity, discontinuity of walls/piers along the height of the building, alteration of the initial structural scheme during the lifetime of the building, inadequate interventions after previous seismic events, low quality construction type of masonry and/or low quality of materials, inadequate connections among vertical elements or between horizontal and vertical elements, lack of any diaphragm action of horizontal bearing elements, etc. Box action results in limiting the deformations imposed to masonry during an earthquake and, hence, preventing extensive damages and collapse. Old masonry structures seldom satisfy the conditions of ensuring box action: floors and roof are rarely well connected with the walls, floors and roof do not behave as diaphragms of limited deformability in their plane, the connections between walls is quite often defective, whereas large openings and openings located close to the corners of buildings lead to further weakening of the box action. It has to be mentioned that masonry walls exhibit enhanced vulnerability to out-of-plane bending (low bending moment capacity mobilized under limited imposed inflexion). This pronounced vulnerability is negatively affected by all the above mentioned conditions that limit the box action of buildings, as well as by the poor quality of construction type of masonry and the poor quality of building materials. Needless to say that previous non-repaired damages, lack of maintenance, decay of materials, etc… further aggravate the effects of a seismic event. The observations of masonry buildings when subjected to earthquakes have shown that the behaviour is strongly dependent on how the walls are interconnected and anchored and to floors and roofs. In old structure the unfavourable effect of insufficient anchorage between walls and between walls and floors was often observed. Irregular structural layout in plan, large openings and lack of bearing walls in both directions often caused severe damage or even collapse. A good quality of the connections between floors and walls, between roof and walls and between perpendicular walls is also crucial to reach a good global seismic behaviour of the building. Good quality connections will drive the collapse of the construction to a configuration that requires a stronger seismic action, (Borri, 2009). Other contributing factors include (1) original configuration and craftsmanship of the masonry; (2) modifications made over time, such as buttresses and ties (which improve the general performance) and additional storeys (which tend to compromise the performance); (3) the characteristics, quality, and condition of the masonry; (4) the appropriate thickness of the bearing and non-bearing walls and discontinuities; (5) the method and configuration of the connection of the floors and roof to the walls; and (6) the materials and design of the floors and roofs themselves. The most important factors tend to differ with the building typology.
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2.2 THE SEISMIC BEHAVIOUR OF EXISTING MASONRY BUILDINGS: MORPHOLOGY OF DAMAGES AND FAILURE MECHANISMS Damage to masonry buildings can be essentially interpreted on the basis of two fundamental collapse mechanisms. According Giuffrè definition, (Giuffrè, 1993a), the “First Damage Mode” is produced by seismic actions perpendicular to the wall (out-of-plane) that cause the overturning of the whole wall panel or of a significant portion of it, (Figure 2.4a). A signature of such damage, short of collapse, can be the shedding of a portion of the exterior leaf of masonry. Another can be the formation of vertical cracks at the corners of a building where the wall began to form a hinge from the swaying. This behaviour represents the highest building vulnerability and in the past was prevented by the use of ties to compensate for the lack of connection between the external walls and the ones orthogonal to them. The effectiveness of the tie consists in involving the walls orthogonal to the façade as containing elements. They resist to the seismic action transmitted by the facades as in- plane action and exert a higher resistance towards such action. When the action overcomes the strength, also the walls stressed in their plane can crack, according to the classic diagonal course which isolates a triangular part of the wind-brace wall and makes it participate to the cracking motion. This further damage modality - called “Second Mode of Damage” - can be checked only when the “first mode” doesn’t occur thanks to metallic connections, (Figure 2.4b).
Figure 2.4 - First mode (a) and second mode (b) collapse mechanisms, (Giuffrè, 1993), (Carocci, 2001).
While the “first mode” is always ruinous, as it implies the complete collapse of the wall and consequent ruin of all supported elements, the “second mode” does not necessarily determine the collapse, though it still implies small, medium and even large cracks of the wind-brace walls. The Second Damage Mode is caused, as said before, by forces acting in the plane of the wall and is usually marked by inclined cracks associated with shear forces that often result in an “X” pattern, but it seldom reaches the total collapse. However, when a full shear crack occurs during an earthquake, the triangular sections of the panel can become unstable, leading to collapse. In historic centres, as well as in building evolved in time the addition of adjacent constructions or portions implies the lack of strong connections between the parts. The “consequence of this organic defect is the particular fragility of the historical house towards the seismic action”, (Giuffrè, 1993), (Giuffrè, 1996). Lack of structural integrity is one of the principal sources of weakness responsible for severe damage leading to collapse. The problems could have origin from all the structural changes, due also to some interventions to adequate building to modern uses and standards. Windows and doors, including garage doors, perforate the exterior walls, leaving an inadequate number of shear walls or connections to support the buildings during an earthquake. Frequently upper story additions had been constructed with heavy concrete floors and roofs, or with new concrete and hollow clay tile floors retrofitting into the
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older masonry structures. The added stiffness and weight of these upper story alterations and additions further increased the risk of failures in the original low-strength masonry walls. The presence of in-plane flexible diaphragms, typically timber floors and roofs as well as thin masonry vaults, is very common in the existing masonry buildings. Even though proper connections between walls and floors allow to prevent local first mode mechanisms, in masonry buildings with flexible floors the global seismic response is quite complex. Since no or little coupling effect can be operated by the horizontal structures, vertical structures (walls) tend to behave independently. However, an acceptable approach in practice could be to analyze separately the in-plane seismic response of each masonry wall as extracted from the global structure with its pertaining loads and inertial masses.
2.2.1 In-plane behaviour Figure 2.5, (Tomasevic, 2000), illustrates that in the plane of the walls, bending and shear cause horizontal and diagonal cracks, respectively. In-plane mechanisms induce the typical shear damage, which often is not sufficient to lead to structure collapse. The limited damage in Figure 2.5 is due to the effective strong connection among the structural components and the presence of floors able to transmit the horizontal forces to shear walls, both characterizing the favourable “box” behaviour of buildings under seismic actions. Unfortunately the layout of historic buildings, their discontinuities, the changing in time, lack of maintenance etc…, led frequently to different behaviours, (Figure 2.6).
Figure 2.5 - Deformation of the building and typical damage to structural walls, (Tomazevic, 2000).
Figure 2.6 - Behaviour of masonry buildings: (a) structural walls not tied together, (b) building with deformable floors and tied walls, (c) building with rigid floors and tied walls, (Tomazevic, 2000).
Observation of seismic damage to masonry walls, as well as laboratory experimental tests, showed that masonry piers subjected to in-plane loading may have two typical types of behaviour, with local cracks according to Figure 2.5, with which different failure modes are associated: Flexural behaviour: this may involve two different modes of failure. If the applied vertical load is low with respect to compressive strength, the horizontal load produces tensile flexural cracking at the corners, (Figure 2.7a), and the pier begins to behave as a nearly rigid body rotating around the toe (rocking). If no significant flexural cracking occurs, due to a sufficiently high vertical load, the pier is progressively characterized by a widespread damage pattern, with sub-vertical cracks oriented towards the more compressed corners (crushing). In both cases, the ultimate limit state is obtained by failure at the compressed corners, (Figure 2.7a).
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Figure 2.7 - Typical failure modes of masonry piers due to horizontal loads: (a) rocking; (b) sliding shear failure; and (c) diagonal cracking, (Calderini, 2009).
Shear behaviour: This may produce two different modes of failure: (a) in sliding shear failure, the development of flexural cracking at the tense corners reduces the resisting section; failure is attained with sliding on a horizontal bed joint plane, usually located at one of the ends of the pier, (Figure 2.7b); (b) in diagonal cracking, when failure is attained with the formation of a diagonal crack, which usually develops at the centre of the pier and then propagates towards the corners, (Figure 2.7c). The crack may pass prevailingly through mortar joints (assuming the shape of a ‘stair-stepped’ path in the case of a regular masonry pattern, or also through the blocks, (Figure 2.8).
Figure 2.8 - Influence of the mortar-brick adhesion in the joints.
The occurrence of different failure modes depends on several parameters: (a) the geometry of the pier; (b) the boundary conditions, (Calderini, 2009); (c) the acting axial load; (d) the mechanical characteristics of the masonry constituents (mortar, blocks and interfaces); (e) the masonry geometrical characteristics (block aspect ratio, in-plane and cross-section masonry pattern). In the past, many experimental tests have been carried out in order to analyse the influence of these parameters on the failure mode of masonry piers. In general, it has been assessed that rocking tends to prevail in slender piers, while bed joint sliding tends to occur only in very squat piers, (Magenes, 1992), (Magenes, 1997). In moderately slender piers, diagonal cracking tends to prevail over rocking and bed joint sliding for increasing levels of vertical compression, (Mayes, 1975a), (Bosiljkov, 2003), (Vasconcelos, 2006). Diagonal cracking propagating through blocks tends to prevail over diagonal cracking propagating through mortar joints for increasing levels of vertical compression, (Lourenço, 2005) and for increasing ratios between mortar and block strengths, (Mayes, 1975b), (Bosiljkov, 2003).
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Increasing interlocking of blocks (block aspect ratio plus masonry pattern) may induce a transition from diagonal cracking through mortar joints to rocking, (Vasconcelos, 2006), (Giuffrè, 1993) to diagonal cracking through blocks or to bed joint sliding. Crushing, in general, occurs for high levels of vertical compression (related to the compressive strength of the material). It is worth pointing out that it is not always easy to distinguish the occurrence of a specific type of mechanism, since many interactions may occur between them.
2.2.2 Out-of-plane behaviour Besides the in-plane failure, the external masonry walls of a house can typically be subjected to out-of-plane mechanisms, (Figure 2.9).
Figure 2.9 - Collapse mechanisms of masonry walls under out-of-plane horizontal action, (Rondelet, 1834).
Direct observation of crack patterns recorded in post-earthquake damage surveys, (D’Ayala, 1999b), (D’Ayala, 2003a), together with the available literature on the survey of stone masonry buildings damage due to earthquake, (Karantoni, 1997), (Tomazevic, 1999), yields to the conclusion that often the most recurrent failure mechanism surveyed is the overturning of the street façade. However, this occurs most commonly involving portions of the party walls as well. The way in which this will develop depends on the quality and strength of the connections with the other elements of the structure, party walls, internal load-bearing partitions, floors, and roof structures. If the structure was not strengthened, it is assumed that the only means of restraint to overturning exerted by other elements to a wall is governed by the friction of the contact surface, and this will give rise to different types of failures, highlighted in the examples of Figure 2.10 and Figure 2.11, (D’Ayala, 2003a).
Figure 2.10 - Overturning mechanisms related to the restraints effectiveness, (Borri, 2004c).
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Figure 2.11 - Mechanisms for overturning failures, (D’Ayala, 2003a).
However, if the building structural capacity has been improved, by means of the introduction of ties or ring beams, then usually the simple overturning is prevented, while mechanisms relying on arch effect develop, (Figure 2.12). The structural behaviour of a very complex building can be analysed in meaningful structural portion. The analysis can be peculiarly complicated in agglomerates of buildings, (Figure 2.13). The first step, then, is to recognise structural portions (structural units - SU) and, then, the damage mechanisms. The developed methodology is referred to the structural portions called macroelement, and to the damage and collapse mechanism.
Figure 2.12 - Mechanism for failures based on arch effect, (D’Ayala, 2003a).
Figure 2.13 - Mechanism for overturning failures, (Giuffrè, 1993).
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More in detail, a macroelement is the building portions structurally recognisable with an autonomous behaviour respect to the whole building. Macro-elements are defined by single or combined structural components, (walls, floors and roof), considering their mutual bond, (potential damage pattern, cracks, borders of poor connections, etc…), and restraints, (e.g. the presence of ties or ring beams), the constructive deficiencies and the characteristics of the constitutive materials. They behave independently as a whole without any support by other portions of the building, but they follow kinematic mechanisms, both out- and in-plane. Thus they are elements in hazardous conditions for possible incipient brittle collapse. The damage mechanism in general tends to separate the masonry portion, distinguishing the macroelement. The damage could involve different geometric shapes according to the action and to the masonry properties. The direct survey of the crack pattern can clearly identify the real behaviour of the structural system. In the case of churches, as an example, the seismic response showed recurrent behaviour, according to local damage and collapse mechanisms of the different architectonic parts, called macro-elements (i.e., facade, nave, triumphal arch, etc.), which behave almost independently, (Doglioni, 1994), (Lagomarsino, 2004a), (Figure 2.14 and Figure 2.15). If masonry shows good characteristics, local damage mechanisms (e.g., out-of plane overturning, rocking) develop as loss of equilibrium of masonry portions capable of sliding and rotating. Focusing on the development of this kind of failures, the initial damage phase may generally be very far from the ultimate condition, because of the structural resources in the post-cracked state (the so-called “dynamic stability”).
Figure 2.14 - Macroelements of a church, (Doglioni, 1994).
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Figure 2.15 - Damage mechanism for churches, (Lagomarsino, 2004a).
Since the ‘80s in Italy, empirical evaluations, by the so called "vulnerability indexes", (particularly for masonry buildings, (Benedetti, 1984), have been proposed, based on weighted sums of vulnerability factors, related both to structural irregularity aspects recorded by rapid systematic or sample surveys and to the actual calculations of the resistance to horizontal actions of the masonry walls. The aim was to compare the vulnerability of different buildings (and thus the priorities for strengthening operations), and to provide damage scenarios for different seismic intensities. Within these methodologies and with specific reference to historic masonry buildings, some procedures have been proposed: they are based on the identification of the values of horizontal static-equivalent forces (and therefore of the values of the masses accelerations) that can activate specific mechanisms of local failure / overturning of structural macro-elements (composed by single walls or subassemblages, as intersecting walls, walls and floors or roof, etc.) in-plane and, especially, out-of-plane, (Bernardini, 1988), (Bernardini, 1990). In these buildings, in fact, the absence of systematic connections between intersecting walls and between walls and horizontal structures may cause kinematic mechanisms related to the loss of equilibrium of structural portions rather than to states of stress exceeding the materials ultimate capacity, (Giuffrè, 1999); this limit analysis approach depends on few geometrical and mechanical parameters and therefore it does not require an extremely accurate survey and time-consuming computation, (Bernardini, 1999). Once the critical structural configuration is defined, the subsequent step is the identification of the most probable collapse mechanisms of each macro-element. The studies based on in-situ surveys after seismic events allowed to create abaci of the typical damages occurring in constructive typologies (buildings, churches, palaces, (BBCC, 1997, 2006)), which led to a consequent systematization of the mechanical models able to describe their behaviour by kinematic models, (Bernardini, 1988), (Bernardini, 1990), (Borri, 1999b), (D’Ayala, 2003a). Kinematic models provide a coefficient c = a/g (where a is the ground acceleration and g the gravity acceleration), which represents the seismic masses multiplier characterizing the limit of the equilibrium conditions for the considered element. In simplified assessment procedures, the mechanism connected to the lowest value of c is the weakest one and, consequently, the most probable to occur: in-plane mechanisms are characterized by c coefficients higher than the out-of- plane ones, (Bernardini, 1990), (Giuffrè, 1999).
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The new methodology for the damage assessment considers the most significant collapse mechanisms in each architectonic part of the building, (Figure 2.16 and Figure 2.17). Several works on seismic vulnerability evaluation of masonry structures through limit analysis procedures have been proposed (Bernardini, 1988), (Bernardini, 1990), (Bernardini, 1999), (Giuffrè, 1993), (Giuffrè, 1997), (Brun, 1999), (de Felice, 2000), (D’Ayala, 1999a), (D’Ayala, 2003a), (Modena, 2004a), (Munari, 2009), but the research was limited to the calculation of the seismic activation multiplier, even if evaluated for complex mechanisms. This approach of limit analysis applied to existing masonry buildings in seismic areas is now provided by the updated Italian seismic code, (PCM, 2003), (PCM, 2005), which finally takes into account the high vulnerability of existing masonry buildings not satisfying assumptions commonly more suitable for new earthquake-proof structures. In this field, another important document is represented by the Guidelines published by the Italian Ministry of Cultural Heritage for the evaluation and mitigation of seismic risk of the architectural heritage, (BBCC, 1997, 2006), (Moro, 2007).
Figure 2.16 - Damage mechanisms for row buildings, (D’Ayala, 1999a).
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Figure 2.17 - Example of damage mechanisms, (Penazzi, 2000).
2.3 MASONRY TYPOLOGIES AND MASONRY QUALITY The construction type, quality and state of preservation of masonry play a fundamental role in determining the capacity of a construction to sustain seismic actions. Actually, the resistance of masonry to various actions is not governed by the mechanical properties of the constituent materials. This problem cannot be studied only in terms of stress and strain: a masonry which can resist and transfer the vertical and seismic forces without breaking up should have geometric and physical characteristics that permit a monolithic behaviour, (Borri, 2009). Among the meaningful parameters to examine in order to classify a masonry as being of “good quality” and introduced by the Italian Seismic Code, (OPCM, 2003), (LLPP, 2008, 2009), it is worth to remember: stones or brick laid in horizontal courses, horizontal courses, vertical mortar joints not-aligned, mortar vertical joints, use of almost square-shaped and big stones of large size, limited volume of mortar as compared to the volume of bricks or stones, in case of multileaf masonry, leaves transversely connected and indeed sufficient mechanical properties of mortar and bricks or stones. The structural performance of masonry can be estimated, taking into account the above mentioned parameters and providing the following factors are known, (Binda, 2000a): (a) its geometry; (b) the characteristics of its texture (single or multiple leaf walls, connection between the leaves), (c) the physical, chemical and mechanical characteristics of the components (brick, stone, mortar); (d) the characteristics of masonry as a composite material. One of the most negative cases is that of masonry that is not monolithic along its thickness, as frequently happens in stone masonry, (Figure 2.18). Given the fact that double- or multi-leaf masonry is in numerous cases used in historic structures, the non-monolithic in-thickness behaviour of masonry becomes a governing parameter for the behaviour of masonry structures. Separation between consecutive leaves may be caused by decay of material in-time, combined with actions (vertical or horizontal) like the eccentric application of normal loads. For instance, floors or vaults can rest on one leaf of a multi-leaf masonry. Vaults, domes and arches are usually less thick than the masonry walls and piers by which they are supported. The situation is aggravated by thrust forces acting along the thickness of masonry elements) may deteriorate further the state of this type of masonry. Masonry with disconnected leaves is extremely vulnerable, especially against out-of-plane actions induced by earthquakes.
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Given the vast variety of construction types of historic masonries and existing cross sections, and significant effect of the construction technique on its structural performance, the starting point for a systematic study on the mechanical behaviour of stonework masonry should begin from systematic survey of the various different geometry and building techniques, including the in-thickness characteristics (i.e. number of leaves and type of connection - if any - among them). Such systematic investigation on the morphology of masonry sections on the Italian territory was started in the early nineties by L. Binda, (Abbaneo, 1993), (Binda, 2000b), as it was considered as a prerequisite for drafting a necessity to define some guidelines for repair or strengthening by grout injection, ((Baronio, 1991), (Binda, 1993) and more recently (Valluzzi, 2004)).
Figure 2.18 - Deformation and failure of a two-leaf wall, (Giuffrè, 1993).
WALL SECTION
Geometrical survey Distribution of stones, mortar and voids in the
detail wall section
100
80
60
[%]
40
20
0
% Stone % Mortar % Voids
2 Distribution of the voids dimension (cm ) 0.25
0.20
0.15
0.10
0.05
0.00 1.00 10.00 100.00
Figure 2.19 - Form representing the wall section and the void calculation, (Binda, 1993).
Contemporarily Giuffrè carried out in the early '90s, (Giuffrè, 1990), the first experimental and analytical studies about the mechanical behaviour of the stonework masonry typologies based on the recognising of "rule of art" characteristics after visual inspection, survey and typological classification. The studies were part of a more general analysis on the vulnerability of some historical centres like Ortigia, (Giuffrè, 1993), Matera (Giuffrè, 1997), Città di Castello (Borri, 1995), (Giovanetti, 1998), Palermo (Giuffrè, 1999). In each case the local masonry typologies and materials are carefully studied and reported in an abacus. The presence of some characteristic, like the connection elements called headers, can be a discriminating parameter for the evaluation of the wall mechanical behaviour, (Figure 2.20). Other parameters can be: dimension of the elements composing the masonry (bricks, stone, ashlar, etc.), shape and cutting of the stones, masonry texture, mortar quality, mortar quantity, presence of wedges, presence of horizontal courses, presence of leaves connections and headers
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(Binda 2000b), characteristic of the masonry section, homogeneity of the materials, (Figure 2.19). The behaviour of each of the considered masonries is then qualitatively evaluated, (Figure 2.20b). Giuffrè, (Giuffrè, 1990, 1999), proposes in fact, a classification based on a parameter which indicates the ratio of the distance d between two subsequent headers to the thickness of the masonry wall. The parameter is representative of the bending resistance of the wall (Figure 2.21 and Figure 2.22). A classification of single and multiple leaf masonry sections according to the number of leaves and their connections and the importance of this knowledge for the implementation of numerical models is commented in (Binda, 1994) and (de Felice, 2001). Results obtained from an experimental campaign carried out on the transfer of shear stresses between leaves of multiple leaf walls subject to vertical and horizontal actions are presented in (Binda, 2003a).
(a) (b)
Figure 2.20 - Header influence and stability of the wall. (a) (Giuffrè, 1993), (b) (Giuffrè, 1990).
Figure 2.21 - Influence on the wall stability of the number and position of the headers, (Giuffrè, 1999).
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Figure 2.22 - Stone masonry survey indicating very good, good and poor connection, (Giuffrè, 1993).
2.4 BUILDING TYPOLOGIES Analytical models are necessary in order to evaluate the seismic behaviour of a building, calculate the load carrying capacity and safety of the structures and to suggest appropriate repair techniques. Nevertheless, due to the complexity of the building geometry and masonry characteristics their use must take into account the many different types of masonry structures which are representative of the building function: houses (isolated, in rows, complex aggregates), palaces, churches, towers, castles, fortifications, etc…, (Figure 2.15 and Figure 2.23). It has been also clarified that the knowledge of the typical features of each historical building, regarding, for example, the constitutive materials, (i.e. masonry typology and arrangement), the structural type (common building, isolated/in aggregate, churches, towers, palaces), is essential for the definition of suitable interpretative models. In addition, the preliminary diagnosis of the building, regarding history, geometry, materials, connections, etc…, should constitute the basis for all safety evaluation and intervention planning, (Giuffrè, 1990), (Giuffrè, 1993), (Doglioni, 1994) and (Binda, 1999a). On the building and structure typology it is much more important that a reliable approach to their restoration and preservation refers to a methodology of investigation and choice for the most appropriate techniques applied to the special type of building and materials, (Modena, 1997a) and (Binda 2003b). It has been shown by earthquakes, that even if the buildings belong to the same age and type of construction the isolated house or dwelling have different failure mechanisms than buildings in a row, (Giuffrè, 1993), (Binda, 1999a, b), (Carocci, 2001) and (Binda, 2004), (Figure 2.16 and Figure 2.17). The analysis of buildings belonging to an aggregation is different than the case of an isolated building, (Binda, 2004), because of the several interactions that the adjacent volumes could act on the structural unit which is analysed. The main structural interaction may be classified in two categories, (Borri, 2009), (Figure 2.24): vertical loads or horizontal actions (especially under the seismic action) from adjacent buildings; buttressing or constraining effects offered by the adjacent buildings. These interactions modify the collapse mechanism of the aggregation and of its structural unit introducing new different actions and changing the constraint configuration. Lack of alignment between façades causes the lack of contrast from adjacent buildings, as in case of head buildings in rows. Another vulnerability source could be the different height of the buildings, with stiff changes that might lead to possible collapse of the higher one. Floors at different levels in adjacent building could act against the common wall.
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Type A) Type B) Row Houses Type C) Palaces Isolated houses and/or dwellings
Consoli Palace in Ms7 axonometric Sel UMI 23 – axonometric view Gubbio Type D) Type E) Arenas Bell-Towers
The Torrazzo of Cremona
plan section The Colosseum in Rome Type F) Churches and Cathedrals
F1) Churches: F2) Churches: central plan plan based on latin cross scheme
S. Maria del Fiore: plan Gothic Cathedral San Vitale: plan and axonometry
Figure 2.23 - Building typologies, (Binda, 2003b).
Figure 2.24 - Hammering from adjacent building, (Borri, 2004c).
High rise towers and bell-towers suffer from similar long-term damage due to vertical loads (to which horizontal actions are added during earthquakes), which can bring them to failure even after centuries. So it is not possible to use the same investigation procedures, modelling and repair measures for Palaces, Churches, etc…, (Doglioni, 1994), (PCM, 2001) and (Sepe, 2008).
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An approach to restoration and retrofitting should be proposed by classes of buildings and structures, (Binda, 2003b). It is frequently impossible to apply the same intervention techniques to all the building classes. The survey of the historic centres produces interesting cataloguing of the building typologies but also of the typical transformation recognised, (Figure 2.25 and Figure 2.26). In fact, each typology but also each transformation can lead to specific damages due to the loss of continuity or to bad connections, (Errore. L'origine riferimento non è stata trovata.). The mechanical behaviour of each typology is then summarised by an expected damage abacus, (Giuffrè, 1996). This approach was very clear in the research concerning the seismic vulnerability of the historic centres of Ortigia (Giuffrè, 1993), Matera (Giuffrè, 1997), Città di Castello (Giovannett, 1998), and Palermo (Giuffrè, 1999).
Figure 2.25 - The row buildings typology of Campi Alto. Buildings have three storey: the first one with an entrance at the lower street (for stables or deposits), one in the middle and the last with the entrance at the upper street (for living places). The lowest storey is partially excavated in the natural rock. The ground floor rooms are covered by barrel vaults that, despite the several seismic events, are still well preserved, even in the partially collapsed buildings, (Binda, 2004).
Figure 2.26 - Damages resulting from historical evolution and transformation; constructive stages in a row, (Carocci, 2001).
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Figure 2.27 - Evolution and damage mechanisms of the row buildings in Citerna, (Borri, 2004c).
A similar project carried out by Bernardini and others, (Bernardini, 1988, 1990, 2000) and partially financed by GNDT, (Bernardini, 2000) developed a software for the evaluation of the vulnerability of buildings, recently updated (Modena, 2008), (Munari, 2009) on the base of the damages extensively surveyed after the Umbria-Marche earthquake, (Figure 2.28 and Figure 2.29), (Cardani, 2004). The seismic behaviour principles are common, starting from the recognition of wall portions and considering their overturning, (Figure 2.30). Even in this case the repair effect is easily computed. In what concerns the analysis of the structures limit state, the requested information concerns mainly the geometric characteristics of the buildings, the presence of ties and and the qualitative evaluation of the masonry monolithicy and texture. The specific weight and the resistance of the material if unknown were suggested in the help menu taking into account the Italian seismic code.
2.1.3 - INTERVENTI DI CONSOLIDAMENTO (anno 1990) DELL'EDIFICIO U.M.I. 20 2.3 - SCHEMA DEI CINEMATISMI E STATO DI DANNO (anno 2001) DELL'EDIFICIO U.M.I. 10
INIEZIONI CON MALTA
1 1 RIFACIMENTO DELLA MURATURA IN LATERIZIO 2 2
3 2
4
01 3 5m PIANTA PIANO TERRA PROSPETTO SUD
INIEZIONI CON MALTA
RIFACIMENTO DELLA MURATURA IN LATERIZIO
1 5 RIFACIMENTO ARCHITRAVE IN C.A.
01 3 5m PIANTA PIANO PRIMO PROSPETTO NORD
1 Danno: lesioni diffuse in corrispondeza delle teste delle 1 Meccanismo: martellamento degli elementi di copertura travi di copertura. per spinta fuori piano.
INIEZIONI CON MALTA 2 Danno: lieve spanciamento della parete con lesioni 2 Meccanismo: spinta fuori dal piano della parete sud a pressocchè verticali in corrispondenza degli elementi di causa della particolare conformazione della pianta, della differente rigidezza. disomogeneità del tessuto murario, e della spinta delle RIFACIMENTO DELLA MURATURA IN travi del solaio. LATERIZIO 3 Danno: lesione verticale in corrispondenza dell'attacco 3 Meccanismo: risposte differenziate all'azione sismica dei due edifici. RIFACIMENTO DELLA MURATURA IN degli edifici adiacenti; nel punto di collegamento, che è la PIETRA zona più debole, si presenta una concentrazione di sforzi di trazione che porta alla rottura del collegamento stesso. RIFACIMENTO ARCHITRAVE IN C.A. 4 Danno: lesione nel maschio snello 4 Meccanismo: schiacciamento dell'elemento snello per forze nel piano della parete.
NOTA: il progetto è stato realizzato dall'architetto F.M. 5 Danno: lesione in prossimità dell'angolata 5 Meccanismo: distacco tra i muri d'ambito per interazione Poggiolini in collaborazione con il geometra Salvatori M. di forze agenti sulle pareti ortogonali. 01 3 5m PIANTA PIANO SECONDO nell'anno 1983.
(a) (b) (c) (d)
Figure 2.28 - Some examples from the survey form with reference to: (a) general survey (plans, sections and photos); (b) masonry structure and typology; (c) past intervention projects and d) damage survey and evaluation of the possible collapse mechanisms, (Binda, 2000b).
The updated version of the software, (Modena, 2008), (Munari, 2009) developed at the University of Padova is based on models calculating the accelerations which activate local collapse mechanisms of macro-elements that can develop in historical masonry buildings. The procedure takes into account two automatic procedures (a) the c-Sisma prcedure allows to carry out ultimate limit state assessments of the most probable severe local kinematic mechanism, as required by the Codes; (b) the Vulnus methodology, based on the fuzzy set theory, provides global vulnerability assessments of individual structural units or groups of buildings, as well as fragility curves related to the achievement or overcoming of the limit state of heavy damage. The automatic procedure was calibrated thanks to the extensive investigation carried out on 4 small typical historic centre in Umbria hit by 1997 earthquake and repaired after a previous event in 1979, (Binda, 1999a, 1999b, 2003d, 2005b, 2008a), (Cardani, 2004). The investigation, supported by GNDT - Italian Group for the Protection against Earthquake - (which involved three Research Units (RU): Politecnico of Milan, University of Padua and the Italian Ministry of Cultural Properties) collected information by a form specifically developed, (Figure 2.28), (Binda, 2000b), (Cardani 2004), and had the strategic aim to: (a) collect information on the effectiveness of the repair techniques, (b) define a methodology for the vulnerability analysis of a building patrimony
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previously considered as minor, but meaningful testimonies of the cultural heritage, (c) calibrate and define a "minimal" investigation program, eventually carried out by the municipality or by the province or region, (d) to set up Databases storing the information useful to prepare rescue plans, (e) to use the collected knowledge for the implementation of reliable models for the vulnerability analysis, (Figure 2.29 and Figure 2.30). This was carried out in order to support the designers in choosing the right analytical model for the safety definition and the appropriate intervention techniques for their projects. The research was based on (Figure 2.28): (a) the survey of the building typology, (b) their geometry and crack pattern, (c) the materials characterisation on site (by non destructive and minor destructive techniques) and in laboratory. The outputs of the research were: a database collecting all the information of each building containing also an abacus of the most typical and frequent collapse mechanisms, guidelines for architects and engineers concerning the diagnostic investigation.
Figure 2.29 - Damage surveyed after the Umbria earthquake, (Penazzi, 2000), (Cardani, 2004).
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(a) (b)
Figure 2.30 - Kinematic models for out-of-plane mechanisms: (a) vertical strips (b) horizontal strips, (Bernardini, 1988), (Modena, 2008), (Munari, 2009).
In previous time, Doglioni, Moretti and Petrini, (Doglioni, 1994), carried out vulnerability analysis, starting from similar basis, for the Friuli churches and bell-towers, (Figure 2.14 and Figure 2.31), damaged by 1976 earthquake. The cataloguing of the typologies, structural details and damages was an important tool for successive researches and interventions on seismic area. Similar analysis were carried out later on by Lagomarsino, (Lagomarsino, 1997), (Lagomarsino, 1998a), (Lagomarsino, 1998b), on the Umbria and Marche churches, damaged by the 1997 earthquakes, (Figure 2.15), on some Catania churches, (Cocina, 1999), on the churches of Lunigiana and Garfagnana (Angeletti, 1997) and Molise (Lagomarsino, 2004c). These studies allowed to verify the appropriateness of simplified structural models, (Brencich, 1998), (Lagomarsino, 2004b) analysing the main church elements and their collapse mechanisms for the seismic actions (Figure 2.14, Figure 2.15, Figure 2.31). The previous research allowed the development of damage survey templates with drawings illustrating the most important crack pattern after earthquakes, (Figure 2.2), (PCM, 2001).
(a) (b)
Figure 2.31 - Example of Bell-Towers typologies in Friuli (a) and (b) crack pattern of a bell-tower and interpretation of the damage mechanisms, (Doglioni, 1994).
An interesting approach for the vulnerability evaluation of churches with no conventional plan (irregular asymmetrical plan, circular elliptic, octagonal plan, etc.) is proposed by Borri, (Borri,
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2004b), considering the geometric aspect, static aspect and masonry quality. It allows determining the vulnerability index of churches with irregular plan, not considered in other methodologies and its effectiveness by a parameter concerning the information quality, which takes into account the origin of the data, if some tests were carried out or not, etc… Starting from the previous experimental research and from the mechanism of collapse survey, several other studies were carried out, recognising even more complex mechanism and interpretative algorithms for building typologies, structural assemblages, etc…, (e.g. Italian Conference of Seismic Engineering, 1999 - 2009). The Civil Protection Department and the Ministry of Cultural Properties in Italy have published damage survey templates with detailed drawings illustrating the most important crack pattern after the earthquake in churches and palaces, (PCM, 2001), (Figure 2.2).
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3 COMPONENT TYPOLOGIES AND DAMAGE MECHANISMS
3.1 MASONRY WALLS AND PILLARS
3.1.1 Masonry typologies In safety evaluation for overturning mechanisms, results may not be completely adequate if the calculation procedure only takes into account geometric parameters and disregards mechanical characteristics (e.g., masonry quality). In fact, some authors, (Giuffrè, 1993 1999), (de Felice, 2001), (Baggio-Carocci in Bernardini, 2000), showed both theoretically and by experimental testing that the lack of transversal monolithic behaviour induces the mechanism activation in case of low values of the seismic multiplier, (Figure 2.18, Figure 2.20, Figure 2.21, Figure 2.28, Figure 3.1 and Figure 3.2). A safety check evaluation for overturning mechanisms around the block edge implies the idea that the compressive stress is unlimited. In order to take mechanical characteristics into account, in particular limited compressive strength, a possible procedure implies translating the strength issue into an equivalent geometric issue: the hinge position can be shifted inward (reducing the virtual work of the resisting actions) proportionally to the decrease in masonry quality, (Figure 3.3). The damage observation has, in fact, highlighted how the masonry quality was fundamental to the understanding of the seismic behaviour of historic buildings, particularly in stone-work masonry. Brittle failures are frequent in stone-masonry walls characterized by two or three leaves without an effective transversal connection. Therefore, this parameter is important to evaluate the seismic vulnerability of historical monuments. The presence of headers, (Figure 3.4), stone elements positioned thorough the wall section, realizes a connection between the different leaves and represents one of those rules of thumb traditionally associated with the good quality of masonry.
Figure 3.1 - Overturning mechanisms of several masonry typologies. Baggio-Carocci in (Bernardini, 2000).
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Figure 3.2 - Overturning mechanisms of monolithic wall and two-leaves wall, (Borri, 1999b).
Figure 3.3 - Overturning mechanism of a monolithic wall: the hinge position can be shifted inward (reducing the virtual work of the resisting actions) proportionally to the decrease in masonry quality, (Borri, 1999b).
Figure 3.4 - Opus quadratum, (Giuffrè, 1991).
3.1.1.1 Brick masonry The wall construction historically is frequently of three solid brick leaves with courses of runners alternated to courses of stretchers, bonding the leaves together. Commonly the thickness of the walls at ground floor is about 450.0 mm reducing to 300.0 mm at the third floor. Old and ancient brick masonries had usually very thick sections (from 600.0 mm on) generally with homogeneous distribution of the bricks in the section then in the outer faces. Sometime only the external leaf of the masonry was made with whole regular bricks, while the internal part was made with pieces of bricks and large mortar joints for economic reasons. The joint thickness was usually much lower than the brick one in a ratio 1-2/5. Nevertheless this was not the case of late Roman architectures and of Byzantine constructions where the mortar joints were much thicker than in older and more
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recent structures. From a survey carried out on Milan Roman walls and on Ravenna Byzantine walls the following classification could be made, (Figure 3.5), (Tedeschi, 2004): solid walls with thin joints; solid walls with thick joints; solid walls with multiple leaves (with different thickness of the leaves).
A) SOLID WALLS WITH THIN JOINTS
128 cm 145 cm
a) St Giovanni in Conca, Milan a) St Nazaro Basilica, Milan
B) SOLID WALLS WITH THICK JOINTS
58 cm 55 cm 66 cm
b) St. Tecla b) St Michele in b) Byzantine Basilica, Milan Africisco, Ravenna Palace, Ravenna
C) SOLID WALLS WITH MULTIPLE LEAVES (WITH DIFFERENT THICKNESS OF THE LEAVES)
128 cm 100 cm
c) St. Simpliciano Basilica, Milan c) St Tecla Basilica, Milan
Figure 3.5 - Examples of brick masonry section.
The brick disposal can have various arrangement, (Figure 3.6 and Figure 3.7). Regular brick masonry, in general shows a good seismic behaviour, related to its easier monoliticity. The possible damages are related to the possible vulnerability of the structure, rather then to the masonry quality. Damage could be found in case of local repairs (expulsions), in case of increasing of the masonry section with a leaf addition or in case of cladding (possible overturning). Thick masonry walls made by solid bricks but with rough construction technique could be covered by brick masonry leaves made with regular bricks and mortar joint. It is evident that this leaf grew up with the wall having regularly courses of header bricks to connect it to the rest of the wall, (Figure 3.8). The other courses are only partially connected with headers and hence in many parts of the structures only the continuing vertical mortar joint is the (weak) restraint between the leaf
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and the internal walls, (Binda 2009a). Errore. L'origine riferimento non è stata trovata. summarizes the possible seismic damages when the connection of the external leaf is not effective, (Carocci, 2004). These typologies are indeed recurring in the whole Mediterranean basin area. For instance, in the Istanbul districts of Fener and Balat, (D’Ayala, 2004b), 4 types of brickwork were identified. The most common building material is the solid brick of dimensions 210 x 105 x 50 mm set in lime mortar to form two leaves masonry walls (88% of sample). A traditional extruded lightweight brick of dimensions 300 x 150 x 60 mm is often used to wall up oriels and jetty in a one leaf wall, or at the upper storeys. Frogged brick imported from France have been surveyed in a minority of cases, while in some very recent alterations, the use of extruded brick tiles, as infill of reinforced concrete frames, has been noted. Commonly the thickness of the walls at ground floor is about 400.0 mm reducing to 300.0 mm from the 3rd floor up. Usually the wall is made up of a solid double leaf with sufficient through thickness connection, while in a minority of cases two leaves brickwork with rubble infill has been recorded. The overall thickness is greater than the previous case, and the two leaves are usually made of double brickwork. In the case of historic brickwork surveyed in the city of Lalitpur, Katmandu Valley, in Nepal, (D’Ayala, 2003c) the most common building material is the brick bonded with mud mortar forming ordinary masonry. There are typically two types of bricks: ordinary sun dried bricks of dimensions 210 x 105 x 50 mm, set in mud mortar, and vitrified fired bricks, called dachi aapa, with same dimensions but a trapezoidal cross section, so that the mud bed-joint is partially covered externally by the brick. This type of brick is usually used in better quality construction for the facing of external walls. Because of the vitrified surface and the overlapping over the joints, this wall construction is substantially impermeable to rain water. The wall construction historically is of three solid brick leaves with courses of runners alternated to courses of stretchers, bonding the leaves together. Commonly the thickness of the walls at ground floor is about 450.0 mm reducing to 300.0 mm at the third floor. Another rather common form of construction is also two leaves brickwork with rubble infill. The overall thickness is greater than the previous case, and the two leaves are usually made of double brickwork.
Figure 3.6 - Examples of regular brick masonry section. (a) Single brick, (b) two headers, (c) one brick and a half.
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Figure 3.7 - Brickwork textures, (Mannoni, 1994).
(a) (b) (c)
Figure 3.8 - View of the cladding connection (a), detail of a cracked brick in the header bond (b) and detachment (c) (Binda, 2009a).
Figure 3.9 - Different configuration of cladding connections and possible failures, (Carocci, 2004).
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Another rather common form of construction is also two leaves brickwork with rubble infill. The overall thickness is greater than the previous case, and the two leaves are usually made of double brickwork. In fact, in case of thick sections, (Figure 3.10), the brickwork can be only an external leaf and hides the inner rubble masonry, which is the load bearing part of the wall. The quality of the inner material could be very scattered. This typology is frequent in roman massive structures were the core was a sort of conglomerate, to middle age structures. Dimension, (Figure 3.11), and physical properties of the bricks are very variable in time and geographically, as well as the mortar composition, its quality and decay. Traditional mortars were produced with lime, with possible additions of cocciopesto (powdered bricks), pozzolan or other natural additives in order to have hydraulic reactions, or natural hydraulic limes. The binder/aggregate ratio is frequently 1/3. Clay or earth mortars could be found in vernacular buildings. Gypsum mortar could be found. It is important to detect the presence of gypsum when injections are planned, due to possible sulphate reactions with grouts. The use of cement mortars is usually related to recent interventions, which could produce salt damages, as well as the adding of new walls made by modern hollow bricks in recent repairs, (Figure 3.12). In this last situation damages could be caused by the different stiffness and/or the lack of effective connections between the portions. In case of openings infilling or local reconstruction, the expulsion is frequent. The recognising of the mortar composition (binder, aggregate origin, and grain size distribution) should be carried out by specific laboratory tests and is an important issue in the formulation of compatible repair materials.
Figure 3.10 - Masonry section of the Civic Tower of Pavia, (Binda, 1992).
Figure 3.11 - Example of brick sizes and dimension surveyed in North Italy, (Carbonara, 2004).
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Figure 3.12 - Examples of modern brickworks frequently used in recent interventions.
3.1.1.2 Stone masonry The worst defect for a masonry wall is to show no monoliticity in the transversal direction; this can happen for instance when the wall is made by small pebbles or by two external layers even well ordered but not mutually connected or containing a rubble infill. This causes the wall to become more brittle particularly when external forces act in the horizontal direction, (Figure 2.18, Figure 2.20, Figure 2.21, Figure 2.28, Figure 3.1, Figure 3.2 and Figure 3.13). The same problem can happen under vertical loads if they act eccentrically or causing the leaves instability, (Figure 3.14), (Giuffrè 1993). It is worth to remark that textures appearing regular on façade often do not correspond to regular morphology in the section, (Figure 3.15). Therefore, a correct analysis of the mechanical behaviour of existing masonry structures, especially when multi-leaf walls are present, cannot disregard the proper investigation of the arrangement of materials in the thickness.
(a)
(b) (c)
Figure 3.13 - (a) Damages after Umbria earthquake: collapse of the outer leaf of the wall, (Binda, 2003c) and corner collapse in a building in (b) Morocco and (c) Greece.
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Figure 3.14 - Deformation and failure of a two leaves wall for vertical loading.
Figure 3.15 - Masonry textures and sections, (Binda, 2000b, 2003c), (Cardani, 2004).
Related to the seismic behaviour of stonework masonries, (Figure 3.16), different typologies can be found, (Abbaneo, 1993) and (Binda, 2003b). Four large classes can be distinguished, each one having subclasses as follows, (Figure 3.17): One leaf solid wall; Two leaves; Three leaves; Dry wall.
The research carried out by L. Binda and others, (Abbaneo, 1993), (Binda, 2000b), (Binda, 2003b), (Binda, 2009b), led to an initial cataloguing of multiple leaf walls based on the percentage of mortar, stones and voids measured on the area of the cross section, (Figure 2.19, Figure 3.18), and to a subsequent classification based on the number of different layers and on the type of constraint between them. Whereas the first kind of classification allows evaluating the injectability of the wall, the second one allows formulating important hypothesis on the static behaviour of the masonry. Addresses survey forms, (Figure 2.19, Figure 3.19), were developed and used to collect information in pilot study aimed to risk mitigation (Lunigiana and Catania area) and in the post earthquakes studies, (Binda, 2000b). A classification of single and multiple leaf masonry sections according to the number of leaves and their connections and the importance of this knowledge for the implementation of numerical models is commented in (Giuffrè,1999).
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Figure 3.16 - Qualitative behaviour of a multiple leaf stone Figure 3.17 - Abacus of stonework sections, (Binda, masonry, (Binda, 2009b). 2003b).
] y [% uenc Freq 90 80 70 60 50 40 30 20 10 0 Friuli Veneto Lombardia Liguria stone Trentino mortar voids Umbria Sicilia Region Toscana
Emila Romagna Emila
Figure 3.18 - Percentage of mortar vs. percentage of stones referred to the area of the cross section of stonework walls in various Italian regions, (Binda, 2000b).
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Figure 3.19 - Example of a masonry survey form, (Binda, 2000a, 2000b).
Following the previous studies several local catalogues were developed, often accompanying the regional versions of the handbooks for the compiling of the seismic venerability forms, (e.g. Ferrini, 2003). Initially referred only to the front texture, (Francovich, 1988), (Mannoni, 1994), (Figure 3.20), the most recent trends take into account the section classification, (Figure 3.21). It is worth to mention the classification proposed in (Gurrieri, 1999), (Avorio, 2002), and introduced in the Region Umbria Code for the post-earthquake financing of the repair intervention, (Angeletti, 2004). Detailed forms explaining the prevalent masonry typologies in Umbria were developed, with a prosed procedure for the masonry quality rough qualification, (Figure 3.22). Furthermore within an Italian National Project RELUIS, a research line was devoted to the definition of the masonry quality based on visual inspection and limited tests, (Binda, 2006d, e), (Borri, 2006). The research is aimed to develop a procedure based on visual inspection and limited tests able to quantify mechanic parameters, after accurate calibrations and statistical correlation. Several parameters were taken into account, like the courses disposal, the vertical joint staggering, the homogeneous dimension of the stones, the presence of tie stones, etc. (Figure 3.23). To this purpose several procedures proposed by in literature are applied and compared.
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(a) (b)
Figure 3.20 - Catalogues of the surface texture of stone masonry proposed in (a) (Mannoni, 1994), and (b) (Francovich, 1988).
Section with leaves connection.
Partial connection of the leaves.
Lack of connection between the leaves.
Figure 3.21 - Classification of the masonry section diffused in Umbria, (Gurrieri, 1999), (Avorio, 2002), (Angeletti, 2004).
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Figure 3.22 - Catalogue aimed to the masonry qualification of the typologies diffused in Umbria, (Gurrieri, 1999), (Avorio, 2002), (Angeletti, 2004).
Courses Disposal
Horizontal Partial Horizontal Irregular
Vertical Joints
Staggered Partially staggered Partially aligned
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Distribution of regular blocks
Prevalence of regular blocks (>75%) Presence of irregular stones 25-50% Presence of irregular stones > 50%
Figure 3.23 - The research developed with a RELUIS project financed by the Italian Government is aimed to developed a procedure based on visual inspection and limited tests able to quantify mechanic parameters, after accurate calibrations and statistical correlation. Several parameters were taken into account, like the course disposal, the vertical joint staggering, the homogeneous dimension of the stones, the presence of tie stones, etc. (Binda, 2006d, e), (Borri, 2006).
According to the previous research, Speranza in (Speranza, 2003) provides a classification of masonry typologies, which is mainly derived by the surveys following the 1997 Umbria-Marche earthquake, but is generally applicable to a large number of Mediterranean and European historic centres. Included typologies, as shown in Figure 3.24, are: Solid masonry made up of long shaped stones, roughly squared and placed along horizontal layers. The connection through the thickness can be considered good (A1). Two leaves of dressed stones, roughly squared with some elements through the wall thickness. The infill between the two leaves is coherent, and the global connection in the thickness can be estimated as medium (A2). Mixed masonry limestone in long dressed elements, small squared stones and rubble. The fabric layout is characterised by alternate layers of long element placed along the bedding surface (stretchers), and other placed through the wall thickness (headers). Rubble is often used to fill gaps between the stones and for the infill, which is incoherent. The overall connection in the thickness can be assumed as weak (B1).
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Masonry mainly characterised by rubble, with layers nearly horizontal. The fabric layout is characterised by three/four layers of small stones, alternating with layers made of higher elements. The cross section is made up of small pieces with a large quantity of weak mortar. The overall connection through the thickness can be assumed as weak (C1). Masonry characterised by rubble of small size with some bigger elements inserted. The tiny dimensions of fabric elements and the weak quality of mortar do not allow any connection through the thickness so that this masonry fabric can be considered of very poor quality (C2). These typologies of masonry were also surveyed in the historic centre of L’Aquila, (D’Ayala, 2010), after April 2009 earthquake. In both cases the less connected fabrics were more sensitive to total and partial collapses, with the masonry units and infill losing cohesion due to dynamic excitation. Furthermore the quality of masonry had high influence on the overall behaviour of buildings: the collapse of horizontal structures was often due to the deformability of the walls, which remained almost undamaged, but caused the floor and roof beams to slip out of their supports.
A 70-75 A 70-75
160 160 B B' 120 B B' 120 A1 A2 80 80
40 40
0 0 A' A-A' A' A-A'
B-B' B-B'
A 70-75
160 BB'120 80 40
B1 0 A' A-A'
5
7
-
0
7 B-B'
A 50 A 65-70 160
160 BB'120 BB'120 80 C1 80 C2 40 40
0 0 A-A' A-A' A' A' 65-70 B-B' 50 B-B'
Figure 3.24 - Masonry fabrics surveyed in Nocera Umbra, (Speranza, 2003).
The use of multi-leaf and dressed masonry is extremely common and widely spread: D’Ayala, (D’Ayala, 2004a), surveyed cavity walls in the city of Bhuj, after the 2001 earthquake, and reports the common practice of local builders of constructing walls with two external leaves of better quality dressed stonework and an internal rubble infill; connections between the two external layers are generally missing. The author also surveyed, in a minority of cases, single-leaf bearing walls, which were made of either random rubble masonry or semi-dressed/ dressed stone masonry. They generally performed better, although the author acknowledges that the use of light-weight iron trusses for roofing could equally be the reason for reduced damage. Conversely, the lack of adequate connection between the leaves of the three-leaf masonry caused extensive damage. Such damage and out-of-plane deformation in general was less in those cases where L shaped corner stones had been used as quoins. Rubble masonry walls were also surveyed in Moroccan foundouks by Houet, (Houet, 2004). Typically walls of buildings in the Medina, the city centre, are composed of rubble of various sizes
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packed together like a dry stone wall. The thickness of the walls varies from building to building, but on average panels are at least 400.0 mm thick. The stones are held together by a lime and clay mortar, which is not very weatherproof and, for this reason, walls are covered with a lime and mud render, called “tadellakt”. This mixture is applied in layers containing more and more lime as it gets closer to the exterior surface. This give the wall a more durable, yet irregular or unlevelled finish, due to thick exterior coat (2.0 to 3.0 cm). The masonry walls of Porto (or Porto region) built from the 18th to the beginning of the 20th century consist of medium to large size stones (50.0 cm to 90.0 cm measured diagonally) often arranged in layers (relatively regular), with stone wedges and lime mortar closing the joints. Although classifications concerning the dimension of the stones already exist, (Binda 2000b), the large dimension of the stone blocks often found in Porto (and outside) walls demanded a new arrangement: large: greater than 80.0 cm; medium: between 50.0 cm e 80.0 cm; small: between 10.0 cm e 50.0 cm; very small: less than 10.0 cm. The stones are granite, changing from yellow (softer, i.e. more degraded) to blue (stiffer), have a rectangular type shape and exhibit reasonable good mechanical properties. The mortar joints show variable thickness (0.5 to 2.0 cm), cream colour and are quite brittle. The geometry of Porto houses, deeper than larger, makes the side walls to be the main vertical elements that support the floor beams. These walls, often shared by two buildings, are made of one leaf and are 30.0 cm to 50.0 cm thick. A large study has been done at LESE on this type of walls, in particular, the quantities of each material were evaluated using image processing resorting to computational tools, and a large experimental campaign was, and still is being performed on these walls. On the contrary, the main façades of the buildings present openings and are made of two leaves, being more than 50.0 cm thick. Figure 3.25 and Figure 3.26 show the example of different textures and cross section of wall panels.
Figure 3.25 - Different textures of stone masonry wall.
(a) (b)
Figure 3.26 - Typologies of cross section: (a) one leaf stone and (b) double leaf stone.
The characterization of such walls needs a thorough investigation of the geometry (elevation and cross-section) and the constitutive materials to identify the walls’ texture and cross-section and the evaluation of percentage of their constituents (stone, mortar and voids). The quantities of each material were evaluated using image processing resorting to computational tools.
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Texture: regular
Wedges: no Stone: granite Shape: regular Size: large
Mortar: - Figure 3.27 and Figure 3.28 show the example of the elevation and cross-section of walls panels after image processing. The cross section of the surveyed walls shows a widespread distribution of voids, more evidenced in the section midline. In fact, the mortar was originally placed along the border lines, without filling the inside where the absence of mortar is quite visible, creating cavities due to the irregular shape of both upper and lower stone faces contact. The stones have a rectangular type shape and exhibit reasonable good mechanical conditions. The size of the stones is very variable.
Walls’ texture Types of elements
Texture: Irregular
Wedges: yes Stone: granite Shape: irregular Size: medium/large
Mortar: lime and sand
Texture: joints irregulars Wedges: yes Stone: granite Shape: irregular Size: medium lime and sand Mortar: (1:3) Texture: regular Wedges: yes Stone: granite Shape: regular Size: large Lime and sand Mortar: (1:3)
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Texture: regular
Wedges: no Stone: granite Shape: regular Size: large
Mortar: -
Figure 3.27 - Examples of elevation survey, one leaf stone masonry.
Cross-section
232 220 22 6
30.0cm thick 40.0cm thick 50.0cm thick Figure 3.28 - Examples of cross-section survey, one leaf stone masonry.
In the past, building stones were material locally available, often round pebbles from rivers and creeks, (Figure 3.29). In this case, the size has a great importance because the reduced dimensions of the construction material implied multi-leaves sections and a worst seismic behaviour, even if empirical rules and detailing were developed to improve the masonry behaviour. Regular ashlar or block textures could hide inner rubble masonry. Similarly to brickwork masonry, massive sections as pillars could have only a regular leaf in the perimeter and a rubble core of variable consistence, (Figure 3.30). Furthermore, ancient monumental buildings were frequently built by monolithic elements, large stone blocks or lintels, (Figure 3.31). Monolithic sections were frequently used for columns, from ancient time up to now. The columns could be composed by drums coupled by metal or hard wood pins and bounded with lead, (Figure 3.32). The column drums were connected by metal dowels which were fixed with lead, and the regular blocks of the walls were fixed by metal clamps and lead as in general horizontal masonry practices, with no mortar used, (Figure 3.33 and Figure 3.34). Another construction feature commonly suggested as an earthquake preventative is the means used to join huge blocks together. It is believed that copper was used at Tiahuanaco, both of which are soft metals. It has also been suggested that these 'ties' were employed to 'ground' structures properly (often made of conducting Quartzite). To make sure everything looked regular and aligned properly, the final carving was saved until all the blocks were in place. The stone was worked down with chisels until it was finally smoothed by small stones and sand. If it was marble, it could be further polished with leather. Completely
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finished stones seemed to be reserved for only the most important buildings, as it took a lot of detailed work. The insertion of metals constraints allowed the tying of large blocks but also the possibility to reduce the building time as in case of bridges. Figure 3.34c shows the section of the Mostar Bridge, built reducing the supporting time by scaffolding due to the torrential river. Architectonic details were often reinforced by metals clamps or rods, (Figure 3.35).
Figure 3.29 - Main stone shapes diffused in Italy, (Carbonara, 2004).
Figure 3.30 - Example of three leaf stone masonry pillars: Cathedrals of Pavia, Noto and Milan.
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Figure 3.31 - Hollowed and extended beams: sections with oblique projection: (a) lintel of temple. (b) Architrave of temple. (c) Architrave of Athenian treasury) column drum. (e) Ceiling beam cross-beam.
Figure 3.32 - Details of column drums.
Figure 3.33 - Tied stone-blocks: a) The U-Shape holes on top, here for levers rather than cranes because of the small stone size. b) The dove-tail clamp connecting the top of the two stones c-f) Preliminary finishing.
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Figure 3.34 - (a), (b) Stone masonries made by tied large stone blocks and (c) detail of the Mostar Bridge.
Figure 3.35 - Architectonic details reinforced by metals elements.
3.1.1.3 Mixed brick-stone masonry Mixed brick-stone masonry was diffused since Roman time, (Figure 3.36). If the brickwork is regularly arranged in the wall and crossing the masonry section it can connect the leaves of the stonework masonry, improving its seismic behaviour, (Figure 3.37a). In the North of Italy, since roman time and particularly during the Gothic period, masonry was frequently built with alternate layers of inclined pebbles and bricks, (Figure 3.38). In other situation, bricks or tiles can be irregularly placed in the masonry, (Figure 3.36, Figure 3.39).
Figure 3.36 - Mixed stone/brick typologies: texture arrangement. 1) sub-horizontal coursed with ashlars and small pebbles between the blocks; 2) with horizontal courses of bricks and ashlars regularly positioned in the masonry ; 3) bricks placed vertically and horizontally around each stone elements; 4) regular coursed of bricks alternate to irregular stonemasonry, (Frankovich, 1988).
(a) (b) (c)
Figure 3.37 - Mixed stone-brick sections. (a) from the Cesariano’s version of Vitruvio treatise (b) (c) from (Gurrieri, 1999), (Ferrini, 2003).
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(a) (b) (c) (d) (e)
Figure 3.38 - Mixed stone-brick textures. (a) brick work layers alternate to ashlar bricks masonry (b) rounded pebbles and (c), (d) insertions of tiles and bricks between stones, irregular texture (e) insertions of tiles and bricks between stones, regular texture.
Figure 3.39 - Church in the Medieval site of Mystras (stone masonry with stones surrounded by solid bricks-the so-called “plinthoperikleisti”). Stone blocks alternated with bricks, is a diffused masonry in Byzantine buildings. However this type of masonry is a three-leaf masonry. The interior leaf is made of rubble stone masonry. Poor quality filling material is placed between the two leaves.
3.1.1.4 Timber reinforced masonry. Other masonry typologies could be the mixed masonry-timber structures. As reported in (Vinzeleou, 2010), the survey of numerous monuments and historic structures in Greece - a country situated in the most earthquake prone area of Eastern Mediterranean - has revealed a systematic and continuous through-centuries use of timber elements within masonry. This observation could be easily extended to whole the Mediterranean countries and not only. The extent of the use of timber in the structures, the contribution of the timber to the overall structural system, the specific measures taken to protect timber from decay and, hence, the efficiency of the developed structural systems do present a vast variety, associated with social and economical factors. This extensive use of timber makes it legitimate to assume that our predecessors were aware of the effect of timber reinforcing systems on the seismic behaviour of structures. (Vinzeleou, 2010) provides information on the use of timber in structures in Greece ranging from the Minoan Crete-2.500 years B.C., to Akrotiri, Thera - 16th century B.C., to Byzantine churches
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and Monasteries, and finally to more than 70 urban nuclei within the country-developed in the 18th and 19th centuries. In several cases the damage revealed unexpected large timber beams imbedded in the walls near the top with a tie function. It is not known how widespread the use of horizontal timber ties is. Timber elements could be used as horizontal tie running along the perimeter of the building at regular intervals or as a frame, which contribute to the overall structure behaviour.
3.1.1.4.1 Timber-tied stone masonry In Greece, as well as in the entire region of eastern Mediterranean, systematic and continuous in- time use of timber reinforcement is observed in masonry structures, (Figure 3.40). Timber-laced stone masonry walls may commonly be found in two storey buildings in Bosnia and Herzegovina as inheritance of Ottoman period in Bosnia and Herzegovina, 15th-19th century. Timber elements are combined with masonry in various structural systems, both in monumental structures (such as palaces and churches) and in residential buildings. The architectural, historical and socio-economic study of those structural systems is quite advanced. Horizontal timber tie consists of more than one longitudinal timber element, running along the perimeter of the building and by transverse timber elements that connect the longitudinal ones at regular intervals. Rubble stone masonry or three leaf stone masonry are the most commonly met construction types. Lime mortar or lime-pozzolan mortar was used; in a number of cases, clay mortar was also found. The thickness of masonry varies between 0.70 m and 0.90 m (depending on the number of storeys, the length of the wall between transverse walls, the actions to be resisted by the building, etc.). In the case histories studied in (Vintzileou, 2008) wood from coniferous or chestnut trees is used. The typical sectional dimensions of the longitudinal timber elements are in the case histories reported in (Vintzileou, 2008) 100 x 100 mm or 80 x 80 mm concerning Greece, or 120 x 120 mm to 140 x 140 mm in the case histories reported in (Humo 2008) in Bosnia and Herzegovin. The size depends generally of the masonry thickness, as well as on the number of longitudinal timber elements within the thickness of the wall. The typical sectional dimensions of the transverse timber elements are 0.06 x 0.06 m. In some cases, only one longitudinal element is used. It is located either close to the longitudinal axis of the wall or close to its interior face (where it serves also as support of the timber beams of the floor). The most common case is that of a pair of longitudinal elements, (Figure 3.41), located either at the two faces of masonry or at a distance from them (mainly, to protect the wood from humidity). There are, however, some cases in which three longitudinal elements were used or, in some extreme cases, the width of the timber elements is equal to the thickness of the wall, (Figure 3.41). In case of two joists (double lacing) in Bosnia and Herzegovin, (Humo, 2008), it is common to find lateral timber joists connecting longitudinal joists typically at 1500 to 2000 mm centres. It is assumed that timber-lacing acts like reinforced concrete ring beams in contemporary masonry and thus improves seismic response of the masonry structure, (Figure 3.42). Timber ties act as flexural and shear reinforcement and increase the deformability of masonry, (Figure 3.43). The main problem of timber reinforced systems is the time degradation of wood. When transverse timber elements are simply resting on the longitudinal ones, connection of the two exterior leaves of masonry may not be sufficient, (Figure 3.44).
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(a) (b)
Figure 3.40 - Timber reinforced building in Greece, (Vintzileou, 2008).
(a) (b)
Figure 3.41 - Arrangement of longitudinal timber elements within the thickness of masonry, (NTUA/EPPO, 2005), (Vintzileou, 2008).
Masonry section with timber lacing: poor or absence of connection between the stone masonry leaves and decay of the timber lacing
Timber roof structure
Timber floor structure
Timber window lintel
Outer timber lacing Timber floor structure Decayed and destroyed outer timber lacing Stone masonry wall, Decayed and destroyed Inner timber lacing outer leaf, outside inner timber lacing Stone masonry wall, outer leaf, inside Collapse of the inner core Stone masonry wall, inner core
Undamaged stage - Three-leaf Start of the collapse - Decay of Collapse development - masonry wall with double timber inner timber lacing leads to Progressive overturning leads to the lacing at floor level. overturning of the upper masonry destruction of the decayed outer assemblage and start of the collapse timber lacing, further collapse of the of the inner core constructed with inner core and separation of outer rubble stone and poor hydrated lime leaves.
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mortar.
Local collapse of the façade wall. Detail of the collapse. View from Detail of the collapse. View from outside. inside
Figure 3.42 - Damages on a timber framed masonry, (Humo, 2008).
Figure 3.43 - Timber ties act as flexural and shear reinforcement and increase the deformability of masonry.
Figure 3.44 - When transverse timber elements are simply resting on the longitudinal ones, connection of the two exterior leaves of masonry may not be sufficient.
3.1.1.4.2 Timber framed masonry A remarkable building technique concerns the wall framing by timber elements. The technique has several local varieties, and it was in general considered very effective for its seismic behaviour. It is worth to remind its diffusion in the 19th century for the extensive reconstruction after catastrophic earthquakes, as in Portugal (Lisbon), Italy (Reggio Calabria), or in Greece (island of Lefkada). In Reggio Calabria, the use of timber reinforcement against earthquakes evolved into a patented system known as the Casa Baraccata, but the most famous case histories is related to “Baixa Pombalina”, the historical downtown of Lisbon rebuilt after the great earthquake of 1755, (Paula, 2006), (Cóias e Silva, 2002).
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“Pombaline” is the term coined after the Marquis of Pombal, the prime minister at the time of the 1755 earthquake, who took most of the decisions regarding the reconstruction of Lisbon. One of the most remarkable features of the buildings which make up this area is their structural concept, the distinctive “gaiola” system, introduced after that earthquake. In line with the post earthquake measures designed by the chief engineer, Manuel da Maia, the new buildings during the first decades of reconstruction after the big shake incorporated a set of features intended to provide them with adequate seismic behaviour, enabling them to resist horizontal loads and to dissipate substantial amounts of energy. Among these measures, the so called “gaiola” or “cage” system stands out, (Figure 3.45). The system consists of a set of timber members embedded along the inner face of the main stone masonry façade walls. To these members and to the ashlars around the openings, an internal timber grid is connected by means of iron cross ties. Further bracing is provided by the timber floors, whose diaphragm action is enhanced by iron ties, bolted to the floor beams and deeply embedded in masonry main walls, and by timber connectors, named “hands”, nailed to the above mentioned timber grid and also embedded in the masonry. The confined facade pilasters are then connected to a two-directional vertical bracing system of timber framed walls with light ceramic and rubble masonry infill, (Figure 3.46). Often small stones and ceramic elements were recovered from the ruins of the earthquake and then assembled with lime mortar, (Paula, 2006).
(a) (b) (c)
Figure 3.45 - Details of the Pombaline construction, (Paula, 2006), (Cóias e Silva, 2002).
Figure 3.46 - Examples of the composite timber-masonry walls: three main types of internal timber arrangement, (Paula, 2006), (Cóias e Silva, 2002).
Another relevant example concerns the island of Lefkada described in (Vinzeleou, 2007), a local structural system was developed before the 19th century in Lefkada. The strong earthquake which occurred in 1821, proved the adequacy of the system to sustain seismic actions. Thus, the British Authorities (ruling the Ionian Islands at that time) imposed rules for the construction of new houses following the main characteristics of this local structural system. These rules, further developed and completed, constituted the Code for Construction, issued in 1827. That Code provided guidance on the selection of building materials, on the thickness of stone masonry in the ground floor, as well as on the maximum storey height. Typical buildings (one- to maximum three-storey buildings) consist of a stone masonry ground floor plus one or two timber framed brick masonry storeys. Intermediate floors and roof are made of timber, (Figure 3.47). The roof is covered with tiles. Openings are practically symmetrically arranged in plan. To protect the timber framed
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masonry from humidity, the upper storeys used to be covered along the exterior façade by timber planks. Bearing walls in the ground floor (typically, perimeter walls) are no more than 3.0 m high and they are made of stone masonry (0.60 to 1.00 m thick). The external leaf of walls is made with roughly cut stones, whereas cut stones are used in the corners of the building, as well as along the perimeter of openings, (Figure 3.47). Rubble stones are used in the internal leaf of ground floor walls. The space between the two leaves is filled with small size stones mixed with pieces of bricks and mortar. In addition to masonry walls, a secondary (timber) bearing system is present in the ground floor which consists of timber columns arranged close to the masonry walls. The timber frame of the perimeter walls is connected to the ground floor masonry through timber beams, arranged along the perimeter of the stone masonry walls. Metal ties (of various geometry and size, are used to connect these timber elements of the floor with the stone masonry and/or with the timber frame of the upper storeys.
(a) (b) (c)
Figure 3.47 - Typical morphology of timber-framed walls, (Vintzileou, 2007).
The peculiarity of this structural system, which has sustained several strong earthquakes, lies in the secondary timber structural system provided to all buildings, (Errore. L'origine riferimento non è stata trovata.). As proven by survey and calculations, the secondary system (too flexible to contribute to the seismic behaviour of the building) is able to safely sustain vertical loads, in case the masonry of the walls between ground and first floor is severely damaged or even collapsed due to a seismic event. Thus, the structure remains safe and gives the population the time that is necessary for repair or reconstruction of the damaged masonry. In several cases, extensive decay of the timber elements was observed due to biological attack (favoured by high percentage of humidity), (Figure 3.49). The form of this damage though obviously not attributed to the earthquake is mentioned herein, since it has affected the seismic behaviour of the buildings. In fact, the reduction, which was observed in some cases, of the sectional dimensions of the timber beam serving as support to the upper storey(s) perimeter masonry walls led to the transfer of loads directly to the secondary timber system of the ground floor. Thus, the upper storey(s) were supported only by the flexible secondary bearing system (i.e., by the timber columns). Secondary timber bearing system (timber columns and connections): In this case too, reduction of the sectional dimensions of the timber elements was observed due to decay. Intermediate (timber) floors: The decay of timber beams due to environmental and biological actions led, in some cases, to a reduction of their cross-sectional dimensions and, by way of consequence, to large permanent deflections of the floors. Timber framed masonry: Similar damages were observed in the timber elements of this system as well. In addition, cracks were observed in several cases between brick masonry and surrounding timber elements, (Figure 3.49a). In a limited number of cases, out of plane collapse of the filling masonry was observed.
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(a) (b)
Figure 3.48 - Typical house of the historic center of Lefkada, (a) The bearing system (schematic, sketch by P. Touliatos), (b) The function of the secondary timber system (schematic, sketch by P. Touliatos).
(a) (b)
Figure 3.49 - Typical damage of the Lefkada masonry and advanced decay of timber elements along the perimeter of stone masonry, (Vinzeleou, 2007).
3.1.1.5 Earth construction: Adobe / Unfired Earth Block / Rammed Earth / Cob Load-bearing earth construction (illustrated in Figure 3.50) can be subdivided into: Adobe or (unfired) Earth Blocks; Rammed Earth, Taipa or Pisé; Cob.
(a) (b) (c)
Figure 3.50 - Different earth building techniques: adobe, rammed earth, cob (from left to right).
These differ in mechanical, physical and chemical properties. While in general terms their seismic behaviour is brittle, parametric studies comparing structural behaviour in terms of building
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technique (whether static or dynamic) are few in the literature. One exception is in (Vargas Neumann, 1993). The understanding and modelling of structural behaviour of earth structures is limited by a lack of consistent testing of the mechanical properties of their constituent materials.
3.1.1.5.1 Adobe / Unfired Earth Block Adobe is a Spanish term indicating earth blocks. For the purposes of this project, the term adobe shall be used throughout. Adobe walls are non-monolithic structural elements consisting of masonry units (adobe) and mortar. In order to produce adobes, soil is mixed with fibers and if necessary with sand. After a certain amount of time in which the resulting paste “matures”, the mixture is formed into blocks by means of moulds, and air dried. Construction takes place by using alternating layers of adobe and mortar, whose binder in adobe structures is usually silt and clay fraction. Some typical masonry units and wall typologies found in historic adobe structures are shown in Figure 3.51. Mortar used in adobe construction is usually based on an earth binder (sometimes combined with lime, CORPUS 2010a), whereas sand, gravel and chopped straw are used as aggregates. Joint thickness (tj) varies from tight, 0.1 tm (tm is masonry unit thickness to broad 0.5 tm < tj = 1.0 tm. Wall thickness varies between 30 and 150 cm. Typical adobe wall typologies are shown in Figure 3.51. In many historical adobe buildings, wall slenderness is (height-to-thickness) is < 5. Walls are in some cases tapering.
Figure 3.51 - Examples of adobe unit sizes and wall (non exhaustive).
Adobe structures are the only earthen structures listed in the 1998 European Macroseismic Scale, (Grunthal, 1998), according to which, together with rubble stone, they are the most vulnerable to earthquakes. Data gathered after the destruction caused by the 1994 Northridge Earthquake indicates that a peak ground acceleration (PGA) of 0.1 to 0.2 g is sufficient to initiate damage in well-maintained unreinforced adobe masonry buildings, (Webster, 2006). Adobe seems to fail from cracks in the mortar, with cracks usually following stepped patterns along the mortar joints, (Vargas Neumann, 2006), (Hardwick, 2010).
3.1.1.5.2 Rammed Earth Rammed earth is also known as taipa (Portuguese) or pisé (French). For the purposes of this project, the term rammed earth shall be used. Unlike adobe walls, rammed earth walls are
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homogeneous, monolithic structural elements. Both the term rammed earth (to ram) and the term pise´ (archaic French, pisér “to crush”) refer to the compaction of the constituent materials. The material (earth, sometimes with straw, lime or gravel), after being left to soak, is placed in the formwork and compacted using a heavy wooden tool, or rammer. Once one layer is compacted, another layer of earth is placed in the formwork and the process repeated until the formwork is full and therefore removed to complete another section of the wall, first horizontally, and then vertically. Historic rammed earth walls therefore consist of modules laid without mortar, the size of which depends on the dimensions of the formwork used. Each layer if known as a lift, and at many historic rammed earth sites material such as lime, straw, stones and bricks is placed between the lifts, (Jaquin, 2006). Historic rammed earth walls are formed by ramming layers of earthen material (usually lime and earth in varying proportions) within formwork. Each layer of rammed earth is known as a lift. Sizes reported (CORPUS, 2010a) for each module (what is rammed within one formwork box) are: 1.0 to 3.0 m length and 30.0 to 50.0 cm height, but the author has observed layer heights up to 1.0 m. According to whether the formwork is fastened externally or internally, holes are present, (Jaquin, 2006a). Different typologies of rammed earth walls are shown in Figure 3.52. No mortar is used between units but in some cases lime, straw, stones, turf, vegetable matter or bricks are rammed between layers. Langenbach in (Langenbach, 2004) and Hurd in (Hurd, 2006) suggest that these ‘mattresses’ between layers are a seismic protection measure since they force diagonal shear cracks to propagate horizontally thus preventing collapse. Even when no layers are placed between lifts, the construction process causes the bond between lifts to be weaker than bonds compacted within the formwork, (Jaquin, 2006a).The thickness of rammed earth walls is determined by the formwork in which it is rammed. Walls are therefore one-leaf. Joints are “staggered”, as in brick masonry. (Vargas Neumann, 1993) describes research carried out to assess the performance of rammed earth walls under seismic loading, with the aim of identifying parameters which influence seismic vulnerability. Parameters chosen and tested were: soil granulometry, humidity content, compactation level, use of natural additives, and “joint treatment”, i.e. presence of materials between layers. The results of the study, which was conducted in order to allow comparison of rammed earth walls with adobe walls, showed a higher resistance and deformation of rammed earth walls in comparison with adobe walls (up to 40%). Current anti-Seismic buildings codes for earth buildings, (NZS, 1997), do not take this difference in consideration.
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Figure 3.52 - Different typologies of rammed earth walls, after (Jaquin, 2006a)
3.1.1.5.3 Cob Cob is a mixture of clay subsoil, aggregate and straw. Walls are built without shuttering on a stone plinth. Literature on the subject of cob in seismic areas is known by the author to be limited to (Langenbach, 2004), but it is documented the presence of cob and its repair methods (as well as adobe and rammed earth) in Central Asia, in relation to seismic and other structural damage. Extensive testing on the compressive strength, density and Young´s Modulus in relation to moisture content was carried out by Ziegert, (Ziegert, 2003).
3.1.1.5.4 Pre-existing conditions Unlike some other historic structural materials, earth buildings are highly susceptible to Cracking, Rising Damp and Salt Damage, Erosion, Termite Infestation, and generally loss of section cause irregularities in strength, stiffness and mass which mainly contribute to poor performance under earthquake conditions. Structures affected by these conditions will clearly be affected at lower pga than the 0.1 to 0.2g reported by Webster, (Webster, 2006), to start damage in adobe buildings. Taking Bam, Iran, as an example, the collapse mechanisms theories proposed in (Kayano, 2004) do suffice in explaining why some of the dwellings were still standing after the earthquake (Langenbach, 2004), (JSEE, 2004). Langenbach´s explanation is that the condition of the buildings at the time of the earthquake was very poor. While (JSEE, 2004), who noticed the good performance of the arch roof of the old adobe buildings, attributes the failure of other buildings to improper and lack of seismic safety consideration in the restoration program and the presence of heavy roofs and walls, as well as the lack of structural integrity especially in newly built buildings, Langenbach (2004) attributes the catastrophic collapse of the Arg-e Bam to internal wall degradation. Cracking: Cracking in earthen buildings is common as from the moment they are built, and is set off by shrinkage straight after construction. Shrinkage cracking is then worsened by conditions such as weathering and differential settlement. Rising Damp, Salt Damage and Erosion: The influence of moisture and salt content on the compressive strength of earthen materials has been extensively researched by Ziegert, (Ziegert,
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2003). A detailed analysis of the erosion process in relation to rising damp and salt damage, can be found in (Fujii, 2009).
3.1.2 Damage mechanisms 3.1.2.1 Walls As demonstrated by the post-earthquake damage surveys carried out after all earthquakes affecting areas where masonry buildings are common, one of the main sources of vulnerability for such structures is associated to local failure modes, mainly due to out-of plane response of walls (Giuffrè 1993), (Doglioni 1994), (Lagomarsino, 1997), (Binda, 1999a, b), (Borri 1999a), (D’Ayala, 1999a, b), (Zuccaro, 2003), (Figure 3.53). The building seismic response can be governed by such mechanisms when connections between orthogonal walls and between walls and floors are particularly poor. This is often the case in existing stone masonry buildings without tie rods, with lack of interlocking at the connection of intersecting walls, presence of simply supported wooden floors and thrusting roofs. Only if connections are improved by proper devices (e.g. tie-rods), local mechanisms can be prevented and a global behaviour governed by the wall in-plane response can develop.
Figure 3.53 - Damage after the earthquake in Messina in 1908, (Giuffrè, 1993).
The kinematic mechanism of a single panel can be classified related to the masonry quality, the boundary conditions or constraints which could involve simple overturning, vertical and horizontal bending and mixed overturning mechanisms. Masonry quality and local discontinuities, such as the placement of chimneys flues in the plane of the masonry bearing walls or new openings and closure of previous openings or the building evolution, affect the mechanism. Furthermore, the damage present in a structure can be the result of continuous modifications taking place over the life time of the building. The position of the load bearing and non load bearing walls could have been modified, new floors inserted, or doors and windows opened, etc., in different time, with different building technologies and materials, as well (Figure 3.54 to Figure 3.57). All these modifications can change the global and local behaviour of the walls under the different actions, especially seismic actions, which tend to separate the subsequent additions or produce hammering, as shown in Figure 3.54. The damages could affect not only both added portions or small volumes but also superstructures. In this last case frequent mistakes, like the construction of walls or pillars directly on floors, or the change of the building technology can have catastrophic effects, (Figure 3.55 to Figure 3.56). Damages caused by the opening infilling or local dismantling and building of the new window/door frames can be surveyed, indicating the low compatibility of the new masonry technology with the previous. Furthermore, the insertion of new opening shows a scarce attention to the load bearing walls or the floor loading, (Doglioni, 2007), (Figure 3.57). The presence of discontinuities can be visually detected or found by non destructive techniques such as thermovision, sonic or radar tests. It is important to identify and map their presence because the lack of continuity will locally change the stiffness of the wall.
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Environmental decay can affect masonry walls, reducing the load bearing section; the decay generally starts from the mortar joint erosion, (Figure 3.58).
(a) (b)
Figure 3.54 - (a) Crack pattern revealing the evolution of the building, (Binda, 2005a). (b) Damage, (Doglioni, 2007).
Figure 3.55 - Crack pattern revealing the superstructure, (Doglioni, 2007).
(a) (b)
Figure 3.56 - Typical additions of upper storeys in Ortigia, (Giuffrè, 1993), and frequent mistake, (Doglioni, 2007).
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(a) (b) (c)
Figure 3.57 - Damage caused by the opening infilling local dismantling and building of the new opening frame. It is remarkable in the case the scarce attention to the floor loading, (Doglioni, 2007)
(a) (b)
Figure 3.58 - Load bearing reduction due to decay. (a) (Binda, 1997a) (b) (Doglioni, 2007).
In plane behaviour In plane behaviour of masonry panels generally involves an effective overall building configuration, as already commented in paragraph 2.2.1, preventing brittle out-of-plane failures. The seismic behaviour could be considered “second way mechanisms”, according Giuffrè definition, (Giuffrè 1993), because the relative damage (shear cracks), generally does not lead the structure to collapse, in comparison with the out-of-plane mechanisms. Nevertheless, the shear damage of a panel could trigger the out of plane mechanism of the connected walls (mixed mechanisms). Kinematics chains describe the in-plane rigid rotation of the resisting structural portions of the building, defined by particular geometrical (dimensions of septa, openings) and bond conditions (connections, presence of ties), subjected to in-plane horizontal actions. In general the shear damages of masonry panels is reveals by crack patterns of Figure 2.5, Figure 2.7, Figure 3.59 and Figure 3.60. Similarly to masonry, the in-plane failure modes of the earthen walls subjected to the vertical and horizontal loads are of three types: Failure by slip: The wall undergoes a relative displacement along a plan of low shear strength such as the joint of horizontal mortar (adobe), the construction joint of built (cob), etc…
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Failure under bending: The wall behaves like a cantilever beam subjected to bending moment and axial force. Shear damage: characterized by the formation of the diagonal cracks.
(a) (b) (c)
Figure 3.59 - In-plane failures.
(a) (b)
Figure 3.60 - From (Valluzzi, 2007). Scheme of the in-plane kinematics model for a wall under in-plane actions: (a) single wall, (b) multiple-wall system, (Giuffrè, 1993). Symbols in figure are as follows: N is vertical load acting at distance L from compressed edge, P is weight of detaching portion and T is tensile force in tie, obtained by difference between the weight cQ of supported portion of wall and counteraction (q) allowed from supporting base of wall.
(a) (b)
Figure 3.61 - The layout of the crack will depend on the dimensions of the units and the overall wall, while the type of mechanism, sliding (a), overturning (b) or mixed (c) will depend also on the loading conditions.
The formation of an in-plane crack is a function of the geometry of the wall and shape ratio of the unit and is independent of the crack angle: this means that, whatever the position of the crack, for
c<b, the total shear strength is constant and equivalent to the portion of a triangular wall identified by the angle b and the height H of the wall considered, (Figure 3.61). For cb, on the other hand, the Ctot becomes independent of the block shape ratio, (D’Ayala, 2003b). The above approach can be applied to a wall with openings, (Figure 3.62), by subdividing it into vertical piers and horizontal spandrels, depending on its number of storeys, while the lateral
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deformability of the wall, whether governed by flexure or shear, will depend on the relative stiffness of piers and spandrels. This in turn will define the redistribution of the external forces among piers, their crack pattern and their ultimate load factor. Hence the problem still reduces to the definition of a collapse mechanism (overturning or shear), a collapse load factor, and an angle of crack. The result will be dependent on the angle of friction chosen, the geometry of the masonry unit, and the geometry of the wall and its openings. In general in a real wall, openings will have different size and ratios, and will not be distributed regularly, so that there will be differences in geometry and hence capacity, among the piers. The spandrels can also have different height. In order to extend the procedure, the assumptions made are that the wall as a whole is defined by one width (L) and one height (Hw); the number of openings at each storey is variable; the size of the opening is constant at each storey (ls) and can be obtained as average of the real sizes. Under these conditions the weakest pier alignment can be identified on the basis of the geometric parameters. However the procedure allows calculating more than one pier alignment in succession if so required, (Casapulla 2006).
Figure 3.62 - Possible crack lines in a multistorey piers and spandrels. A uniformly distributed load due to the live loads and horizontal structures is assumed to act at each storey level. hf and hs are the inter storey height and the height of the spandrel, respectively, Hp=(Hf-hs/2) is the variable height of the pier, with Hf=nhf the variable height of wall considered at each iteration depending on the number of storeys n, and hence hf≤Hf≤Hw, with Hw total height of the wall.
A relatively common case is represented by facades with porticos or piers at the ground floor. In this case for the in plane mechanism the most vulnerable part tends to be the ground floor. The mechanism development and calculation are included in Figure 3.63.
Mechanism Load factor
L C p tot G H W p p m WG 2
m = number of pillars; Lp= width of pillars in the plane of the facade.
Hp= height of pillars; Wp= weight of each pillar;
WG = weight of facade portion above colonnade;
= Number of connection between façade wall and orthogonal edge walls (0-2); M = Number of internal bearing walls orthogonal to the façade;
CtotG = Total shear strength exerted by orthogonal edge walls (lateral and internal)
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L
G 2 3 HG W 1 HG Ctot ( ) Ctot ( ) W F G H W N-z
G p G p G p Hp Wp Wp Wp
Lp Contribution of restraint by orthogonal walls Figure 3.63 - Mechanism of collapse for in-plane overturning for presence of porticoes, (D’Ayala, 2003a).
Simple overturning The simple overturning of external walls could be considered as one the most frequent and brittle collapse mechanism. The mechanism involves a rigid rotation of a wall or of a portion of a wall around a horizontal hinge, (Figure 3.64). The out of plane actions due to the earthquake start the mechanism.
(a) (b)
(c)
Figure 3.64 - Overturning of the whole façade and of a portion (a) (Giuffrè, 1993). (b) (Borri, 2004c). (c) (Doglioni, 1999).
In the worst situation, the wall is free on top, without any restrain, and not connected to the lateral orthogonal walls. Weak connection could activate the mechanism, as well as shove actions from beams, tie beams, etc. The boundary conditions are, in general, the lack of effective connections and constraints. The mechanism is easy recognisable by vertical crack patterns between the wall and the orthogonal lateral walls and the presence of horizontal cracks, (Figure 3.66). In some cases, the floor beams collapse, (Figure 3.64). Different masonry portions, local damages and opening geometry could start the overturning. The mechanism could be limited to one or more building floors, (Figure 2.11, Figure 3.64, Figure 3.65), related to the floor connection, masonry typologies, restrain geometry etc. In the Italian code, the collapse index has to be calculated for several position of the horizontal hinge.
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In multiple leaf masonry the collapse could involve only the external leaf, with a reasonable decrease of the collapse index, (Figure 3.1, Figure 3.2 and Figure 3.53). Figure 3.67 shows some example of damaged structures. Mechanism Load factor
j 2 r j Ti hs 2 l TN L j sb kL hs j Ti T hs j i i112 3 l r 2 i1 (0), j j 1 j hs LTi j i kL j j i i11 2 i
is the number of internal bearing walls orthogonal to the façade, and effectively A connected to it as to provide restraining action due to friction
2 j1 r j1 Tmj Tmj L h l L 2 Lj kL kh tan j i (1 ) s j 2 sb jkh h tan j i s j s s j 2 2 i1 L 3 l 1 r 2 3 i1 (0), j j1 LTmj hs 2 2 j j kL khs tan j j i 3 2 3 i1 as above
L width of facade in between party walls. D Tmj average thickness of wall over height of overturning portion
j 2 r j Ti 1 2 l TN Lvar hs hop hs j sb kLvar hs j Ti T hs j i i112 3 l r 2 i1 (0), j j 1 j hs Lvar Ti j i k j j i 2 i11 i as above as above = (1,2) E is the number of vertical discontinuities within the façade non coincident with the façade edges = (1,2) integer =(0,1) provides the number of active side connections Figure 3.65 - Possible mechanisms of collapse for simple overturning depending on boundary conditins and connections between piers and spandrel, (D’Ayala, 2003a).
(a) (b)
Figure 3.66 - Overturning of the outer leaf (a) (Borri, 2004c). (b) (Binda, 2006a).
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(a) (b) (c) (d)
Figure 3.67 - Separation of wall due to out-of-plane bending.
Vertical out-of-plane bending A common situation in masonry buildings concerns a wall restrained at the extremity and free in the central area. This could happen in case of irregular layout of restraints or tie beams, (Errore. L'origine riferimento non è stata trovata.a, Figure 3.69). The ties prevent the wall global overturning but other actions or the floor hammering or the masonry quality could start the vertical instability and the wall bulging, (Errore. L'origine riferimento non è stata trovata.c, d). The upper and the lower bonds in general are effective to prevent the global overturning of the wall. The collapse index is higher then in the unrestrained wal. In a damaged building, a relevant out of plumb or the wall bulging reveals the mechanism that could involve one or more floors. The mechanism often concerns repaired building by r.c. tie beams not passing thought the wall section, (Errore. L'origine riferimento non è stata trovata.c, d). As in previous situation, in multiple leaf masonry the collapse could involve only the outer leaf, (Figure 3.70). Figure 3.71 summarizes kinematics models for out-of-plane mechanisms involving vertical strips, (Valluzzi, 2007).
(a) (b)
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(c) (d)
Figure 3.68 - Separation of wall due to out-of-plane bending.
Mechanism Load factor
r1 r2 Ti Ti TN Ti 2sbhv 1 l l Ti Tk k hu khs 2 1 lr l r 4 hv 2 3l 0 r1 r0 1 r2 r1 h h 1 k 1 v T T T T h h h 4 2 i j k j v i j r0, r1, r2, u T 2 T 2 1 T 2 h 6 i k j v h 1 k 1 v T T T T h h h 4 2 i j k j v i j F number of courses above upper hinge, middle hinge, lower hinge respectively real number (0,1)
hv height of portion of wall subjected to mechanism
Kinematism Contribution of friction forces along the sidewalls Figure 3.69 - Development of vertical arch mechanism and load factor calculation, (D’Ayala, 2003a).
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Figure 3.70 - Vertical bending and damage of a multiple leaves wall, (Borri, 1999b).
Figure 3.71 - Kinematics models for out-of-plane mechanisms involving vertical strips, (Valluzzi, 2007).
Horizontal out-of-plane bending - Arch mechanism Restrained panels to orthogonal walls but not in the upper side could be damaged by bending in the horizontal plane. Floor or roof beams could thrust the wall but are restrained to the side walls connection. The general behaviour involves an arch mechanism within the wall section caused by the out of plane actions, (Figure 3.72). The condition of limit equilibrium is characterised by the developing of three hinges, one in the middle and the others close to the connection of the lateral walls, (Figure 3.72c). In case of good quality of the lateral masonry and effectiveness of the connections, the collapse does not happen but the inner side of the wall could be compressed, (Figure 3.72b). The mechanism is typical of walls restrained by ties. The beam hammering or the roof thrust and the low quality of the masonry could produce the whole mechanism or local damages. More in detail in multiple leaf masonry, the mechanism could involve only the external leaf that could collapse, (Figure 3.73 and Figure 3.74). Chimney flues, internal masonry discontinuities like holes for rainwater pipes or ducts for other technical plants reduce the load bearing section, localising the hinges for the kinematic mechanism. Their mapping should be one of the keypoint of the building survey addressed to the seismic structural analysis, (Figure 3.75). Furthermore, the opening geometry and position could affect the behaviour and the extension of the damaged area, (Figure 3.75). In slightly damaged structures the interpretation of the mechanism could be carried out by the crack pattern survey, which reveals the macro element geometry. In most cases, in undamaged structures it is difficult to evaluate correctly the probable kinematic mechanism, (Figure 3.73 and Figure 3.76). Figure 3.77 summarizes kinematics models for out-of-plane mechanisms involving horizontal strips, (Valluzzi, 2007).
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(a) (b) (c)
Figure 3.72 - Arch mechanism (a) (Borri 2004c, d).
Figure 3.73 - The roof beam hammering can start the kinematic mechanism, as well as the window position.
Mechanism Load factor
2 r r T T kL l T T L l l l l u i u i sbT T s b l 4 2i 2 4 u i r r l11l ,hi 2 2 l L Tu Ti kL 5l 5Ll L 8 2i 3l l 2 l L hi 2tg i valid for 2l l L L
G The index identifies quantities associated with internal bearing walls
Kinematism and contribution of internal orthogonal walls to the restraint
Figure 3.74 - Development of horizontal arch mechanism and load factor calculation, (D’Ayala, 2003a).
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Figure 3.75 - The presence of chimney flues reduces the load bearing section, localising the hinges for the kinematic mechanism.
Figure 3.76 - Arch mechanism due to horizontal bending: layout of the damaged area a) parabolic, b) triangular c) rectangular, (Borri, 2004d).
Figure 3.77 - From (Valluzzi, 2007): kinematics models for out-of-plane mechanisms involving horizontal strips, (Bernardini, 1988).
Complex overturning mechanism
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In most cases, the mechanisms are triggered by complex damages. In this case, the overturning of a wall acts on the orthogonal walls and on the corners, which could be damaged by in plane action (Figure 3.78 to Figure 3.82). The mechanism involves the lack of constraints at the panel top but effective side connections with the orthogonal walls. In general, this could happen when wall connections are effective, in contemporary building portions or in strengthened buildings but with irregular disposal of ties. The geometry of the damaged area could change with the floor characteristics. Traditional floors involve the façade overturning and the diagonal cracking of the orthogonal wall. Stiff floors, instead, produce a double diagonal shape of the cracks in the shear wall. Many factors affect the mechanism, such as the masonry quality, the openings geometry e positions in the shear walls, the discontinuity localisation (e.g. chimney flues, plants, etc.). In case of shear walls without openings the inclination of the diagonal crack increases with the masonry quality corners, (Figure 3.79). The presence of openings close to the wall intersection affects the geometry and the shape of the damaged area. The collapse index decreases with the decreasing of the area of the shear walls involved in the mechanism, up to the value of the simple overturning of the façade. The macroelement geometry derives from several factors and could involve many building floor, in relation to the presence of ties or other constraints. The mechanism frequently affects good quality masonry. Contemporary shear damages of the perpendicular walls - the restraining walls - could start the mechanism during the earthquake, (Figure 3.80 and Figure 3.82). The mechanism is the frequent cause of the corner damages, particularly if coupled with roof thrust. Furthermore ground movements or simply the lack of constraint at the base of buildings in slope can increase the overturning movement. Errore. L'origine riferimento non è stata trovata. and Figure 3.83 present examples of the overturning mechanism in buildings which are located in a slope.
Figure 3.78 - Overturning of the façade with damage of the orthogonal wall, (Borri 2004c).
Figure 3.79 - The overturning of the façade in case of effective connection damages the orthogonal walls with a variable angle with the masonry quality, (Borri, 2004c): a) good quality masonry (angle 30° and 45°), b) average quality (15° and 30°); c) low quality (0° and 15°).
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(a) (b)
Figure 3.80 - Mixed overturning mechanisms. (a) (Avorio, 2002a). (b) (Doglioni, 1999).
Mechanism Load factor
j j T 2 h h T i L ( ) s j 2 tan T s tan T kL N h j T T h j i 2 2 j s 3 j i 2 s i s i1 i1 (0), j j 2 j 1 hs 3 hs LTi j i ( ) j tan j kL j j i 2 3 i11 i is the number of edge party walls orthogonal to the façade under exam which can provide restraining B1 action as above
Ts thickness of party walls and internal bearing walls j j T 2 h h T i L ( ) s j 2 tan T s tan T kL N h j T T h j i 2 2 j s 3 j i 2 s i s i1 i1 (0), j j 2 j 1 hs 3 hs LTi j i ( ) j tan j kL j j i 2 3 i11 i as above B2 as above
2 2 j1 2 2 j1 2 TN T j j jh T j TN T j j jh T j k j i h k j i h jf jf js js 2 2 i0 3 2 2 2 i0 3 2 (0), j j1 2 2 TN T j hs j j jf js k jf (1 ) js j i 3 2 2 i0
C jf =tan jf
js =tan js integer identifies the load bearing wall =(0,1) Figure 3.81 - Development of overturning mechanisms for coupled walls and load factor calculation, (D’Ayala, 2003a).
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Figure 3.82 - Mixed overturning mechanisms.
(a) (b)
Figure 3.83 - The slope can worsen the wall overturning. (a) (Borri, 2004c). (b) (Carocci, 2001).
3.1.2.1.1 Adobe Structures At a global level, the failure of adobe is so described in (Vargas Neumann, 2006): Significant cracking starts in regions subjected to tension; Out of plane rocking causes large vertical cracking that separate walls; Walls overturn; Walls break into separate pieces, which may collapse independently.
Seismic damage to adobe buildings, (Tolles, 1996), is not limited to the typical damages observed in other masonry (in-plane X-cracking, out-of-plane gable wall failure, out-of-plane collapse of wall subjected to high confining forces). It also includes, assumingly at lower stress than for fired brick masonry, the damage typologies shown in Figure 3.84.
Out-of-plane flexure in load bearing walls: This type of damage to earthen buildings was reported, in the case of load bearing walls, to mainly result in cracking, leaving walls stable. Slippage between walls and wood framing: Roof, ceiling, and floor framing often slips at the interface with the adobe walls due to inadequate connection to the adobe walls. Horizontal upper-wall cracks: horizontal cracks: may develop near the tops of walls when there is a bond beam or the roof is anchored to the beam. Moisture damage contributions to instability: Out-of-plane instability (or a contribution to instability) is caused by weakening or erosion, usually at the base, or saturation or repeated wet/dry cycles resulting in weakened slip-planes at the base of the wall along which the wall can slip and collapse. Corner damage: Damage often occurs at the comers of buildings due to the stress concentrations that occur at the intersection of perpendicular walls. Instability of comer sections often occurs because two sides of the comer are unrestrained. Therefore, the
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comer section is free to collapse outward from the building. Vault / Dome damage: From photographic documentation, seems attributable to failure of side walls. Figure 3.84 - Damage Typology for Adobe Buildings.
An attempt to define collapse mechanisms was made by Kiyono and Kalantari (Kiyono, 2004), who attributed the catastrophic collapse of Bam (Iran) to “improper” bonding strength of mortar. The collapse mechanisms proposed are three: Overturning of wall (monolithic) - as per Section entitled “Simple Overturning”; Slippage; Failure of bond between bricks, proposed in (Kiyono 2004), Figure 3.85.
Kα: overturning moment caused by seismic force n : no. of bricks W : Weight of each brick p : bonding strength in the normal direction A : Area of bond Figure 3.85 - Collapse mechanism proposed in (Kiyono, 2004).
An important parameter concerning overturning is the wall slenderness (height-to-thickness ratio), on the basis of which Ginell et al. (Ginell 1995) categorised the seismic performance of adobe walls. Similar evaluation could be done for rammed earth buildings, (Figure 3.86), and cob buildings, (Figure 3.87).
Figure 3.86 - Damage Typology for Rammed Earth Buildings.
Figure 3.87 - Damage Typology for Cob Buildings.
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3.1.2.2 Pillars Slender structural elements like pillars and columns frequently show overturning damages caused by out of plane bending. Besides the overall overturning, seismic action could produce the drum shifting, (Figure 3.88), in stone columns. Bending action during earthquakes could act at the base of slender structures producing local stress concentration, (Figure 3.89 and Figure 3.90) The coupling of stress concentration due to the seismic bending with high compression stress could increase or start the damage due to long term behaviour. It has been shown that the behaviour of masonry under the action of persistent loads can evolve over a relatively long time to its collapse, (Binda, 1992). This can happen under lower stress values than those corresponding to the nominal material strength subjected to standard monotonic compression tests. The creep phenomenon can start at 45-50 % of the nominal strength value in some soft stones like limestones. Several case histories show that significant crack patterns, clearly due to vertical compression caused mainly by the dead load, often appear on the walls of ancient towers as well as on particularly slender or heavily loaded elements like columns, pillars, etc., which turn out to be overloaded by heavy persistent compressive stresses, (Binda, 2008b). Furthermore, this happens any time significant concentrations of stresses take place in some portions of the material due to non-uniform stress distributions. The available experimental data collected to date tends to show an evident increase of lateral deformations developed in time caused by the development of the typical vertical cracks due to compressive stresses. The dilation phenomenon, an apparent increase in volume, can lead to collapse due to crack propagation. The effect can also be coupled with synergetic stresses caused by cyclic wind action and temperature variation. Additional minor shocks, like storms, low intensity earthquakes, etc., may contribute to increase the damage. The damage can easily develop when the material used for the construction is rather weak (weak bricks and mortars with irregular joints, soft stones) or the technique of construction is such that the internal core of the masonry can settle and deform more than the external leaf. In all these cases the damage can start early, even a few years after the construction, or after some partial reconstruction or even repair, and continue very slowly for decades, until a sudden collapse happens.
(a) (b) (c) (d)
Figure 3.88 - (a) Cathedral of Syracuse: drum shifted after an earthquake in XVI century, (Binda, 2007c) and (Giuffrè 1993); (b) block shift as reported by Azevedo and Guerreiro in (Sousa Oliveira 2008); (c) rocking and overturning of columns.
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Figure 3.89 - Local damage concentration due to bending action during earthquakes, (Binda 2010).
Figure 3.90 - A damaged pillar in the courtyard of the Spanish fortress in L’Aquila.
3.2 ARCHES / VAULTS / DOMES
3.2.1 Introduction Among the structural components in masonry buildings, arches and vaults deserve particular attention for being widespread in European historical centres but also outside Europe e.g. in Middle East; therefore, their preservation as part of the cultural heritage is a topical subject. Along the times, the building technology evolved taking into accounts the local environment, problems and materials. The case of the Roman and Byzantine domes could be cited, made by light clay elements as pipes and diffused also in area with soil settlements like Ravenna. Vaults and domes have been used extensively in the roofing of ancient buildings, often with large span in the case of the Churches or Mosques, (Figure 3.91 to Figure 3.93). Knowledge of the behaviour of these structural elements under load is fundamental for the safety evaluation of most buildings and to plan structurally compatible and economic conservation programmes, (Theodossopoulos, 2004).
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ARCHES BRICK ARCHES
REGULAR STONE ARCHES
UNREGULAR STONE ARCHES
Figure 3.91 - Examples of arch typologies, (Binda, 2003b).
These structures can suffer several types of damage, due to many causes (such as earthquakes, age, etc.). Soil movements and deformations can disturb the equilibrium and cause instability. In practice, the pattern of failure can be established through in-situ observations. A careful examination of the changes in the geometry induced by progressive failure of the supporting system is essential for the early diagnosis and accurate treatment of the problem. This requires a deep knowledge of the behaviour and characteristic of each arch, vault or domes typology. Furthermore, the correct structural evaluation and the modelling should also take into account the influence of some traditional reinforcement or building characteristics, like metal ties or infilled rubble materials at the extrados, (Theodossopoulos, 2004). Hence, the contribution of strengthening materials and repair techniques is often required to re-establish or enhance their performances and to prevent a brittle collapse of the masonry in possible future hazardous conditions. Many factors affect their structural behaviour, such as geometry, stiffness and mass distribution, building evolution, damage and repair interventions.
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VAULTS
BARREL VAULT GROINED VAULT RIBBED VAULT
PENDENTIVE COMPOSED CLOISTER VAULT FAN VAULT VAULT VAULT
Figure 3.92 - Examples of vault typologies, (Binda, 2003b).
DOMES SOLID MASONRY LIGHTENED ROMAN CONCRETE
HEMISPHERICAL SAUCER DOME RIBBED DOME (CALOTTES)
Figure 3.93 - Examples of dome typologies, (Binda, 2003b).
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3.2.2 Typologies There are different structural typologies of vaults from ancient times to today. Arches and vaults can be classified by the typology of structure (e.g. barrel vault, cross vault, pavilion vault, fan vault, etc) and depending on the pattern used for laying bricks (e.g. stretcher bond, Flemish bond, herringbone pattern, etc). The most common typologies of vaults are the barrel and cross vaults; the former, in particular, has been object of the scientific community’s interest because, together with the semicircular arch, from which it derives, it is the most elementary shape of spanning curve structure and, as such, it well reflects the average performance and the typical pathologies of the whole class of horizontal curve structures. The vaulted structures can be typologically divided, according to its geometry, which is one of its most important parameters, essential to determine the proper structural behaviour. According to (Carbonara, 2004) the vaults can be typologically classified as (Figure 3.94 to Figure 3.95): i. Translation Vaults, generated by the movement of a straight line (generatrix) along a curve, (Figure 3.94); Barrel vault Groin vault Barrel vault (with or without) lunettes Cloister vault Volta a schifo (composed vaults) ii. Rotations Vaults, defined the rotation of a curve around an axis, (Figure 3.95). Ribbed vault; Dome; Circular barrel vault.
(a) (b) (c)
(d) (e) Figure 3.94 - Translation vaults. (a) Barrel vault, (Carbonara, 2004). (b) Groin vault, (Carbonara, 2004). (c) Barrel vaults with lunettes, (Isawi 2001). (d) Cloister vault, (Isawi 2001). (e) Volta a schifo, composed vault, (Isawi, 2001).
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(a) (b)
(c) Figure 3.95 - Rotation vaults. (a) Ribbed vault, (Isawi, 2001). (b) Dome, (Isawi, 2001). (c) Circular barrel vault, (Carbonara, 2004).
3.2.2.1.1 Structural behaviour From the Roman age to the Renaissance, the problem of stability and correct design of arches were mainly faced under a geometrical point of view, (Figure 3.96), even though the presence of horizontal thrusts was already perceived at least in the 1st century B.C., as reported by Vitruvius within De Architectura, (Benvenuto, 1981). Wide reviews of the historical progress of studies on masonry arches and vaults can be found in (Benvenuto, 1981), (Heyman, 1982), (Boothby, 2001) (Carbone, 2001), (Theodossopoulos, 2004). An intact masonry arch is statically indeterminate. It becomes determinate when three hinges form in the arch ring under live load. The thrust action transmitted by arches and vaults to walls and piers is usually the only cause of damage to the supporting walls or piers or the vaults and arches themselves. If the thrust is too high, it might cause out of plumb, overturning and deformation of the supporting structure. Heyman (1995) observes that a structure must satisfy three main structural criteria: strength, stiffness and stability. Moreover, three standard assumptions can be made when dealing with historic masonry structures: Masonry has no tensile strength as mortar is weak or lacking; Stresses are so low that masonry has effectively an unlimited compressive strength; Sliding failure does not occur. Thus, failure of masonry in compression is unlikely to affect the whole structure, despite local cracking and spalling, and deflections are negligible, because of the stiffness of masonry. Hence, instability is the cause of failure in a masonry structure.
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For a voussoir arch, instability involves the formation of hinges that allow the creation of a mechanism. It is common to observe cracks in specific locations on a vault; however, as long as there are only three hinges, the structure is statically determined and no further displacement occurs. Conversely when four hinges form, due to movements in the supports or change of load bearing on the vaults, the vault will be subject to the creation of a kinematic mechanism and will hence collapse, (Figure 3.97).
Figure 3.96 - Geometrical design rule still used during the 18th century, (Benvenuto, 1981).
Figure 3.97 - Voussoir arch: stable state of cracked arch (left) and collapse under point load (right), (Heyman, 1995).
The formation of three hinges is inevitable: the structure naturally tends to accommodate the small displacements in the abutments due to differential settlements or the shrinking and wear and tear of mortar. On the one hand the formation of a three-pin arch in fact imply that one of the infinite number of balance positions of the structures has been determined by external conditions, so that the line of thrust can be calculated. On the other hand, the creation of an unstable mechanism means that a change of loading or an imposed displacement occurred and a seismic event can cause either or both these effects. In structural analysis, four hinge positions are selected to search for the minimum collapse load taking into account all forces acting whilst still fully containing the thrust line within the arch ring. In fact, four hinges are needed to allow a kinematic mechanism (Figure 3.97 to Figure 3.98), and then the rotation of the supporting elements (pillars, masonry walls, columns, etc.). In general the mechanisms could be due to asymmetric or excessive vertical loads, or to a relative displacement of the abutments, as it happens in case of earthquake. The vaults structural mechanism (similar to the arches) takes advantage of the natural crushing of the elements that composes it, provided that two basic conditions that ensure the equilibrium are guaranteed: Vaults stability: a generic anti-funicular that balances the loads must be within the profile on every section causing a compression stress state compatible with the composing materials; eccentric compression would lead to unsustainable masonry bending (due to its almost absent tensile strength); Global stability: the piers on which the vault is supported should be able to accommodate the horizontal thrust generated by it.
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These conditions, essential for the stability of the element, are the two principles that guide the design of a vaulted structure that function well, and in case of an existing building, that allow assessing the structural safety. The "pushing" that characterizes these structures, makes them intrinsically prone to instability/damage, propensity that can be enhanced by any seismic phenomena.
Figure 3.98 - Damage mechanisms of arches, (Rondelet, 1834).
3.2.2.2 Damage mechanisms and their causes It is possible to classify the vaults collapse mechanisms based on its causes. Among the possible sources of damage it is possible to distinguish: The relative displacement of the supports; a. Displacement of the supports on the orthogonal direction to the generatrix line of the vaults; b. Differential settlement of the piers; c. Longitudinal sliding.
The variation of the load to which the vaults and piers are subjected; The decay of masonry. The first mode mechanisms involve the rotation of the supporting elements (walls, columns, pillars, etc.), (Figure 3.99, Figure 3.100, Figure 3.101, Figure 3.102), while the second mode mechanisms involve in plane loading, then a relative sliding and deformation due to shear stresses, (Figure 3.99). The masonry texture plays a key role in the structural system, being base on equilibrium of mutual thrust of the blocks. Other example could be the texture of barrel vaults, discriminating the behaviour as 3D shell or set of adjacent arches, (Cattari, 2008). Texture influences the crack pattern, affecting potential sliding planes. Other factors are the presence of infilling, structural backing and constraint boundary conditions, deriving by building traditions or damages. Tie-rod insertion in church macro-elements is very common, but their effectiveness is related (in addition to the quality of the anchorages) to internal prestress and position (e.g., at the arch haunches or lower), (Figure 3.101 and Figure 3.102). Lunettes in barrel vaults are often built without connection to lateral walls: in this condition when the vault and the supporting walls perform out-of-phase oscillations, they can act as struts or counterforts if compressed and without damage if the wall tends to rotate, (Cattari, 2008).
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Figure 3.99 - I and II mechanisms in vaults and strengthening intervention, (Giovanetti, 1998).
Figure 3.100 - Vault with extrados infilling damaged by the supporting wall movement, (Doglioni, 2007).
In single-nave churches, the structural macro-elements made by the arch-pillar system (dividing one bay from others) or the triumphal arch (separating the nave from the presbyterial area) are often present: typically, the seismic damage is represented by a four-hinge mechanism. It may involve both the arch pillars, or only the one subjected to an outward thrust. Sometimes the pillars of the triumphal arch work as shear walls, as they are generally quite wide because of the minor width of the presbytery. This macroelement is characterized by in-plane seismic response in spite of its slight stiffness out of plane; the presence of the church hall and of the presbytery and/or apse prevents the activation of an out-of-plane collapse mechanism. In Figure 3.102, typical collapse mechanisms of the triumphal arch structure in a church are described. Effects of excessive thrust values that can cause damage to arches and vaults are shown in Figure 3.103.
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Figure 3.101 - Damage mechanism for unreinfornced and tied arches, (Avorio 2002a).
Figure 3.102 - From (Lagomarsino 2009). Three-block kinematic chain for the triumphal arch in the churches: (a) mechanism A-two-pillar overturning mechanism; (b) mechanism B-one-pillar overturning mechanism.
Figure 3.103 - Frequent damage mechanism on barrel vaults, cloister vaults and domes, (Croci, 2001).
The survey templates of Figure 2.2 illustrate with drawings illustrating the most important crack pattern after earthquakes in churches, including damage on vaults and domes, (PCM, 2001). Although the seismic behaviour of masonry structures has been extensively studied experimentally, analytically and numerically, still little is known about the structural performance of vaulted structures undergoing earthquake actions. It is certainly clear from the survey of damaged vaulted structures that seismic action induces bending in the vault ring as well as rocking of the abutments at the base. The onset of failure mechanisms and their evolution in the large-displacement field was investigated via the principle of virtual works in order to define capacity curves (Abruzzese, 1999), (Housner, 1963), (Lagomarsino, 2004b) and out-of-phase rocking of the abutments (Como, 1991). Giuriani et al. (Giuriani, 2007) proposed instead a simplified analytical method, relying on limit analysis, for the assessment of the seismic vulnerability of transverse arches undergoing rocking of the abutments. The study highlighted how in at-rest conditions, the arch lateral thrust can be
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largely, or entirely, resisted by the buttress action of the abutments, whereas the arch thrust is increased by the rocking mechanism, which is also responsible for the drop in the confining action of the buttress. This implies that strengthening ties must be not only dimensioned to resist the thrust exceeding the abutments capacity in at-rest conditions, but need indeed to provide an adequate response to the whole arch thrust during seismic events, (Giuriani, 1993, 1995, 2007). Such result was also achieved by Lagomarsino et al. (Lagomarsino, 2004b) by application of the principle of virtual works. The rocking excitation that barrel vaults experience under horizontal load was experimentally investigated by Marini et al. (Marini, 2008) through full scale cyclic testing of a masonry arch representing a unitary-length slice of a barrel wall confined at the arch springing by an intrados metallic tie. The structure proved to be relatively ductile in terms of displacements, even though little energy dissipation was achieved and the behaviour was highly self-centring. Failure was reached by formation of four hinges at the crown of the arch and in the abutments. The tension measured in the tie significantly changed due to the loss of the buttressing action of the abutments and the increase of the arch span. Results were validated by FE modelling: the locations of the cracks resulted to be function of the geometry and applied vertical loads. The intersection of two equal semicilindrical barrels generates the simplest form of groin vault; the resulting bay of vaulting is square in plan, and the diagonals of the square define the location of the groins, (Heyman, 1995). Because of difficulties in cutting the stones meeting at the groins, especially when the two intersecting barrels had different length and height, Romanesque builders started the construction of vaults by first erecting the masonry arches along the diagonals, which were then embedded, either fully or partially, within the masonry of the vault webs. Later on the use of ribs became common: the ribs had aesthetic functions - they helped concealing the joints at the groins - as well as structural features: the discontinuity due to the intersection of the two curved surfaces of the barrels indeed causes a localised increase of stresses and requires reinforcement. However, it’s not infrequent that ribs are purely decorative; this can be observed where no variation of the curvature is present at the groins, meaning that the vault doesn’t present localised stresses in such position. In quadripartite vaults, three different typologies of cracks can be observed, (Figure 3.106): 1) cracks in the main barrel of the vault in the region of the crown, which correspond to plastic hinges at the extrados of the vault, 2) fissures the Sabouret and 3) cracks separating the vault webs from the bearing walls. Sabouret cracks open because of a movement in the supporting walls: the vault cannot completely accommodate such displacements and the formation of the three hinges provokes the separation of the masonry of the vault from the walls. Such phenomenon also involves the masonry of the buttresses with appearance of the cracks of the third type. As said above, several typologies of vaults are present in historic structures, even if some are frequently neglected, like fan and pavilion vaults, because of the intrinsic difficulty of applying simplified theories to their complex shapes. D’Ayala and Tomasoni (D’Ayala, 2008) investigated in detail the behaviour of pavilion vaults by computational models and, by the use of the concept of thrust surface, overcame some of the limitations typical of the analysis of masonry vaults. One major simplification that is normally carried out for the study of vaults is indeed the reduction of the three-dimensional structure to a series of adjacent arches, without transversal connection nor the mutual interaction between the ‘slices’. For such hypothesis the results, in spite of being conservative for uniform load distribution, are limited to specific loading conditions and are not exhaustive in respect to the assessment of the global behaviour of the vault. The authors’ analytical method, which account instead for finite friction and is based on a lower bound approach, allows obtaining the crack pattern, the stress field and horizontal thrust at the supports of the pavilion vaults considering the three-dimensional behaviour of the vault, (Figure 3.104 to Figure 3.105).
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Figure 3.104 - Horizontal trust along the supports obtained by the simplified arch model (a) and the actual horizontal thrust obtained by the limit state analysis (b) (D’Ayala, 2008).
Figure 3.105 - Possible collapse mechanism of vault for each slice, (D’Ayala, 2008).
Analytical results demonstrate that even vaults with rigid boundary conditions at the support are affected by cracks along the diagonals, unlike the common opinion that ascribes the diagonal cracks to the walls overturning. Furthermore, the vault web, despite diagonal cracks, develops a series of natural arches that define an alternative load path from the centre of the web to the diagonals, allowing for the transmission of load between adjacent arches.
Figure 3.106 - Crack patterns of quadripartite cross vault as described in (Heyman 1995).
Plastic hinges may form at the intrados towards the springs of the vault and at the extrados in the crown area; moreover, the vault could fail by sliding. This latter failure mode is not included in the traditional approach, which considers friction infinite and sliding impossible, whereas in the reality sliding can be observed because of the deterioration of the binding materials or contact surfaces.
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The structural behaviour of pavilion masonry vaults can vary with the rise-span ratio: in shallow vaults the geometry is closer to the thrust surface generated by the gravity load distribution and compression hoop stresses are present for a larger portion of the surface, meaning that the spread of cracks that develop along the diagonal and in the centre of the web depends on the vault rise. The horizontal thrust also increase if the vault rise reduces and the trend along the perimeter walls tend to become constant for shallower vaults. Therefore reducing the rise/span ratio, the natural arches that develop in the pavilion vaults and that causes the transfer of meridian stresses from the centre of the web to the diagonal aft a smaller area near the diagonal itself. Barringer (2006) gives a relevant contribution to the topic of the structural behaviour of vaults by investigating the performance of herringbone vaults, which are almost completely missing from the literature. The herringbone pattern is formed by a zigzag pattern consisting of columns of short parallel lines, with all the lines in one column sloping one way and all the lines in the next column sloping the other way (Oxford English Dictionary). This method of laying bricks has many names including Fish Backbone (Spinapesce) and opus spicatum; the latter name comes from the Roman period and described the way in which bricks could be laid, although the pattern was used to decorative purposes rather than with structural functions. There is also evidence that the pattern was popularly used by the Vikings during the 8th ad 11th Centuries. According to the Viking Heritage Magazine, (VHM, 2002), the pattern derived from the carpenter’s efforts to hew the timber always in the same direction, which achieved a smooth and water repellent finish. Furthermore the pattern was used in the U.K. during the 16th century as infill for timber frame houses. This was a replacement for wattle and daub, and although it was cheaper due to the lower labour’s skill required to lay bricks, it created draughtier buildings due to the gaps left when put against timber. However the Herringbone pattern had a widest use for the construction of vaults and domes around the world, and especially in Italy. One of the most famous examples is the Cupola of Santa Maria del Fiore by Brunelleschi. Barringer, (Barringer, 2006), simulates a herringbone semicircular vault undergoing different vertical loads by a three-dimensional FE model, where material behaviour is defined by the Drucker-Prager relationship. The herringbone pattern seems to be more efficient than a voussoir arch since it allows for a better distribution of stresses and displacements. However the author doesn’t provide direct comparison with a model of a voussoir arch nor analyses the performance for horizontal loads.
3.2.2.2.1 Adobe domes A research carried out at Politecnico of Milan concerning Arg-e-Bam (Iran), (Licciardi, 2008), heavy damaged by an earthquake in 2003, catalogued recursive damages in domes, both ancient structures and repaired/rebuilt structures. The typical adobe domes of this area are frequently built without wood scaffolding. There are a lot of versions of this typology, but they can be grouped in four main types: i. Domes with concentric courses; ii. Domes with jointed pendentives (first type); iii. Domes with jointed pendentives (second type); iv. Domes with isolated pendentives. The first typology are the rotation domes, the most simple structures, hemispherical, with the bricks arranged radially, (Figure 3.107), and jointed by a clay-gypsum mortar (60.0% of clay, 30.0% of fine sand and 10.0% of gypsum as quick setting). These domes are locally called colombu’ and their structural behaviour is very effective.
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Figure 3.107 - Scheme of the layout of the brick courses of the first typology dome recognised at Arg-e-Bam, (Licciardi, 2008).
These domes have squared base, and they are supported by hemispheric or hemispheroidal pendentives, (Figure 3.108). This typology showed the better seismic behaviour during the Bam earthquake. In fact few cracks were observed after the earthquake, usually localised in the mortar joints, therefore set concentrically, or seldom, oblique. Few domes collapsed and most damages were probably caused by the collapse of the supporting pillars or walls rather then to the structural ineffectiveness of the dome itself, (Figure 3.109).
(a) (b)
Figure 3.108 - (a) Barracks, Arg-e-Bam, Iran. Hemispherical dome built on pendentives with concentric brick courses - April 2006 (Licciardi, 2008). (b) Bazaar, Arg-e-Bam, Iran. Restoration and rebuilding of the central dome by the technique of concentric courses and plastered arches - before the earthquake of 2003 (Source: Iranian Cultural Heritage and Tourism Organisation, Teheran).
Figure 3.109 - Scheme of the courses layout and of the typical crack pattern surveyed on domes of the first typology (Arg-e-Bam), (Licciardi, 2008).
The second dome typology includes the domes with pendentives, which are locally called colombu’-e-gushvar. The single pendentives are laid on the median axes of the open sides and the bricks are arranged in inclined arched courses, (Figure 3.110). Furthermore, each course lies on
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truncated cones, the section has not constant height and the key stone line does not follow a linear directrix, but an arc of a circle, (Figure 3.111).
Figure 3.110 - Scheme of the courses disposal in a dome of the second group (Arg-e-Bam), (Licciardi, 2008).
Figure 3.111 - Complex of Mirza Na’im, Arg-e-Bam, Iran. Restoration and rebuilding of a dome of the second group before the 2003 earthquake (Source: Iranian Cultural Heritage and Tourism Organization, Teheran).
This dome typology is very diffused at Arg-e-Bam, also thank to the decorative effect of the joint layout. Unfortunately it showed an ineffective structural behaviour, since the four segments of the dome are badly jointed to each other and the brick courses intersect each other at right angle at the median axes, causing local weakness and, then, possible damage concentration. In case of earthquake the cracks are concentrated where in the courses line changes and, in highly damaged cases, along the diagonals as well, (Figure 3.112).
Figure 3.112 - Scheme of the courses layout and of the typical crack pattern surveyed on domes of the second group (Arg-e-Bam), (Licciardi, 2008).
In serious cases the whole structure collapsed except the impost of the pendentive, connected to the supporting walls. In the previous typology the damage was often due to the collapse of the supporting elements, pillars or walls; this typology instead is generally supported by continuous walls and their collapse is mainly related to the building technique which involves local vulnerability caused by the courses discontinuity. Similar to the previous case, the course bond is very poor as well as the general brick layout is not effective with joint alignments, (Figure 3.113).
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Figure 3.113 - Arg-e-Bam, Iran. Dome of the second typology with pendentives. In case of nearly complete collapse the poor masonry configuration is clear - April 2006 (Licciardi, 2008).
It is also important to point out some interesting use of this dome typology. Since they are built as a half dome, they are often used together with other vault typologies, local called zarbi, in order to build a conch, supporting the vault in absence of a wall, (Figure 3.114). These domes are also used to build jutting out conches, which are placed to protect porches. In these cases their use allowed to cover rectangular spaces and to have an arched front, creating refined decorations, (Figure 3.115).
Figure 3.114 - Bazaar, Arg-e-Bam, Iran. Restoration and partial rebuilding of a conch to close a zarbi vault, by a dome of the second typology called colombu’- e- gushvar (Source: Iranian Cultural Heritage and Tourism Organization, Teheran).
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Figure 3.115 - Complex of Mirza Na’im, Arg-e-Bam, Iran. Use of a dome of the second typology called colombu’- e- gushvar, to cover a rectangular porch - April 2006, (Licciardi, 2008).
The third group of domes includes domes with pendentives laid down parallel to the supporting walls, (Figure 3.116). The bricks are jointed by earth mortars and gypsum and they are arranged in inclined arched courses, laid on conic surfaces; the orientation of the laying cones, (Figure 3.117) is opposite in comparison with the previous case; however the vulnerability factors are the same.
Figure 3.116 - Scheme of the layout of the bricks courses in a dome of the third group (Arg-e-Bam), (Licciardi, 2008).
Figure 3.117 - Arg-e-Bam, Iran. Layout of the bricks courses of a dome of the third typology (Source: Iranian Cultural Heritage and Tourism Organization, Teheran).
In fact, the damage localisation is similar to the one of the domes of the second type, with cracks along the diagonals, close to the pendentives joints, where the bricks courses change the alignment angle. In the most damaged cases cracks along the median axes but also parallel to the supporting walls are visible, (Figure 3.118).
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Figure 3.118 - Scheme of the course layout and of the typical crack patterns surveyed on a dome of the third group present at Arg-e-Bam, (Licciardi, 2008).
The last typology is composed by domes with isolated pendentives locally called colombu’-e- chahar tac. They are a combination of the second and third type, with arched course following a trend similar to that of the colombu’- e- gushvar (second dome typology) and with a sharp change of the course alignment along the diagonals, like in the third type of domes, (Figure 3.119).
Figure 3.119 - Scheme of the brick courses layout in a dome of the fourth group (Arg-e-Bam), (Licciardi 2008).
In this case, much more than in the previous, the pendentives are isolated and they show two discontinuities. The first is the same of the previous two typologies and related to the courses discontinuity close to the diagonals. The second is along the medians of the vault, where the pendentives are completely isolated to each other due to the joint alignment in this position, even if they have parallel brick courses. This building solution is apparently incomprehensible and it is difficult to find a clear explanation. For the previous reasons this group has the highest intrinsic weakness. The first cracks are concentrated along the medians, as a result of ineffectiveness of the joint connections. In the most damaged cases cracks along the diagonals and parallel to the median axes are documented, (Figure 3.120).
Figure 3.120 - Scheme of the courses layout typical of a dome of the fourth typology present at Arg-e-Bam, (Licciardi, 2008).
Lightened domes are often used to cover large span structures, as in the stalls of the citadel Arg-e- Bam. They are built following the technique of the first, second and third dome typologies in order to have wide spaces. For example, in a case of the stalls at Arg-e-Bam, the domes create a
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complex covering structure; they are supported by pillars and allow reinforcing a larger dome at the impost, without over load the vertical elements. At the same time, this allows to have the ceiling at a constant height, (Figure 3.121). In these cases heavy damages were observed, often collapses, due to the peculiar weakness of the structural solution with the dome intersecting other domes above the pillars.
Figure 3.121 - The lightened domes of the stalls at Arg-e-Bam, Iran - April 2006, (Licciardi, 2008).
3.2.2.3 Types of interventions Next is presented a brief overview of the traditionally used types of consolidation according to (Giardina, 2006), (Figure 3.122). a) Buttress; b) Counter vaults in reinforced concrete; c) Counter vault in lime mortar and FRP net; d) Transversal vertical diaphragms, (frenelli); e) Tie at the intrados; f) Tie at the extrados; g) Vertical tie; h) Curve tie; i) Suspension tie; j) Cross ties at the extrados; k) Overcoat with composite material strips. l) Reinforcement arches.
Figure 3.122 - Traditional arch reinforcements, (Croci, 2001).
3.3 FLOORS Traditional floors are usually built in timber loaded directly on the walls. Since the end of XIX century iron/steel beams with fired clay elements or small shallow barrel vaults had been diffused.
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During the earthquakes the beams could hammer directly on the walls, thrusting the whole panel or only one leaf. Structurally a floor has to: support the vertical loads without collapsing and without excessive deformation, (the criteria of out-of-plane strength and stiffness); have adequate in-plane stiffness in order to distribute the loads to the vertical structures; ensure an effective connection to the perimetrical vertical structures in order to ensure box type behaviour of the entire building.
3.3.1 Wooden floor typologies In terms of typology, on a wooden floor it is possible to distinguish two main parts, with different structure and function, the supporting structure and the planking, (Angotti, 2005): The supporting structure consists of one or more frames, arranged in directions orthogonal to each other (usually, a main frame composed by beams and a secondary frame composed by joists). The frame has a static function of resisting vertical actions due to its self-weight, the weight of the deck elements, the weight of the completing parts at the intrados and extrados, and also of any existent partitions and variable live loads. The deck is, in its simplest configuration, composed by a plane element made with one or more layers of wooden boards joined adjacently to each others, Figure 3.123a), or made by tiles, supported by the beams, (Figure 3.123b). The deck has a static function of resisting vertical loads applied directly on it and to distribute them between the supporting frame elements, and also, of assigning transversal stiffness to the frame, in order to transfer the horizontal loads to the structure vertical elements.
(a) (b) Figure 3.123 - Types of wooden floors: (a) deck with wooden boards; (b) deck with tile flooring, (Angotti, 2005).
The wooden floors can be divided according to the arrangement of the supporting beams and to the material that is placed above the beams, boards or panels.
In what concerns residential buildings under service loads, the wooden floors can be divided into two main categories: i. With the main bearing frame equal to the length of the span to be covered; ii. With the main bearing frame with a length inferior to the span that needs to be covered. To the first group belong the simple frame floors and the composed frame floors. While the second group includes: Serlio type floors, compartmented floors, polygonal floors, ray floors and all the floors that have a bearing frame composed by non-parallel beams, crossed in different directions and between them. Apart from these types of wooden floors it will also be presented next, in more detail, the new generation wooden floors. 3.3.1.1 With the main bearing frame equal to the length of the span to be covered Simple unidirectional floors This type of floor is the most common in historical constructions in Italy. The main beams are arranged according to one direction, Figure 3.124. The maximum span that this type of floors with
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main beams, can cover is 4.0 to 5.0 m. It is composed by elements with rectangular section arranged parallel to the smallest side of the environment where they are inserted, and with the longer side parallel to the vertical section. The beams section existent in the historical constructions ranges from solutions with simply peeled truncated circular beams, to beams with square and rectangular section. In Germany and Austria, the floors composed by square section beams were used up to the 800’s, in the rest of Europe, and particularly in Italy, the relation base / section height equal to 0.7 was introduced since 1600. This ratio derives from optimizing, in terms of bending resistance, the beams made from a cylindrical trunk, (Barbisan, 1997a). For this type of floors Rondelet (Rondelet, 1834) defined a height of the main beams equal to 1/24 of the span to be cover, (Munafò, 1990).
Figure 3.124 - Simple unidirectional floors, (Rondelet, 1834).
The Renaissance treatises (Palladio, Serlio, Scamozzi, Milizia) and the subsequent technical manuals, propose a distance between main beams equal to its width. This arrangement appear due to aesthetics reasons, when the objective is to confer unity to the underlying environment, and to static reasons, so that between the heads of the beams remains a quantity of wall sufficient that does not weaken too much the wall and granting an adequate resistance to the floor. The resistance of these floors is however considered exuberant. For this reason, for practical reasons and due to the availability of the wooden species, in the manuals of the second half of the 800’s, the space between the main beams increased to 0.4 to 0.5 m, except in cases where tiles areused for the deck. This modification originated an increase on the beams thickness. Nowadays, the elements that are normally used, are the joists, which present a section with a base between 3/4 of the height and dimensions between 9.0 x 12.0cm and 12.0 x 18.0 cm, or the “panconi” (boards), that come from the French constructive tradition, presenting a cross section with a restricted base, between 5.0 and 8.0cm, and height equal to 3.0 or 4.0 times the base, (Ceccotti, 2007). Normally the joists or the “panconi” are arranged with a distance variable between 30.0 and 50.0cm, and with a fixing support on the walls of approximately 10.0cm. The deck can be completed with boards or tiles (the kind widespread in central Italy) or with boards and “panconi” with thickness varying between 20.0 and 30.0mm (the most common type in Northern Italy is with chestnut or oak boards). In the first case it is placed directly over the joists, kept at a distance of 30.0 to 35.0cm, with single or double board layer. On the second case the length of the boards is in accordance with the distance of the joists, so that their extremities are in the middle of them and the unions are offset (Ceccotti, 2007). Composed frame floors This type of floor is normally used when the span to be covered is larger the 4.0 to 5.0 m above mentioned for simple unidirectional floors, or whenever the beams available do not have sufficient length or section to constitute by themselves the supporting structure of the floor, (Ceccotti, 2007).
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It is composed by main beams placed along the shorter side of the floor where they are inserted and secondary beams (joists) parallel to the major side and orthogonal to the main beams that support them. For the design of the beams that are a part of the composed floors, Milizia and Rondelet proposed a height of the main beam equal to 1/18 of the span to be covered, (Munafò, 1990). Usually, main beams are spaced of 3.0 to 4.0 m, present a length ranging from 4.0 to 6.0 m and a fixation length on the walls of at least 25.0 cm. The technical-constructive evolution of the composed floors is the connections between the primary and secondary frames. The oldest mode to connect joists to main beams are through: overlapping of the joists, arranged head to head or offset, the connection of the joists to beams throughout their entire thickness or overlapping with V shaped connection, to prevent the joist displacement out of their supports, (Munafò, 1990). The Rondelet, (Rondelet, 1834), instead recommends the use of “filarole”, attached to the main beams and bound with metallic bandages, while, since the end of 19th century were introduced metallic angular elements replacing the “filarole” or the seconday wooden beams (“correnti in legno”). Bidirectional floors with double frame Most wooden floors in Italy are unidirectional; it’s the obvious solution in order to better exploit the wood characteristics. The bidirectional proposal, Figure 3.125, from a constructive point of view is more elaborate and thus possesses a higher degree of complexity, due to the complicated intersection of the beams being, therefore, much less used. However, this solution presents some advantages, especially when considering the behaviour of the entire structure, both for the uniform distribution of loads on all the supporting walls, and for reducing the stresses on the entire partition, namely for its plate behaviour, (Barbisan, 1995).
Figure 3.125 - Bidirectional floors with double frame, (Rondelet, 1834).
Mixed floor with timber beams and tile elements The practice of applying tile elements on top of the main and secondary beams of reduced thickness is documented in occidental structures from the XIII century. Despite the numerous proposals found in the manuals of the late 800’s, this building system started to be widely used only in the nineties, especially with the new building systems for mixed wood-tile-concrete floors, such as the patent Lear that uses laminated timber beams, smooth tiles and a layer of reinforced concrete on top, connected to the wooden beams through a metallic truss inserted in a groove created at the beams extrados. Another type of mixed floor wood-tile, common in the occidental construction since the XVIII century, is the floor with timber beams and tile vaults. It consists of beams, roughly squared,
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resting on the supporting walls, with a maximum span of 4.0 to 6.0m and placed at a distance of 90.0 to 100.0cm, with the section placed diagonally, Figure 3.126. In the case of a diamond arrangement, the inclined beams surfaces serve as a support to the small vaults between each beam, (Ceccotti, 2007). This floor is very simple and is used mainly in poor buildings, typically rural and agricultural buildings.
Figure 3.126 - Mixed floor with timber beams and tile vaults, (Ceccotti, 2007).
3.3.1.2 With the main bearing frame with a length inferior to the span that needs to be covered These floors, which are described in detail in the manuals of the first half of the 800’s, with the appearance of the steel floors began to lose importance until their almost complete disappearance from the manuals of the early 900’s, (Munafò, 1990). Therefore, to find some information about this type of floors it’s necessary to perform an historical research. Serlio Floors - "Solai alla Serlio" This type of floor, Figure 3.127, was designed by Sebastiano Serlio, Bolognese architect of the XVI century, although it was very similar to the type of floor designed by Barbarano-Palladio, (Barbisan, 1995). Cavaliere di San Bortolo and Milizia reported "solai alla Serlio" that consists of eight beams connected with overlapping with tongue and groove joints. Rondelet (Rondelet 1834) proposed "solai alla Serlio" composed by four beams of length equal to 2/3 of the span to be covered, connected to each other at mid span. On the point of connection between beams arranged in the orthogonal direction, there is an extension of the length equal to 1/3 of the span to be covered with the same dimensions of the main frame, (Munafò, 1990). Rondelet, (Rondelet, 1834), proposed tongue and groove connections or overlapping connections in which the elements have only half section in the connection area.
Figure 3.127 - Serlio floors, (Munafò, 1990).
It can be assumed, the hypothesis of floors for the towers, this bidirectional floor were born from the necessity of creating sufficiently rigid horizontal diaphragms, and from the difficulty in rising to altitudes as long as the span to be cover, (Barbisan, 1995). In the manuals from the 800’s, with the term "solai alla Serlio" are wrongly reported many types of bidirectional floors.
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Compartmented floors - “Solai a comparti” In the compartmented floors the four diagonal beams, called angular beams, are used to layout the floor and are supported, on both sides, by the perimetral walls, thus dividing each side of the room into three parts. The resultant triangular spaces are completed by placing the joists parallel to the angular beams and may eventually be reinforced at the intrados by a secondary beam arranged in the orthogonal direction, (Munafò, 1990). Beam to beam connections and the beam to secondary beam are of tongue and groove type and can be reinforced with metallic bandages. The floor’s intrados is usually visible while on the extrados is expected a planking to stiffen the floor, (Munafò, 1990). Polygonal floors The polygonal floors have Dutch origins. This way of organizing the floor frames was revived in Paris giving life to various forms. Emy distinguishes it in two types: (i) concentric regular polygons with parallel sides and (ii) concentric polygons offset in such a way that the corners of one match the side of the other. The connections of the beams are formed by V shaped connections or by a joint at half thickness. As for the other floors it’s expected planking at the extrados, (Munafò, 1990). Ray floors The main framework of the ray floors can be composed: by some beams that meet in the middle of the room with one king post, while others are composed by different shapes bars or beams that converge all in a central king post. The secondary frame is arranged parallel to the walls of the compartment to be covered and is positioned head to head over the main frame. This type of floor is proposed to cover circular rooms or regular polygonal rooms with many sides, (Munafò, 1990). Others In historic structures in Greece, typically, floors are made of timber. Due to the use of timber ties in most cases, there are several construction details in the connection between floor elements and timber ties. Furthermore, there is a vast variety of construction details in the region of cantilevers, Figure 3.128.
Figure 3.128 - The beams of the floor rest only on the interior longitudinal element of the timber tie, the beams of the floor are extended to the outer face of the wall, thus resting on both longitudinal elements of the timber tie; the beams of the floor protrude, in order to give support to balconies.
3.3.1.3 New generation wooden floors A great progress was attained with the arrival of laminated wood and wood-based composite materials such as Parallam, Intrallam and finally Microllam. The lamellar is formed by overlapping wooden layers, with an ideal thickness of 25.0 mm, glued to each other at controlled humidity. The laminated beams are competitive, when compared to the reinforced concrete and steel beams, to
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cover spans over 10.0 to 20.0 m, for reasons of cost, lightness and ease of assembly. The Parallam is produced by cut veneer logs to form sheets from which are eliminated the main faults; the sheets are dried, fragmented into individual elements of 2.0 to 3.0 mm thick, oriented and glued under pressure with liquid hydrophobic resin. Derived from the research on Parallam, the Intrallam is made of poplar wood splinters approximately 30.0 cm long, bonded with resin under pressure. The Microllam instead consists of very thin veneers made of wood pressure glued all together one on top of the other, (Bazzana, 1999). These technologies mainly used in North America are slowly gaining ground in Europe; although they were born to be applied in buildings constructed with the Platform system with the necessary adjustments may also find use in the Italian construction, (Dattomi, 1997) and (Ceccotti, 2002), (Figure 3.129).
(a) (b) Figure 3.129 - Use of wood base materials on the construction of floors, (Dattomi, 1997).
3.3.1.4 Damages: Symptoms, Causes and Intervention Criteria
3.3.1.4.1 Symptoms and Causes In the following the manifestations of damage and degradation found in the elements that compose wooden floors: extrados, intrados, primary and secondary supporting frames will be explored. The biggest part of the degradation and damage possibilities that can be visually surveyed on wooden floors are manifested through: (i) the deformation of the bearing frame, (ii) the failure of the floor supports or through (iii) material degradation. Next are going to be identified the causes that led to these symptoms, suggesting some criteria, methods and intervention techniques. Deformation of the bearing frame This damage can express itself through the loss (local or complete) of the floor plane horizontality and can affect both the primary and secondary supporting frame alignments. This type of instability is due to the loading state of the floor, and consequently to the beams carrying capacity, (Munafò, 1990). The damage on the beams can be located at mid span or at the supports. The first have almost always a vertical movement, and are due to bending moment that induces a tensile state on the inferior fibers of the beam. The collapse of the beam at mid span can be either brittle, and as so proceeds from the tensioned fibers up to the neutral axis, or vertical on the external layer of the tensioned fibers after which becomes longitudinal according to the fibers direction. The damage on the supports can be vertical, if they are caused by bending moment, inclined at 45° if they are due to shear, or the result of the combination of the two previous actions. Excessive deflection of the floor evolves through various phases, the last of which, the formation of cracks in the tensioned areas of the beam, is the most dangerous because it involves a reduction in the section inertia moment and thus less resistant capacity. Failure of the floor supports
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The damage in the beams supports are directly related to (i) the transmission of loads from the horizontal to the vertical structures and (ii) the connection between floor and walls. Concerning the supports failure, it is possible to distinguish several types of damages, which can affect both the horizontal structure, and the walls, (Munafò, 1990). On the beams, as mentioned previously, the failure occurs with the formation of lesions at 45° on either side of the beam and due to shear. These cracks may be caused by an increase in the static or dynamic loads, degradation of wood materials (defects, abnormal degradation or fire) or by factors related to the construction of the floors (non homogeneity of the sections, inexistence of load distribution elements). In the masonry the typical manifestation is the inverse hyperbolic damage due to crushing of the wall material under the beam support. Another manifestation of damage in the masonry is caused by the supports vertical rotation which causes also the movement of the floor supporting beams. This phenomenon is manifested by the loss of horizontality of the floor, or by longitudinal damage following the beams arrangement. Kinematic mechanism with horizontal components (earthquake, foundations displacements, etc...) may cause a horizontal movement of the support with consequent detachment of the floors from the walls, the sliding of the beams from its supports or punching of the wall. The causes of damage relative to the structures in elevation concern: the high slenderness of the walls, the modification on the static state of the walls (removal of some load bearing elements, creation of openings), degradation of materials that compose the masonry (water infiltration, aging of the mortars, etc...) or an increase of the static or dynamic loads.
3.3.1.4.2 Intervention Criteria Nowadays, a significant role in the stability of the entire building is assigned to the floors. These structures are required, in addition to an adequate performance level, a remarkable rigidity and an efficient connection to the supporting walls, (Tampone, 1996), especially in what concerns seismic actions. In the recent past, according to several international codes, the wooden structures were substituted by r.c. elements. The recent earthquakes showed the failure of the strategy that frequently involved damages and brittle failures in ancient masonry buildings. For this reason, the restoration of wooden floors is a key point in the seismic mitigation strategies, improving the behaviour and efficiency of the whole structure. Whatever system is adopted it is essential to preserve the building configuration, not to increase the height of the floor, in order to maintain the inter-storey height, not to create level differences with adjacent environments, stairs, etc…, and also not to change the dimensional relation of the environments, (Tampone, 1996). The criteria for intervention are as follows: Reduction of accidental loads and live loads that caused the damages; Rehabilitation and/or increased of the load capacity; Remove the causes of material degradation; Modification of the static scheme. The improvement of the bearing capacity can be made by: Regeneration of the structural element, i.e., retaining the original structure and intervening on the existent materials to restore their mechanical properties; Increasing the section resistance of the floor supporting structure; Replacement of degraded elements with other similar elements. An example of consolidation of the main beams is through the application or the inclusion at the intrados of metallic elements, which increase the tensile strength. This intervention, in most cases, is insufficient and must be done with devices resistant to compression and placed at the extrados, through high complexity operations, (Tampone, 1996). Another set of interventions is based on the use of structural elements individually resistant such as steel profiles. However this solution, although desirable in special cases and provisional
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measures, in addition to the imposed visual alteration, it will release the original structure from all load-bearing function, ignoring any reserves of strength in it. The solutions that consist on eliminating completely the original wooden beam masking the structural elements that replace it should be condemned. Solutions that have some success are based on the "message" that the wood works in certain situations, (Barbisan, 1997). If it is not possible to do otherwise, it is preferable to add a wood element to the entire section, possibly laminated, although it is always advisable to try to recover, by regrouping and consolidating also with any reinforcements, the original nucleus, (Tampone, 1996).
Table 3.1 - Main intervention techniques applied to the main damage causes on wooden floors, (Munafò, 1990).
Restoration and/or increase on the bearing capacity: Insertion of a laminated timber beams or of a steel elements; Substitution of the degraded part with another one made of wood. Increase in the section resistance: Creation of new supports with: lintels, cantilever or metallic supports, metallic elements placed at the intrados or extrados of the existent beams, construction of a reinforced concrete beam; Repair of the beams with metallic elements at the intrados; Application of a wooden beam to the intrados or adjacent to the one to be consolidated; Application of a flat steel or fiberglass strips at the intrados of the beam; Beam substitution. Removable of the degradation causes: Treatments against insects and attacks from fungus. Modification of the static scheme: R.C. beam projecting from the wall as a support for the of R.C. beams; Connection of the floor to the perimetral walls with reinforced concrete beam, V shape elements and steel rods; Stiffening of the floor extrados with steel ties, reinforced concrete slabs or additional boards; Replacing the existing floor.
3.3.1.5 Seismic behaviour of wooden floors On the seismic behaviour of existing masonry constructions, the role of the floors is, first, to provide an effective connection for the walls subjected to perpendicular seismic actions, and second, to transfer and distribute the horizontal actions to the supporting walls arranged parallel to the seismic action. For these reasons it is useful a limited stiffness of the floors, for which should always be carefully assessed the effects, inevitably associated to an increase in the elements resistance, that can improve the structural robustness. If the increase in the floor’s stiffness, in the in-plane actions, is such that allows a significant distribution of the seismic action among the different masonry walls, this should be taken into account by carefully considering the effect of this distributions; this concerns expecially the more stiff elements, on which the forces tend to concentrate, and of the perimetral elements, in the case of planimetric irregularities that accentuate the torsion effects. In any case the quality and consistence of the walls and of the floor-wall connections must be carefully assessed. In agreement with the presented purposes, the floors with wooden structure should be preserved as much as possible, also given their low weight and compatibility with whole structural configuration and mass distribution of historic masonry buildings.
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The floors must be effectively connected to the masonry walls in order to avoid the out-of-plane deformation, through a sufficiently wide supported and through connecting elements that prevent slippage. In general it is therefore advisable that a wooden floor presents a sufficient stiffness and resistance to in-plane action in both directions, and that the connections to all the walls are well conserved. It is important to ensure, taking also into account the masonry quality, a good distribution of the anchorage forces between floors and walls, and, in each floor, a tensile resistant boundary. Beam slipping frequently hammers in the walls, Importance of in-plane stiffness and strength (Horizontal Forces) The main function of a floor in seismic areas is to transfer, through its strength and stiffness, the in- plane seismic forces that are created in the floor to the walls parallel to the seismic action. If the floor is deformable, it transfers the seismic action partly to the walls parallel to this action and partly to the orthogonal ones, Figure 3.130. Another function of the floor is to distribute the seismic actions among the different supporting walls. If the floor is stiff and with low deformability, the seismic forces are distribute among the different supporting walls parallel to the direction of the seismic action, in accordance with the walls stiffness, and considering the distance between the resultant of the in-plane forces and the center of stiffness, Figure 3.131a. Instead, if the floor has high deformability, the seismic forces are distributed among the different supporting walls parallel to the seismic action, in accordance to their influence areas regardless of the stiffness of each septum, Figure 3.131b.
(a) (b) Figure 3.130 - Schematic of the seismic forces generated in-plane in: (a) a rigid planking and in (b) a deformable planking, during a seismic event.
(a) (b) Figure 3.131 - Distribution of the seismic forces to the supporting walls considering: (a) rigid planking, and (b) deformable planking.
Importance of the connection to the supporting walls
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During a seismic event the floor acts as constraint to the supporting walls, linking them together and ensuring box type behaviour to the building. This constraint reaction should be transferred from the floor to the walls parallel to the seismic action, soliciting the floor’s in-plane bending; to this end it is however required a certain strength and stiffness of the floor to avoid the rupture of the walls orthogonal to the seismic action after excessive movements. In the absence of an adequate connection between floor and walls, the seismic actions can overcome the friction forces and the beams may slip causing several vibrations modes between the various wall elements, which could cause local hammering damages on the wall. In what concerns the capacity of the floors to distribute the in-plane seismic actions among the various supporting walls, there are two types of buildings: Buildings of the first type - Buildings with the floors effectively connected to the masonry but without adequate in-plane resistance and stiffness (floor without reinforced concrete slab, or floor with simple or double planking). Buildings of the second type - Buildings with floors effectively connected to the masonry with suitable in-plane strength and stiffness (floors with reinforced concrete slab). The floors with plywood or with bracings (in metal or in composite materials), even in the absence of an R.C. slab can be considered adequately resistant and rigid in thier plane, but these characteristics must be verified through accurate modelling of the entire building. What is certainly desirable in both new and existing buildings is to create effective connections between the walls and the horizontal structures in order to prevent the out-of-plane overturning of the walls, Figure 3.132.
Figure 3.132 - Scheme of the damages caused by an inefficient connection between the main beams and the walls.
3.3.1.6 Seismic improvement of wooden floors In most cases, the wooden floors present in historical constructions, are considered deformable until some elements are added to increase the in-plane stiffness (ex: reinforced concrete slabs), and until appropriate connections with the perimetral walls are established. Indeed, in the historical constructions, the floor can hardly be considered as an in-plane bracing element, but rather as a dead load that unloads horizontally, during a seismic event, on the masonry, (Barbisan, 1997b). The in-plane stiffening of the floors, even limited, allows to distribute diversely the seismic actions among the vertical elements, and implies, generally, an increased of resistance, which improves the structure robustness, (Barbisan, 1997b). The need to stiffen the floors in their plane, ensuring the connection to the walls, is almost exclusive of wooden floors, because in the cases of floor solutions in mixed steel-tile or in reinforced concrete-tile, the in-plane stiffness is usually always assured by the concrete layer, while the connection to the perimetral walls is ensured by a R.C. beam, (Bazzana, 1999). However, it should be considered that the transformation of flexible floor into in-plane rigid floors leads to a redistribution of the horizontal loads over the walls that can have positive or negative
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effects, depending on the geometry of the structure and on the mechanical characteristics of the materials that compose it. Indeed, the stiffening of wooden floors, unless accompanied by adequate monitoring and improvement of the mechanical characteristics of the supporting walls that result more loaded (in-plane) with this redistribution, can even be harmful. In this regard, it is known how important is for the global behaviour of a structure to consider the stiffness of each material that composes it, because the combination of materials with very different stiffness causes abnormal and fragile behaviours if the structure is subjected to dynamic actions, as in the case of seismic actions.
3.3.2 Other typologies Since the second half of the XIX century other floor typologies were diffused, thanks to the development of the iron/steel industry. More in detail, mixed iron/steel beams with fired clay elements or small shallow barrel vaults in brick or stone were used frequently also in case of short span, (Breymann, 1903), (Figure 3.133 and Figure 3.134).
(a) (b) (c) Figure 3.133 - Example of mixed iron/steel beams with shallow barrel vaults built made by solid bricks (a) (Ferrini, 2003) (b) with different brick positioning (Aveta, 1987) or (c) stones, (Aveta,1987).
Figure 3.134 - Example of mixed iron/steel beams with clay hollow elements, (Breymann, 1930).
The XIXth century iron beams usually have shorter and thicker flanges compared to the recent sections. In general size and characteristics change in each country, being related to the local industrial production. The distance between the beams could range between 80.0 to 140.0 cm, supporting short span barrel vaults in solid bricks/stones or hollow bricks of various shape and size. Flat floors could be built by shaped clay elements clamped between them and frequently patented. The use of stones, usually soft stones likes tuff, calcarenite or pumice is diffused, as well, (Figure 3.133c). A conglomerate layer completes the floor structure, (Figure 3.133). Generally they are considered effective load bearing floors, enough stiff in their plane; they are often kept without any drastic intervention, preventing hybrid behaviour with substituted structural elements. The seismic behaviour of such structures is, in general, quite, good. This could be related also to the effective structural detailing of the XIXth century building.
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Possible damages are related to hammering, which could trigger overturning mechanisms of the walls, and to slight movements of the metals beam which could produce deformation in the vaults, (Figure 3.135).
Figure 3.135 - Example of damage on floor built with mixed iron/steel beams and shallow barrel vaults caused by the beams shift, (Croci, 2001).
3.4 ROOFS The roof structure is usually composed by timber beams or trusses. The roof structure could thrust directly the walls or simply supported as in case of trusses (Figure 3.136 and Figure 3.137). In some case, wooden tie rings mitigated the thrust from the roof, and constrains the walls on top.
Figure 3.136 - Example of roof structures diffuse in Italy.
Traditional roof structures include several configurations, spanning from about 5.0 to 20.0 m, according to complexity of the structure, (Figure 3.138). The configuration of the secondary structure supporting the covering influences the truss distance, (Figure 3.139). Wooden structures could have further configuration, as wooden vaults, (Figure 3.140). Roof damage could be revealed by a movement of the joints or of the ridge. Local damages caused by the movement of the tile coverings are frequently causes of the beams decay. In Figure 3.142 the decay of the connection between strut and tie causes a sliding of the strut which thrust directly the wall. Strut-tie decay is frequently surveyed due to the biological attack or moisture conditions, (Figure 3.143). The damage could involve the truss collapse or an anomalous loading of the supporting walls. Thrusting elements could contribute to the local or global overturning of unrestrained walls, (Figure 3.144 and Figure 3.145).
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Figure 3.137 - Classification of the roof structures diffuse in Italy according their thrust; a) Thrusting structure; b) Reduced thrust; c) not thrusting. From the post-earthquake damage survey form (GNDT, 1999), (Aedes, 2000).
Figure 3.138 - Example of roof trusses from (Rondelet, 1834).
Figure 3.139 - Example of roof covering structure, (Giordano, 1993).
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Figure 3.140 - Example of wooden vault, (Giovanetti, 1998).
Figure 3.141 - The ridge movement produces the struts pushing on the side walls, (Doglioni, 2007).
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Figure 3.142 - Thrust to the load bearing walls caused by truss damages, (Tampone, 2001).
Figure 3.143 - Truss collapse due to the decay of the strut-tie node, (Doglioni, 2007).
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Figure 3.144 - Roof thrust damage on a multiple leaf stone masonry, (Doglioni, 1999).
Figure 3.145 - Hammering of the roof structures to the load bearing walls, (Doglioni, 1999).
3.5 SUB ASSEMBLAGES (WALL-WALL, WALL-FLOOR, ETC.) / CONNECTIONS A good quality of the connections between floors and walls, between roof and walls and between perpendicular walls is also crucial to reach a good global seismic behaviour of the building. Good quality connections will drive the collapse of the construction to a configuration that requires a stronger seismic action. On the basis of several survey activities in Italian areas struck by seismic events, it can be reasoned that the out-of-plane overturning of the facade is one of the most recurrent damage mechanisms. This structural behaviour is strictly connected to the presence and to the quality of the constraints between walls and floor/roof structures. The presence of ties is another relevant factor in the effectiveness of the constraints. The influence of the connection between orthogonal walls in the overturning mechanism has been studied by various authors (de Felice, 2001), (D’Ayala, 2003a). As detailed in Figure 3.146, unstrained wall could globally overturn, while the presence of the floor constraint due to friction changes the kinematic mechanism triggered by a higher energy. Similar effects are due to the constraints of roofs structures and ties.
Figure 3.146 - Effects of the constraints imposed by floors and vaults, (Carocci, 2004). A unconstrained wall, B floor constraint; C wall not restrained by roof structures; D roof restraining; E vault thrusting; F tied vault.
Rondelet in his treatise (Rondelet 1834) explored the collapse mechanisms of an unrestrained wall and the effects of one side and two sides constraints. The effectiveness of the constraints and, then, of the global box behaviour of the masonry structure is related to the wall slenderness, as well.
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The mechanisms A, B, C of Figure 3.147, related to unstrained walls, are sensible to low intensity earthquakes. In the following mechanisms D, E, F, G, H of Figure 3.147, the kinematic mechanism is related to higher actions, due to the presence of constraints.
Figure 3.147 - Effects of the constraints imposed by ties and buttresses on the box behaviour (Carocci, 2004). I, II, III Rondelet’s mechanisms A without constraints, B one side constraint; C one side and floor constraints; D two side constraints; E wall restrained only by the floor; F presence of buttressed; G vertical bending of a wall; H III Rondelet’s mechanism with tie restraining.
In Figure 3.148 the quasi linear vertical crack suggests that the connection between the walls was defective, producing the walls separation due to out-of-plane bending. Traditional masonry structures were restrained by corner tie stones, (Figure 3.149); their effectiveness in the wall restraining, (Figure 3.150), is related to the dimension, (Figure 3.151), but also to the masonry quality of the walls. In fact, this influences the restraining possibilities of the orthogonal walls, as appears clearly in Figure 3.79. Openings close to the corner decrease the effectiveness of the constraints. Several structures show the blocks cut with an internal angle, so as to 'fold' the stone around corner's. It is suggested that this was incorporated as an earthquake prevention measure. It is worth to mention the architecture of Armenian buildings or other buildings made by large stone blocks as in Egypt or in Peru. It is interesting to note in Figure 3.152 that the stones have been cut so as to continue only a short distance around the corner which hints at the idea that style might have been involved (rather than, or as well as, function). Similar features were surveyed also in Italy. The basement, the enlarged base, has an importance to ensure a stable base on which rises a building, (Figure 3.153), a historic monument for example. The vertical reinforcing chain made out of cut stone, is another mean of reinforcement and consolidation of the building corners, which is generally used in colonial architecture and fortified architecture. The reinforced basement and the rebuilt angle chaining using building materials different from the original masonry, are very apparent actions on the frame and could belong to indicators suggesting knowledge of damages caused by an earthquake. Similar problems affect the constraints of the transversal masonry walls inside the building which have shear wall functions, (Figure 3.154).
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Figure 3.148 - Separation of wall due to out-of-plane bending. The quasi linear vertical crack suggests that the connection between the walls was defective.
Figure 3.149 - Corner details: a) absence of tie stones, b) ineffective constraints; c) good quality connections (Ferrini, 2003).
Figure 3.150 - Effects of the quality of the side walls connection, (Giuffrè, 1999).
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Figure 3.151 - Out-of-plane overturning of the church facade: (a) mechanism 1; (b) mechanism 2. Possible failure surface in mechanism 1: (c) facade and lateral wall made up of blocks of similar dimensions; (d) facade and lateral wall made up of blocks of different dimensions, (Lagomarsino, 2009).
Figure 3.152 - Stone blocks cut with an internal angle to improve the corner connections. Picture from Egypt.
Figure 3.153 - Al Kamra Tower in Asila medina and rampar and tower in Salé medina.
(a) (b) Figure 3.154 - Possible mechanisms related to the wall to wall constraints. a) (De Felice, 2001) b) (Doglioni, 2007).
In the building survey aimed to the seismic vulnerability evaluation, a deep control of the corner detailing and of the wall to wall and wall to floors/roof connections is one of the keypoint (Figure 3.155 and Figure 3.156). The presence of effective constraints and ties influence the kinematic mechanism. Furthermore the building evolution could produce discontinuities between walls, or weak constraints. It is worth to remind the frequent use of decorative stone elements at the corner, which have not any structural connection effect but their shifting can damage the masonry (Figure 3.157); similar damage could be produced by apparently effective details disposed only superficially or not extended in the depth.
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Figure 3.155 - Disposal of the tie stones surveyed in Syracuse, (Giuffrè, 1993).
(a) (b) Figure 3.156 - Layout of the tie rods in a Palermo’s historical building, (Carocci, 2001).
(a) (b) Figure 3.157 - The stone block at the corner could be (a) only decorative elements or (b) only superficial, (Doglioni, 2007).
The wall/floor connection is an important detail to survey. If the horizontal actions are not correctly transferred and distributed to the whole masonry wall by an effective floor connection, out of plane displacements of the wall or local hammering can occur, (Figure 3.158a). These could contribute to the beam movement and possible beam falling, (Figure 3.158).
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(b)
(a) (c) Figure 3.158 - The lack of connection between floor and wall can led to the beam falling and hammering increasing the wall displacement, (Doglioni, 2007).
In timber reinforced buildings, quite often, the connection between walls is ensured by the timber elements (well connected in those regions, as shown in the Figure 3.40 and described in point 3.1.1.4.1). This is the reason why, the most critical (out-of-plane) failure does not take place exactly where walls meet but after the strong region of the connection. The effectiveness of timber tied are extensively reported in (Vintzileou, 2008) and (Humo, 2008). Typically, a horizontal timber tie consists of more than one longitudinal timber element, running along the perimeter of the building and by transverse timber elements that connect the longitudinal ones at regular intervals (Figure 3.41, Figure 3.159 and Figure 3.160). In timber reinforced structures, special care was given by the constructors to the connections between timber elements in the roof, within the timber-framed masonry of the upper storeys, as well as between timber beams and columns in the ground floor. Figure 3.160 shows examples of connection of timber elements in the corner of a building. Furthermore, some construction errors are recorded in several structures. Those errors increase the vulnerability of timber reinforced systems, (Figure 3.161). If timber ties do not continue in the corner zone of masonry, their tying role cannot be activated and cracking (due to out of plane bending) cannot be prevented, (Figure 3.161a). Poor splicing detailing can contribute to the unsuccessful of the technique. When the nail that connects the two wooden elements is corroded, the tying action of timber ties is lost, (Figure 3.161b). Timber reinforced systems pose rather difficult problems, when their preservation is sough. The main problem is that it is quite difficult to replace or to substitute rotten member ties that are- quite often-located away from masonry faces and, hence, difficult to reach.
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Figure 3.159 - Typical transverse connections of timber elements, (NTUA/EPPO, 2005), (Vintzileou, 2008). (Sketch by P. Touliatos).
Figure 3.160 - Connection of timber elements in the corner of a building.
(a) (b) (c) Figure 3.161 - Building errors increase the vulnerability of timber reinforced systems. (a) If timber ties do not continue in the corner zone of masonry, their tying role cannot be activated and cracking (due to out of plane bending) cannot be prevented; (b), (c) Poor splicing detailing can contribute to the unsuccessful of the technique. When the nail that connects the two wooden elements is corroded, the tying action of timber ties is lost.
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4 BUILDING TYPOLOGIES AND DAMAGE MECHANISM
Generally, the structures in the most damaged area are residential buildings with a variety of configurations, sizes and openings. Frequently in the historic town centres the buildings are organised into irregularly shaped blocks (usually with a linear layout). The buildings are connected to each other as a result of growth and transformation over the centuries. In historic centres streets can be very narrow, with high risks of injury of the occupants from falling debris, and difficulty of access for rescue vehicles. In addition, there is often a shortage of refuge areas, such as parks and squares, and the steep slopes and density of the buildings makes site access difficult. A general problem to all the building typologies and concerning the several modification in time, could be related to the change of use of the building or of portion of building. Typical example is the insertion of commercial activities at the ground floor, as in the case reported in (Vintzileou, 2007). Excessive lateral displacements, (Figure 4.1), were observed in a limited number of buildings in Lefkada in which the stone masonry was partly demolished at the ground floor, before the event, when the use of those buildings was modified from residential to commercial. The demolition of masonry was done without previous design, as happened frequently in the past. Thus, the secondary bearing system of the ground floor became primary. Since, however, its stiffness was very limited, this system could not prevent large (permanent) horizontal displacements that led to distortion of the building as a whole at the ground storey.
Figure 4.1 - Lefkada Permanent interstorey drift in the ground floor, (Vintzileou, 2007).
4.1 ISOLATED BUILDINGS Isolated buildings, in general, are the simplest typology, unfortunately not very frequent in historic centres. The general behaviour is reported in point 3. The structural behaviour is related to the building evolution and to the technological detail.
4.1.1 Earth constructions Isolated buildings are quite frequent in earth construction. Typical floor plans are shown in Figure 4.2. Single as well as multiple storey buildings are found. Originally, earthen buildings are characterised by simplicity in plan and elevation. With the exception of colonial buildings in California and South America, possibly inspired by fired brick architecture in Europe, buildings are found to be round and square in plan.
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L ≈ 50-60m
Fujian Tulou Temples and Rapaz Missionary Las Flores Adobe Collective dwellings Religious Building Ranch House Housing India Peru California China Rammed Adobe Adobe Rammed Earth Earth and Adobe Figure 4.2 - Some plan typologies of isolated earth buildings.
4.1.1.1 Multi-Storey Buildings 4.1.1.1.1 Adobe brick walls, load bearing, topped by timber beams at storey/roof levels Adobe houses with timber bond or ring beams belong to the vernacular architecture of different countries. Those of Kashmir and Nepal are interesting examples of vernacular architecture showing signs of seismic-resistant features. Taq houses (Kashmir): consist of adobe walls held together by horizontal timber bond beams, or runners, which are continuous around the perimeter of the building, and present at each floor and roof level. The floor beams and the wall beams lap over each other, so that the walls are tied together with the floors. Gosain and Arya (Gosain, 1967) suggest that the weight of the masonry prestresses the wall, contributing to its resistance to lateral forces. Some of the characteristics of these buildings oppose some of today's commonly-accepted practices: mortar of negligible strength was used; masonry leaves are poorly bonded; roofs are heavy. However, both Neve (Neve, 1885) and Gosain and Arya (1967) reported relatively undamaged survival of buildings 3-5 stories after earthquake damage. The literature suggests that this is due to the damping resulting from the friction induced in the masonry of Taq walls, estimated to be in the order of twenty percent, compared to four percent in uncracked modern masonry (brick with Portland cement mortar) and six to seven percent after the masonry has cracked (Gosain and Aryia, 1967). The timber runner beams and floor diaphragms keep the individual piers from separating, which would cause the house to break apart, so that even though the mortar is extremely weak, causing the wall to yield under a much smaller load, the masonry continues to have a good chance of holding together. D´Ayala, (D´Ayala, 2009), has thoroughly studied a similar housing typology, the newari, in the Kathmandu Valley, Nepal (D´Ayala, 2003c). Traditional newari houses are usually independent, rectangular, three-storey houses, in plan about 6.0 m by 4.0 to 8.0 m. A rather common feature of Nepalese traditional construction is the insertion of pegs, called chokus, to restrain floor joists from sliding over walls. Two vertical pegs are usually inserted through a joist on each side of the wall. Typically this will occur on every two or three joists. From an external visual inspection, the chokus are easily identified at roof level, due to the presence of the overhang; however they are also present at intermediate storeys on joists passing over the internal wall. For the intermediate storeys the common practice is for the joists to be anchored with pegs on the internal face of the external wall and in between the two leaves. This practice is very effective in preventing relative sliding of the floor structure on the walls in presence of lateral forces and hence create a box effect, while at the same time, given the flexibility of the pegs and their position, does not prevent other movements associated with temperature gradients and other environmental effects. The
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presence of the pegs is also effective in limiting any substantial out of plane movement of the external walls due to uneven settlements. Most commonly, the pegs butt on a timber wall plate running along the width of the façade, on which the joists sit. In most cases the timber wall plate is positioned directly above the level of the window frames, spanning the openings, and runs the entire width of the façade. While the best traditional practice uses wall plates on both leaves of the façade, (Figure 4.3), connected by transversal struts dovetailed into them, as can be seen in some of the oldest and better built examples, nowadays the common practice is for only one wall plate spanning over the internal leaf of the wall. From a structural point of view the double wall plate is not only effective in redistributing the vertical loads more evenly across the wall, but, in the original arrangement has the double function of tying together the two leaves of the wall and, in presence of lateral load preventing shear cracks in the masonry from running from one floor to the next.
Figure 4.3 - Facade mechanisms of failure, (D´Ayala, 2009).
The most common cause of seismic vulnerability increase in houses in the Kathmandu valley was reported in (D´Ayala, 2006) to be the use of the dalan, a timber frame used in conjunction with adobe building, consisting of columns pinned to the ground below and pinned to the beams above. The use of pinned connections means that the use of the dalan can be compared to that of a soft-storey structure, and its failure mechanism as that of a soft-storey as well (Figure 4.4).
Figure 4.4 - Soft-storey failure mechanism in newari house with dalan wooden structure.
Rammed earth multi-storey buildings: The Tulou is a unique building typology found in South-East China. The massive rammed earth peripheral walls are up to 4-storeys in height and enclose storehouses, wells and bedrooms for hundreds of dwellers More than twenty thousand tulou are still standing and some are still inhabited. Tulou are usually rectangular or round in plan, Figure 4.5. One of the largest is Zaitianlou in Zhaoan, with 2.4 m thick walls and a diameter of 91.0 m. Tulou houses have been said to be earthquake-resistant buildings, (Minke, 2006).
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Figure 4.5 - Tulou housing, different typologies after Huang Hanmin (1991).
4.2 ROW BUILDINGS The generalized characteristic of the historical centres layout is the structural continuity of the buildings, (Carocci, 2001). In fact, excluding exceptional cases, frequently masonry buildings are structurally connected with the adjacent ones in order to form a block, (Figure 2.13). The latter can be synthetically defined as a buildings system - also of remarkable dimensions - delimited by public and/or private un-built spaces. The buildings can evolve in curtains along a street, in rows, (Figure 4.6). The reconstruction of the row evolution is a keypoint in the vulnerability evaluation because it can clarify the effectiveness of the restrains between the walls and locate discontinuity between masonry portions, (Figure 4.7). This peculiarity of the historical layout is the reason why the analysis of the behaviour of each portion does not result sufficiently exhaustive when not associated to a wider interpretation involving at least the buildings directly bordering the one which is object of the analysis. The analysis of a structural unit belonging to an aggregation is different than the case of an isolated building because of the several interactions that the adjacent buildings do on the structural unit which is analysed, (Figure 2.16, Figure 2.17 and Figure 2.24). The basic structural interaction phenomena involve both negative and positive actions; adjacent buildings can induce vertical loads or horizontal pushes (especially under the seismic action) or, on the contrary buttress or constrain the adjacent buildings, (Figure 2.24). These interactions modify the collapse mechanism of the building introducing new different actions and changing the constraint configuration, (Figure 2.17). As an example, it can be easily understood how some peculiarities of the structural behaviour derive from the particular location of the individual building within the building system in the block. This is the case of the extremity portions, often more damaged, (Figure 4.8), and in the past preventing reinforced by buttresses, counterforts or ties.
Figure 4.6 - Row of buildings, (Binda, 2005a).
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Figure 4.7 - Evolution of a row of buildings, (Carocci, 2004) 1 isolated building, 2 building built adjacent to the previous one; 3 contemporary buildings built adjacent to the previous ones; 4 building built between existing buildings, 5 demolished building.
Figure 4.8 - Damages of the last building of a row. (a) (Gurrieri, 1999) and (b) (Avorio, 2002b).
Damage often affects whole rows of contiguous buildings, with the damage concentrated at the base of the structures on the up-slope side of the buildings, (Figure 3.83). This indicates that the rows of buildings lagged the ground motion as a unit, rather than pounding each other, which would have caused the most damage at the upper story points of collision. Instead, as the earthquake waves cause the buildings to sway, shear cracks opened in the ground floor walls. With each loading cycle, the buildings toward the ends of the rows are forced away from the center of the rows, or at least away from the section of the row with the most resistant or most heavy and flexible building. This spreading of a row out from the center resulted in enough displacement of the outermost building to cause it to collapse, (Giuffrè, 1993), (Carocci, 2001). The extent of the damage to rows of buildings is sometimes worsened by modern top story additions which impose lateral force, not just on the subject building, but on the whole row. The damage to individual structures could often be a disruption of the masonry in the highest points of the structure, with an outward spreading of the walls as a result of a dearth of wall ties through the building or effective tension at the connections to the floor diaphragms. When assessed for vulnerability, buildings that are physically attached to each other in rows must be treated together. The risk is increased if buildings in the row are modernized or replaced in ways that make them taller and heavier. Irregularities and weak points due to various reasons are assessed; namely, due to the morphologic characteristics (staggering of the foundation level), to irregular elements both horizontal and vertical (adjacent wall cells of very different dimensions or height, (Figure 2.11, Figure 4.9 to Figure 4.11), elements resulting from the progressive closing of open-spaces), to the integration of the buildings with pre-existing structures (castle or urban walls, ground supporting walls, terracing supporting structures). Missing or poor connections are pointed out; missing, namely, because of: a) the transformation phases, b) the existing discontinuity (flues, chased plants, openings in breach), c) the opening position (proximity to the corners, excessive width and length of the spaces, lack of alignment, reduced distance between openings), d) the elimination of building load-bearing elements (lengthwise partition walls between two close cells), e) the introduction of un connected additions.
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Contiguities are defined between different building systems which introduce unfavourable interactions due to previous structural interventions that have changed the stiffness characteristics of walls or floors, or to the ratio between their masses, or to substitutions carried out with load- bearing structure different from the masonry one, (Figure 4.9 to Figure 4.11). The previous vulnerable elements should be interpreted in order to understand the overall mechanical behaviour, including the slenderness of the individual walls and their connecting conditions. Hammering, (Figure 4.12), between adjacent cells is a diffuse damage which could lead to the overturning of the smallest adjacent volume. If the height of adjacent cells is very different the higher one frequently overturns due to the stiffness change. Other relevant problems are related to the lack of alignment of the floors or vaults: the seismic action can push on the common wall, (Figure 4.13). Damage abacus of cells within rows were developed by Giuffrè, (Figure 4.14) (e.g. Giuffrè, 1993, 1997, 1999) and (D’Ayala, 2003a). The damage prevision of a row building should take into account all these factors, (Figure 4.15). Figure 4.15 shows the seismic damage scenario of a row of buildings in a historical centre evaluated from the inspection and interpretation of the building vulnerabilities.
Figure 4.9 - Damages due height changes in a row and of the floor alignment, (Gurrieri, 1999).
Figure 4.10 - Damages due to the lack of alignment in a row, (Gurrieri, 1999).
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Figure 4.11 - Damages due to stiffness changes. Rotation, partial overturning and hammering, (Binda, 2006a).
(a) (b) Figure 4.12 - Interaction between adjacent volumes. (a) (Doglioni, 1999). (b) (Doglioni, 2007).
Figure 4.13 - Interaction between adjacent buildings, (Borri, 2004a).
Figure 4.14 - Cell position within a block and possible damage, (Giuffrè, 1993).
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(a) (b) Figure 4.15 - Damage prevision in a row of buildings. (a) (Carocci, 2001). (b) (Giuffrè, 1999).
It is worth to mention also the case of the wide enlargement of Barcelona designed by Cerdà (Paricio, 2009) and recently studied by Universitat Politècnica de Catalunya. The “Eixample” covers a very significant part of the today’s urban texture of Barcelona, (Errore. L'origine riferimento non è stata trovata. to Figure 4.19), with problems typical of many cities all over the world and related to the continuing changes in time, particularly in recent time of the buildings. This construction period includes some 80 years beginning around 1860 and reaching until the end of the Spanish Civil War in 1939. The typical building from the Eixample consists of a masonry load bearing wall structure. In most cases, the load bearing walls are in the façades and parallel inner walls, (Errore. L'origine riferimento non è stata trovata.). The façades are 11.0 to 14.0 m long, allowing for two flats per story. The load bearing walls are complemented with secondary perpendicular walls to grant stability. Unfortunately, these secondary walls were not always built, leaving its role to the thinner partition walls. In some cases, the interlocking between the load bearing walls and the secondary ones has been damaged or has been lost even due to cracking caused by soil settlements. Generally, the load bearing walls of the façades are about 28.0 to 30.0 cm thick, while the inner load bearing walls or the secondary ones are only about 14.0 to 15.0 cm thick. There are to different types of buildings. The first one is the chafer building, at the corners of the square blocks, while the second is the typical joint ownership house placed along the sides of the blocks. The cross-sections highlight four typologies, including buildings built above ground level, buildings with basement, buildings with a basement just under the ground level and buildings with a semi- basement. To characterize the earthquake collapse mechanisms, it is necessary to understand how the structure works as a supporting element of the whole building. One must also understand how the wall systems are articulated and how these create a system of “enclosed space”. The load bearing walls are built of bricks set in lime mortar and plastered also with lime mortar. Remarkably, a criterion in the specification sheets of the time indicated that once a existing building was demolished, all those materials that might be usable were left for the builder. Therefore, many current elements of these buildings came from other constructions, leading to a first process of sustainable construction.
Figure 4.16 - Current view of Figure 4.17 - Current urbanism of Eixample. Figure 4.18 - Typical inner Barcelona´s Eixample. Distribution of typical blocs. Courtyard.
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Figure 4.19 - Street façade showing typical buildings, (Gran Via de les Corts Catalanes).
The buildings of the Eixample show some problems defects which may affect their overall strength and seismic performance and that could be similarly found in several cities, even in recent expansions. Frequently, the adjoining properties share the same dividing wall. For many years, and due to an urban density problem, many buildings experienced additions of new floors, causing overload in the building’s structure, and the setback of the upper part of the building, (Figure 4.21). In more recent times, the inner cores of buildings have suffered several changes at the ground floor level, such as openings at the walls. These changes often waken the structure. The buildings, and specially those built along the second period, have a significant amount of external ornamentation. Many ornaments are not anchored to the structure and may fail in the case of a seismic movement. In some cases, an interlocked connection may not have been formed between the bearing walls and the secondary perpendicular walls. Initially adequate connections may have been lost due to cracking caused by differential soil settlements and other actions. The Ground Floors are weak points with regard to the seismic action, because these levels, intended to be used as commercial spaces, are more diaphanous. Often, the vertical structure at these levels consists of cast iron pillars. The pillars support a grid of iron or steel beams on which the upper load bearing walls are supported. Hence, the strength and stiffness of the inner walls is interrupted or compromised because they do not reach the foundation, but are only supported on a set of pillars. On the base of the previous detailed observations and studies concerning the building technology and most frequent changes, an abacus of the possible mechanisms was developed, (Figure 4.22).
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Figure 4.20 - Geometric features of two different building typologies in Figure 4.21 - Cross-section showing the Eixample - Barcelona. additions of new floors.
Figure 4.22 - Collapsing mechanisms for the typical “Eixample” masonry buildings.
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4.3 COMPLEX BUILDINGS A specific problem of urban historical centres is the issue of the seismic vulnerability of aggregation or blocks of masonry buildings. They are the result of the progressive growth of the urban tissue, (Figure 2.13, Figure 4.23, Figure 4.24, Figure 4.25), in which elevations are added to existing buildings and enlargements or progressive growths in plan are made by adding structural cells or units in contact with the previously existing ones, so that often adjacent units share the same boundary wall. Within a similar situation, the distinction of structurally independent units is problematic if not impossible, and any structural analysis that aims at evaluating the “global” response should in principle either model the whole block or model the structural unit with suitable boundary conditions that take into account the effect of the adjacent ones, but both options present high difficulties for professionals. Complex building could have the same or amplified vulnerability of row buildings, due to the possibility of addition in several directions. In row buildings, in fact, portions interact only with lateral cells. In complex buildings the aggregation could be at several sides, as well as the possible lack of connection of walls which causes overturning, the discontinuities, the lack of constraints, the different height of the portions, the different height of the floors, etc. The evaluation of the expected damage should be carried out both in terms of direct vulnerability - originating from the structural and transformation characteristics of each building of the block - and in terms of induced vulnerability - originating from the mutual interactions between buildings placed side by side, and from the damage effects on the open spaces, (Figure 2.13), (Carocci, 2001).
Figure 4.23 - Complex buildings in Umbria, (Binda, 2006a).
Figure 4.24 - Building typologies within the historic centre of Castelluccio, (Cardani, 2004), (Munari, 2009).
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Figure 4.25 - Ksar Ait-Ben-Haddou was added to Unesco's World Heritage List in 1987. The Ksar, a group of earthen buildings is a traditional pre-Saharan habitat in Ouarzazate province, a striking example of the architecture of southern Morocco.
4.4 PALACES Palaces are special building typologies. Usually they are listed as monuments, with more refined details, lodges, wide halls, vaults, chapels and different functions. They could be fruit of an original design or progressive additions of parts, including also connection to close buildings. In the first case, the structure can have good connection between walls. In the second case, the building can have the vulnerability of a complex structure. Palaces frequently host public functions, e.g. museums, town halls, library, government offices, schools, etc. The change of function could involve heavy intervention of seismic adequacy and floor changes due to public access. In this last situation, incompatibility between the old and new structures could produce unexpected seismic failures. The Italian Civil Protection Agency developed a post-earthquake damage survey form, which summarizes the most frequent damages, (PCM, 2005), (Figure 4.26).
Figure 4.26 - Complex buildings in Umbria, (PCM, 2005).
4.5 CHURCHES From a systematic observation of the structural damage caused by the Friuli earthquake, (Doglioni, 1994), the seismic response of churches may be described according to recurrent behaviours, traceable to the damage modes and collapse mechanisms of the different parts, called macroelements, which demonstrate structural behaviour that is almost autonomous. Typical examples of macroelements are the façade, the bell tower, the apse, the transept and the side chapels. This approach allows a very effective qualitative interpretation for churches, due to the recursive architectonic portions “standardise” by liturgic and functional rules.
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Most of the damage mechanisms are related to specific portions of the church (macroelements) as exemplified in Figure 2.14 and Figure 2.15. It is worth noting that the concept of macroelement is aimed at a more effective understanding of the seismic response of the fabric, but it is not strictly necessary; the landmark of the methodology is the singling-out of the collapse mechanisms. In the damage survey templates published by the Civil Protection Department and the Ministry of Cultural Properties in Italy, (PCM, 2001), (Figure 2.2), for each macroelement, by considering its typology and connection to the rest of the church, it is possible to identify the damage modes and the collapse mechanisms. During the inspection operations, the surveyors must indicate: (a) the actual macroelements; (b) the damage level; and (c) the vulnerability of the church to that mechanism, related to some specific details of construction. Slender walls, large, heavy architectural elements, such as domes and vaults, and the lack of intermediate horizontal floors create a vulnerability that has caused damage greater than that observed in ordinary buildings, even in low-intensity seismic shaking. The main churches have two or three naves, an apse and one or two side lateral chapels, a bell tower, and sometimes a transept. Frequently the main churches are placed in the centre of the main square and sometimes have staircase in front of the façade. In some cases, houses were built around the main church up to the growing of a village. Minor churches can have a single nave, sometimes with an apse, a two-pitch roof, and smaller dimensions than those of the main church, (Lagomarsino, 1997), (Lagomarsino, 1998a, b), (Lagomarsino, 2004a). In some cases, masonry discontinuities are clearly recognisable and due to the several building phases, repairs and interventions, as well. The most frequent damages could be the following: (1) cracking and collapsing of vaults (owing to their limited thickness and the lack of tie-rods); (2) damage to the pillars due to bending or shear in churches with more than one nave; (3) sliding or overturning of the spires over the bell towers; and (4) out-of-plane failure of the façade gables or apses. Out-of-plane failures of gable end walls are common in structures with either wood or concrete roofs, where the wall is not well connected to the roof. Typically, the inertial forces corresponding to the weight of the wall itself are thought to ensure failure. In this case, the failure shows the effects of the hammering of the roof on the masonry below. In single-nave churches, the structural macro-elements made by the arch-pillar system (dividing one bay from others) or the triumphal arch (separating the nave from the presbytery area) are often present: typically, the seismic damage is represented by a four-hinge mechanism. It may involve both the arch pillars, or only the one subjected to an outward thrust. Sometimes the pillars of the triumphal arch work as shear walls, as they are generally quite wide because of the minor width of the presbytery. Figure 4.27 summarizes the typical collapse mechanisms of the churches (Lagomarsino 2004a). More in detail, according to Lagomarsino in (Lagomarsino, 2004a), the following 18 collapse mechanisms, affect the church macroelements:
THE FAÇADE
1. OVERTURNING OF THE FAÇADE damage: separation of the façade from the lateral walls, in proximity to the corner or with curved cracks in the lateral walls vulnerability: 1. bad connection between the façade and the lateral walls 2. lack of longitudinal tie rods or buttresses
2. OVERTURNING OF THE GABLE damage: separation of the top of the façade into parts vulnerability: 1. presence of wide openings that weaken the façade (rose window)
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2. lack of connection with the roof covering (hammering of the ridge beam)
3. SHEAR MECHANISMS IN THE FAÇADE damage: cracks in the façade with X trend; central vertical crack; arched crack near the corner vulnerability: 1. presence of wide openings, even if closed with masonry 2. roof thrusting on the lateral walls and lack of transversal tie rods
THE NAVE AND THE TRANSEPT
4. TRANSVERSAL VIBRATION OF NAVE OR TRANSEPT damage: cracks in the structural arches; rotation of the lateral walls, with crushing or opened cracks near the base of the pillars vulnerability: 1. lateral walls too slender 2. lack of transversal tie rods or buttresses
5. LONGITUDINAL VIBRATION OF THE CENTRAL NAVE damage: cracks in the longitudinal arches; crushing or opened cracks at the base of the columns; diagonal shear cracks in the aisles vulnerability: 1. slender columns and central nave very high with respect to the lateral ones 2. lack of longitudinal tie rods
6. VAULTS OF THE CENTRAL NAVE damage: damage in the vaults, with disjointedness from the stiffening arches vulnerability: 1. vaults too lowered and/or too thin 2. presence of concentrated actions from the roof covering (due to wooden props of the roof)
7. AISLES AND VAULTS OF THE TRANSEPT damage: damage in the vaults, with disjointedness from the stiffening arches vulnerability: 1. vaults too lowered and/or too thin 2. presence of concentrated actions from the roof covering (due to wooden props of the roof)
THE TRIUMPHAL ARCH
8. MECHANISMS IN THE TRIUMPHAL ARCHES damage: formation of hinges in the arch, with opened cracks, crushing of masonry and sliding of stone ashlar vulnerability: 1. thickness of the arch too thin or presence of masonry of bad quality 2. lack of tie rods or badly positioned; insufficient propping up by the lateral walls
THE DOME AND THE TIBURIO
9. COLLAPSE OF THE DOME AND THE TIBURIO damage: formation of a continuum arched crack in the dome; cracks in the drum vulnerability: 1. drum very high and slender (with big openings) 2. lack of hoopings and of external buttresses
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THE APSE
10. OVERTURNING OF THE APSE damage: vertical cracks in correspondence of the windows; inclined arched cracks vulnerability: 1. lack of hoopings or of longitudinal tie rods 2. hammering roof covering or weakening as a result of the presence of big openings
11. VAULTS OF THE APSE AND OF THE PRESBYTERY damage: damage in the vaults vulnerability: 1. vaults too lowered and/or too thin 2. presence of concentrated actions from the roof covering (due to wooden props of the roof)
WIDESPREAD MECHANISMS
12. OVERTURNING OF OTHER WALLS (TRANSEPT FAÇADE, CHAPELS) damage: separation of the end wall from the orthogonal walls vulnerability: 1. bad connection between the end wall and the orthogonal walls 2. lack of tie rods or buttresses
13. SHEAR FAILURE OF THE WALLS damage: inclined cracks in masonry; disjointedness in the lack of continuity (closed windows) vulnerability: 1. masonry of poor quality or too thin 2. weakening as a result of the presence of too many openings
14. HAMMERING AND DAMAGE IN THE ROOF COVERING damage: cracks in proximity to the support of the beam; disjointedness from the R/C ring beam and the masonry vulnerability: 1. hammering roof, with absence of a link between the wooden beam and the masonry 2. increasing of the weight and the stiffness of the roof (substitution with a R/C slab)
15. INTERACTION BETWEEN ELEMENTS OF DIFFERENT BEHAVIOR damage: cracks due to the hammering between different parts vulnerability: 1. significant difference in the global stiffness of the two parts of the fabric 2. lack of a good connection between the masonry in the two parts or of tie rods
THE BELL TOWER
16. GLOBAL COLLAPSE OF THE BELL TOWER damage: cracks near the connection with the church; vertical cracks below the bell cell vulnerability: 1. bell tower too slender and made of walls of limited thickness 2. masonry of poor quality and lack of connection between the walls
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17. MECHANISMS IN THE BELFRY damage: cracks in the arches; rotation and sliding of the pillars vulnerability: 1. lack of tie rods or hooping ties 2. pillars too slender and roof too heavy and/or thrusting
BELL GABLE, SPIRES, AND PROJECTIONS
18. OVERTURNING OF PROJECTING ELEMENTS damage: global permanent rotation or sliding; cracks at the base of the element vulnerability: 1. lack of an effective connection with the church 2. element too slender
Figure 4.27 - Table of the seismic damage mechanisms in the macroelements of the church, (Lagomarsino, 2004a).
The methodology and the abacus were used in the analysis of the damaged churches at the Açores Island as reported by Azevedo and Guerreiro in (Sousa Oliveira, 2008) or the most relevant Gothic churches built in the city of Barcelona, namely the basilica of Santa Maria del Mar and the church of Santa Maria del Pi (Cassinello, 2005), (Vendrell, 2008), studied by Universitat Politècnica de Catalunya, (Figure 4.28).
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Byzantine churches are frequently octagonal with a main dome supported by four main arches (one to four in Figure 4.29). In the four corners, four oblique arches (5 to 8) are arranged (allowing for the formation of four quinches). Thus, the support of the central dome becomes octagonal. In addition to the central dome, there is a system of domes and arches, supported by vertical piers. Damage observations and analytical work have proven that this structural system is very vulnerable. Actually, even under vertical loads alone, there are tensile stresses developing in the region of supports of the curved bearing elements. The problem is typically more pronounced along the North-South axis, mainly because the total area of the piers along this axis is significantly smaller than along the East-West axis. This is the reason why the pathological image of those churches is typical. The methodology could be probably applied to other type of buildings with similar architectonic characteristics.
Figure 4.28 - Mechanisms of the churches of Santa Maria del Mar and Santa Maria del Pi studied by Universitat Politècnica de Catalunya.
Figure 4.29 - Plan of the main church (Katholikon) of Dafni Monastery.
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4.6 SLENDER BUILDINGS (TOWERS, MINARETS) Although characterized by different stylistic decorations, age of construction and original function, their comparable geometrical and structural ratios yield to the definition of an autonomous structural type. In a very concise definition, they can be described as monuments where the total height is the prevalent dimension. After the extensive survey of the damaged religious buildings, including bell-towers, carried out after the 1979 earthquake in Friuli and reported in (Doglioni, 1984), (Figure 2.31), several databases obtained by direct surveys outline some major features of Italian towers and sub-types, (Sepe, 2008). In the case of towers, several observations could be done, e.g. about the distribution of the slenderness, taking into account minarets, towers, bell-towers, with circular base or squared, adjacent to other buildings. Beyond this, a third class of likewise towers can be identified, which is the so-called tower house (in Italian called ‘Case Torri’), typical defensive house, shaped as a bulk tower. Besides the general characteristic of a building (architectonic, geometric, structural), the survey records the post earthquake damages and/or the presence of structural damage of different nature. In the case of towers, it is very common the interaction with other buildings (e.g. castles, churches, etc.) and the several transformation and changes in use (e.g. the including of water tanks). Another important point in the assessment of the seismic vulnerability and in general in safety evaluation is the urban position (Figure 4.30, Figure 4.31). Similar to buildings, the position of the tower with respect to the urban context is very important, as it can strongly influence the possible damage mode under a seismic excitation.
Figure 4.30 - Position of the tower in the urban context, (Sepe, 2008).
Figure 4.31 - Mantua: the façade of St. Andrew is connected to the bell-tower.
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Furthermore the presence of non structural elements, as turrets, bells and the presence of possible decorations or furniture increasing the value need also to be surveyed and finally to quote their level of conservation. The load bearing walls of towers have generally massive thickness. Intermediate floors are usually present, dividing the total height in storeys but frequently, being timber floors, can be in bad conditions due to lack of maintenance. The staircase is in frequently in timber, or running within the wall thickness. In defensive towers roofs could be flat and accessible, while in bell-tower are generally thrusting wooden structure and host the bell-fry on top. Frequently very small openings are present on the perimeter walls. These buildings are featured by notable slenderness, and this also represents one of the main differences from most of historic monuments (churches, palaces) or even ordinary buildings. Their geometric, constructive and structural features are so peculiar that specific vulnerability/risk functions require to be formulated, (Sepe, 2008). Frequent mechanisms in buildings, palaces, or churches such as the overturning of the façade are not present because associated with lack of efficient connection at corners rather improbable for towers. From mechanisms common to buildings, as reported in (Sepe, 2008), it is possible to outline the overturning of a wall with lateral side walls and corner failure, the horizontal arch effect and shear failure. Figure 4.32 summarizes the main structural damages of towers. The former is an overturning with a vertical crack in the middle of the body, with development of frictional stresses along it. This damage mode is particularly feasible when a vertical array of openings is present on the wall surface enabling the onset of a vertical crack, so as to reduce the frictional strength. Other mechanisms include failures caused by stresses very near to the compressive strength at the base, and this can occur particularly in the presence of out of plumbs or long term compressive behaviour and might be independent from seismic effects. Tensional effect can be activated only in the presence of asymmetries in the geometrical and/or mechanical characteristics or due to the presence of adjacent buildings of different height. Some damages could be partially independent of the seismic effect and is associated with geotechnical/ geological problems but that could be active during the earthquake. Other failure modes concern the collapse of either additional parts or turrets or decoration standing elements or to the belfry, as previously highlighted by Lagomarsino’s survey form for churches (Lagomarsino, 1997) and by Curti (Curti, 2007).
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Figure 4.32 - Crack pattern of towers and interpretation of the damage mechanism, (Sepe, 2009).
The mosque minarets of Agadir were subjected to strong shaking of Al Hoceima Earthquake in 2004. A simple model for non-structural damage to the mosque lantern is shown in (Figure 4.33) All mosque minarets are built to similar geometric specifications; a RC frame with no diaphragm action save for the spiral stairs. Lantern is a non-structural element on the roof. In some of these mosques, the lantern collapse, the shear damage to the tower base and the plastic hinging at base of tower have been observed.
Figure 4.33 - Failure model: lantern rotation and collapse and plastic hinging, shear damages at the base of the tower.
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5 COLLAPSE MECHANISMS OF REPAIRED BUILDINGS
In general, the interventions should be aimed at improving the structural connections, and reducing horizontal diaphragm deformability, at increasing masonry strength; furthermore they should improve the behaviour of vaults, arches, pillars, etc. Nevertheless, in numerous cases very invasive structural modifications have been applied, probably as a result of the assumption that they should provide a higher safety level, but without any definite proof of their effectiveness. There are many cases in which roof or floor reconstruction - during which original timber trusses or floors were replaced by new elements of reinforced concrete or steel - were probably the main cause of damage. Substitution of wooden roofs with reinforced concrete slabs is a very common rehabilitation technique in most recent intervention. In most cases it is difficult to categorise the intervention in good and bad. Their seismic performance is, in fact, referred to the weak compatibility with the old structure or the poor application/workmanship. As currently reported in literature, the repair techniques are distinguished as traditional and modern repair techniques, discriminating the use of techniques extensively used in the past and sharing the same expertise of existing buildings. Modern techniques were carried out after 1945, but introducing techniques and materials, referred mainly to r.c., previously not used in the building rules. The use of r.c. structures to retrofit historic masonry buildings was strongly supported by the changes in the building process in Europe, excluding masonry and turning to the extensive use of r.c. structures and cement mortars. Furthermore, the calibration of numerical procedures and research developed national codes mainly addresses to r.c. which increased the neglecting of masonry building tradition but also of the repair materials.
5.1 TRADITIONAL REPAIR TECHNIQUES One of the first aspects to be taken into account when dealing with the seismic behaviour of existing masonry buildings is the lack of good connections between structural elements. Hence, to allow the structure to manifest a satisfactory global behaviour, it is necessary to improve the connections between masonry walls, and between walls and floors and walls and roofs (Tomazevic, 1994), (Tomazevic, 1996a, b) This goal was in the past achieved by inserting ties or confining timber rings at the top of the building (Figure 3.156, Figure 5.1 and Figure 5.2) or at the floor level. An effective connection between floors and walls is useful since it allows a better load redistribution and applies a restraining action towards the walls overturning. In the case of timber floors, a satisfactory connection was provided by fasteners anchored on the external face of the wall. As mentioned above, the bracing is one of the most significant aspects of the earthquake-resistant design. A basic principle is the monolithic behaviour, according to which the various parts of the structure must be suitably connected between themselves to avoid the dissociation of their elements under earthquake loads. This technique results essentially by the bracing arch which binds the two different masonry structures to the arcades and which become bind together. They are consolidated by a wooden beam system which connects the frontages of the galleries to those of the load-bearing walls to ensure the stability of the walls with those of interior and avoiding their opening and collapse. Their distribution is rather regular and is particularly observed in the traditional courtyard houses in Morocco, (Figure 5.3). The use of this linking system shows again the manner of the earthquake loads resistance. Nevertheless, an irregular layout or an insufficient disposal, accompanied to other techniques e.g. floor/roof substitution could be not enough to prevent wall overturning, as in Figure 5.4: the tie rod could not prevent the façade partial overturning due to the r.c. tie beam pushing. Similar structural layout explains the damage in Figure 5.5.
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Figure 5.1 - Ties disposal, (Giovannetti, 1998).
(a) (b) (c) (d) Figure 5.2 - Timber ties (a) from (Giuffrè, 1999), (b) corner reinforcement (c) arch tie in Umbria (Italy), (d) arch tie House of Gamal El-Din El-Dahbi in Cairo.
Figure 5.3 - Use of bracing and wooden tie in Rabat and Meknes medinas.
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(a) (b) Figure 5.4 - (a) The tie was not sufficient to prevent the façade overturning caused by the pushing of the r.c. tie beam, (Binda, 2003c); (b) the insufficient tie dimension locally punches the masonry wall, (Doglioni, 2007).
Figure 5.5 - The (short) ties between longitudinal and transverse wall, arranged at the level of the floor, was (expectedly) not able to prevent the out-of-plane collapse. This closer photo allows us to observe that (reinforced?) concrete slabs were constructed at floor levels. The distance between the end of the roofing and the protruding part of the slab indicates that the slab was supported by the collapsed wall. The effect (perhaps negative) of the slabs cannot be assessed just by visual inspection, (Andravida earthquake, 2008).
Building repaired in the past are characteristically heavy and robust at the ground level, with added buttresses and added iron or steel rods that run through the buildings to plates on the exterior, (Figure 5.7). Over the centuries, such strengthening was probably done to stabilize and strengthen masonry as the buildings were continuously repaired and adapted to new uses. When a frontage wall or a great wall presents a cant, the response of the Community to this problem is almost always the same one: the use of a buttress, a mass of masonry built against a wall to strengthen it. This system is a consolidating element to the existing structure and it is generally added to an older masonry building. Sometimes, the buttress is constructed at the same time as the building construction, a voluntary and premeditated act to reinforce this construction, generally at the corners of the structure. In the areas subjected to the seismic risk, the buttress frequently accompanies the stone frame and becomes an essential element to achieve the building stability. Sometimes, the buttresses were used both as utilitarian and decorative forms. They can be also used as staircase ensuring the access to the dwelling, built to play the role of the confortement, a judicious way to associate reinforcement technique with comfort, (Figure 5.6). The unreinforced masonry structures have very low stress level and their stability, not strength, governs the safety, but the geometry changes may threaten stability of the structure. For high
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vaulted buildings, the arch will collapse and the buttress will remain standing in most cases. A masonry buttress will fracture at collapse, reducing its load capacity.
Figure 5.6 - Meknes medina rampart with buttresses and se of buttress in Chechaouen medina.
(a) (b) (c) Figure 5.7 - (a) Tie rods and buttresses disposal (Giovannetti, 1998) (b) buttress damage by the wall bending, (Doglioni, 2007).
Intervention on the materials in the past was aimed to improve the original mechanical properties of materials or to improve their performance by local intervention such as local repointing or local reconstruction. It is important to stress that intervention event by traditional technique should make use of materials with mechanical and chemical-physical properties similar to the original ones (Binda, 2006b), (Valluzzi, 2008a). The local dismantling and rebuilding (“scuci-cuci”) methodology aims to restore the wall continuity along cracking lines (substitution of damaged elements with new ones, reestablishment of the structural continuity) and to recover heavily damaged parts of masonry walls. The use of materials that are similar, in terms of shape, dimensions, stiffness and strength, to those employed in the original wall is preferable. Adequate connections should be provided to obtain a monolithic behaviour. The effectiveness of the intervention is strictly connected to the recovering of the previous wall properties; otherwise the seismic actions could expel the intervention as in Figure 5.8. A discharging arch or relieving arch is an arch over a door, window, or other opening, designed to distribute the pressure of the wall above Figure 5.9. An opening in a frontage constitutes a vulnerable point in the event of deformation of the frame. The cracks of the front walls are found in the contours of the openings where the stresses are most significant and in particular close the restrained angles. The earthquake-resistant design codes recommend, for masonry structures, rigid reinforced concrete, steel or wood framings of the openings and, in principle, must be connected to the links of the walls. The wooden framings must be effectively connected to the masonry. The openings in the medina, doors and windows, are framed by wood and well connected to masonry. Cut stone arches are also located at the top of these openings.
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Figure 5.8 - Local repairs re-damaged by the earthquake, (Binda, 2003c).
Figure 5.9 - Discharging arch in Sale medina and Rabat extramural.
Interventions aiming at enhancing the in-plane stiffness of existing floors must be carefully evaluated, since it changes the redistribution of horizontal seismic action to the load-bearing walls, and this is seldom the objective of structural interventions. The role of diaphragms in the dynamic behaviour of masonry buildings consists in transferring seismic actions to the walls parallel to the earthquake direction, (Tomazevic, 1991); therefore, an effective connection between floors and walls has a large importance as this can limit undesirable overturning of walls. Providing a further layer of wooden planks is a limited intervention, that does not modifies the overall behaviour and the force redistribution, and increases the wooden floors stiffening, (Parisi 2002). The historic centres mainly of middle age origin in Europe as well as the medina urban structure were very affected by the constraints of the site and were organized in an irregular network of narrow streets rarely rectilinear, which surround all sides the blocks, separate them from each other allowing them a dynamic behaviour during the earthquakes. These separating spaces play the role of an empty joint of separation. This urban morphology and building construction with narrow streets are probably a solution to reduce seismic damage and prevent the houses from collapsing. The houses are semi-detached, overlapping and leaning against each other forming a compact unit. Some streets are covered by galleries on top of which the houses extend and thus creating roofed passageways called ‘the sabats’ in Marocco ma diffused in Italy, as well. This extension can also be done in height thus from the starting of the sabats, which are extensions of the houses on the top of the public space, which cover it and form passageway, (Abdessemed-Foufa, 2005). Those are elements of cuts in the linear continuity of the frontages, realized either in vaults built out
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of stones or bricks, or flat with wooden logs incorporated. Those are elements of reinforcement which play a determining role in the bracing of the blocks between themselves. In addition, the medina urban framework is characterised by a certain number of arches built out of stone or bricks, called “confortement” arches or contrasting arches whose relative flexibility and elasticity allow the transmission of the horizontal stresses and their transfer to the ground, (Figure 5.10). The buildings are not considered any more as isolated elements but as a compact dynamic block. This bracing is always present in the narrow streets of the medina and allowed the constructions not to collapse.
(a) (b)
(c) Figure 5.10 - (a) Contrasting arches in the Fez medina. (b) Contrasting arches in the Azzemour medina. (c) Roofed passageways or ‘Sabats’ in Chechaouen and Rabat medinas.
5.1.1 Earth constructions 5.1.1.1 Traditional means of repairing earthquake damage to earth construction Crack repair
Technique Nature of technique Material Location/Source Demolish adobe and Traditional, currently used Adobe Navarro Grau rebuild 2006 Soft stiching Traditional, currently used Cob and Rammed Hurd 2006 but only reported earth
Demolish and rebuild: (Navarro Grau, 2006) describes the repair of a severely cracked corner by means of demolition and new build, and justifies it by explaining that the current state of the art of injection techniques does not guarantee structural integration of walls, which is necessary to reinstate a monolithic behaviour. (Navarro Grau, 2006) also claim that the use of grout injections made with stiff material, such as cement mortars, is not recommended in earth construction
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because it creates stiffness discontinuities in the masonry which would result in stress concentration and thus new cracking in case of seismic loading. Soft stitching: Soft stitching is a repair procedure carried out in (Hurd, 2006) which makes use of various materials such as flat adobe bats, thick tiles, hemp, fibre mats or stainless steel expanded metal lath introduced into a groove cut across the crack needing repair. The groove is cut to about half the wall thickness, over the crack, with deep returning ends in the form of a staple and continually wetted down with water during the construction process to eliminate suction. The chase is then filled with alternate layers of fibres and adobe blocks until the top course, which is 10-15 cm deep, is reached. This is then wetted down and dry packed with loose material identical to that of the adobe blocks. So as to form a dense rammed fill. This procedure is carried out internally and externally. (Hurd, 2006) recommends the use of stitches of varying length to allow for stitching of subsidiary cracks and to prevent the formation of the new cleavage planes that may develop from stitches of same length. Soft stitching is not known by the author nor by (Hurd, 2006) to test or examine the engineering performance of soft stitching under dynamic loading. (Hurd, 2006), (Figure 5.11), however, claims to have observed the use of still - functional stitches, i.e. after an earthquake of unknown magnitude.
Figure 5.11 - Soft stitching carried out by Hurd in 2004. Photo: (Jaquin, 2008).
5.1.1.2 Means to reduce earthquake damage
5.1.1.2.1 Vernacular means
Additional elements Bond beam To prevent: Lack of connection between perpendicular walls External horizontal wall ties, Nepal (Chaudhry 2006) Vertical wall ties, Nepal (Chaudhry 2006) Buttresses (ZRS Photographic Database, Oman) Males, or horizontal mattresses or layers in rammed earth or cob
5.1.1.2.2 Design considerations Thickness
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It has been claimed in the literature that in historical buildings, where height/thickness ratio is often < 5 (Tolles, 2002), wall thickness is often sufficient to prevent overturning as the walls are difficult to destabilize even when they are severely cracked. 1. Roof Support provided at the top walls by a roof system may add additional stability to the walls. This point is contended by researchers. While a flexible and rigid roof is often claimed to be an appropriate solution, the presence of additional loading on the walls might provide additional stability (Webster, 1995). Openings
5.2 MODERN REPAIR TECHNIQUES The post-earthquake surveys after the recent earthquake have shown poor performance of heavily reinforced buildings that often made them more vulnerable. It appears that a large number of the damaged buildings had been modified over the years to meet the needs of their users also following in Italy the seismic code, but that such measures had rarely improved their resistance against seismic events. As a matter of fact, these functional modifications, (remodelling, raising, enlargement, opening or closing of new windows or doors) frequently added vulnerability. A common form of upgrade has been the replacement of the old wooden floors with new floors of heavy reinforced concrete beams supporting hollow clay tiles. This was often done without upgrading the strength of the masonry bearing walls. Sometimes bearing walls were even removed to open up spaces, resulting in beams located where shear walls had once existed. The post- earthquake survey found that alterations and remodelling was rarely done with any heed of regulations or seismic design criteria. The compatibility or incompatibility of materials and systems is a good indicator of seismic vulnerability. Recent earthquakes were particularly ruthless on masonry buildings that had been retrofitted relatively recently with reinforced concrete floors (usually with recast beams with large hollow clay tile elements spanning between the beams). One might have thought the stronger and stiffer diaphragms would have improved performance, but these floors were not properly tied through the masonry walls, and thus did not contribute to holding them together. A significant problem with this kind of construction is that the absence of wall ties is not easy to verify, while the presence of the new floors may give one the false sense of security that the structure has been improved. In the highly seismic Umbria-Marche region, seismic retrofitting may have been the principal intention in replacing roofs. The added masses and the stiffness of the new roof cracked the supporting walls and frequently causing their total collapse; or damaged the external leaves of multiple leaves masonry walls, (Figure 5.12). The increase in the weight and the stiffness of the roof, whose retrofit usually involves a reinforced slab insertion over the wooden ledger board, can lead to an increase in the horizontal seismic forces that induce collapse of the masonry walls; moreover, the stiffness of the top most tie-beam can obstruct the natural vibration mode of the masonry, thus inducing local high stresses in the masonry. Concrete ties, floors and roofs were frequently adopted to substitute timber floors and roofs. The tie is positioned along the four sides of the structure as a connection floor to walls. In an existing building while the roof concrete tie can be positioned on the whole thickness of the top wall, the r.c. ties at each floor can only be inserted in part of the section after partial demolition of it. In this case it is very difficult to realise a stiff connection to the existing wall. In general this connection is very difficult when the wall is made of a multiple leaf irregular stone masonry. Figure 5.13 and Figure 5.14 show the effects of the tie beam insertion, often hammering or building rotation but also wall collapses. Conversely, introduction of tie-beams in the masonry thickness at intermediate storeys should be definitely be avoided, due to their damaging effects on perimeter walls, often causing also uneven
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load redistribution among masonry leaves and/or pounding effects on the external masonry leaves in case of seismic excitation, Figure 5.15.
(a) (b) Figure 5.12 - Structural failure due to the insertion of a stiff r.c. roof (a) (Avorio, 2002b), (b) (Binda, 2006c).
The damage observed more frequently were the following: (i) partial eccentric loading of the walls, (Borri, 2004a), (ii) lack or poor connection of the tie beam to the walls (Binda, 2003c), (Binda, 2006a, b). The seismic events, then, showed that these elements cannot transmit the horizontal actions to the walls and neither can connect the two masonry leaves, of which one remains free and can rotate freely and overturn, (Figure 5.16), (Binda, 2003c, 2006b), (Borri, 2004a). The collapse mechanism of the masonry is not for in plane shear as expected after the floor substitution, but a partial overturning mechanism of the external leaf of the wall which starts for lower values of the expected collapse coefficient. In most case application of modern technique does not prevent overturning or other kinematic mechanism but acts only on the shape of the damaged area, (Figure 5.16). Unsuccessful of the vault straightening by adding a r.c. structure (Gurrieri, 1999), (Avorio, 2002a) was surveyed as well, (Figure 5.17). In recent years, experimental research focused on the behaviour of masonry vaults strengthened by new composite materials, as carbon or glass FRPs, placed at the intrados (inner surface) or at the extrados (outer surface) of the structure, (Valluzzi, 2001b), (Panizza, 2008). A multilayer system of adhesion based on epoxy adhesives and designed to provide a support as homogeneous as possible for the fibers has been adopted. Nevertheless, further research is needed. An abacus of damages related to these diffused building retrofitting technique is proposed in Figure 5.18. Furthermore several techniques applied to repair or to strengthen the masonry improving the masonry quality or compactness showed their ineffectiveness, as jacketing and grouting, as well, (Figure 5.19), (Binda, 2003c), (Binda, 2006c).
Figure 5.13 - Damages due to the introduction of r.c. tie-beams surveyed after the Umbria earthquake, (Binda, 2002c).
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Figure 5.14 - A r.c. tie beam is constructed under the opening. Disintegration of the pier and horizontal sliding is observed.
(a) (b) (c) Figure 5.15 - Damages due to the introduction of r.c. tie-beams in the masonry thickness at intermediate storeys (a), (b) from (Gurrieri, 1999) and (Avorio, 2002b); (c) (Binda, 2003c).
(a) (b) Figure 5.16 - The presence of r.c. tie beams does not prevent the wall overturning but changes the shape of the damaged area, (Borri, 2004a).
Figure 5.17 - Unsuccessful of the vault straightening by adding a r.c. structure, (Gurrieri, 1999), (Avorio, 2002a).
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Figure 5.18 - Modern techniques often change the expected seismic behaviour, (Binda, 2006a).
(a) (b) (c) (d) (e) Figure 5.19 - (a) Failure due to insufficient steel mesh overlapping and (b) insufficient transversal ties confining action; (c) lack of connection between nets; (d) absence of connectors, (e) unsuccessful of grouts due to the lack of diffusion: only some spots were injected in the case of this wall with a very low percentage of voids.
5.2.1 Earthen buildings 5.2.1.1 Crack Repair Hard, i.e. stainless steel stitching Cob (Hurd, 2006), (Schröder, 2010) Rammed earth Grout injection Adobe (Chaudhry, 2007), (Webster, 2006)
5.2.1.2 Strength-based retrofit Strength-based retrofit is based on introducing and independent structural frame of reinforced concrete or steel, requiring the removal of large amounts of historic material, (Hardy, 2006). The use of reinforced concrete is a practice which in the past was and is sometimes still applied to earthen constructions; according to the same engineering strength-design principles which lead do the use of reinforced concrete for the restoration of other masonry structures. For instance, prior to the publication of the CHBC California Historical Building Code (CHBC), made mandatory in 1985, seismic retrofitting for adobe buildings in California was based on the UBC, which does not recognise any seismic load resistance for adobe. Therefore, retrofitting consisted in adding shear walls and constructing independent steel or concrete structures designed to carry all loads from horizontal members. This resulted in the destruction of historic fabric and removal of stabilising loads at the tops of adobe walls.
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The validity of applying strength design concepts to earthen structures has not been proven in practice to be effective. On site observation of seismic damage to earthen structures, (Langenbach, 2004; Cancino, 2009; Navarro Grau, 2006), indicate, on the contrary, that the presence of reinforced concrete elements has lead to the damage, and not the protection, of earthen structures. Strength design is also at the basis of the use of steel reinforcement bars as a means of reinforcing earthen walls. Since earthen walls do not bond with conventional rebar and therefore, the yielding stress conditions on which strength design is based cannot be met, the use of steel as internal reinforcement has also been opposed, (Webster, 1995). Reinforcement (steel) can bond to the adobe if embedded in cement mortar or grout, (Chaudhry, 2007), but might results in stress concentration and thus new cracking in case of seismic loading. It is claimed, (Barrow, 2006), that thick adobe walls do have energy dissipation characteristics in the post-elastic phase. As well described by Langenbach (2004), if both the restored houses at Arg-e-Bam and the new houses suffered more than the untouched ancient abandoned earthen ruins in the desert nearby, as also reported by JSEE (2004), then the problem had less to do with earthen construction per-se than it had to do with the particular form of earthen construction that was practiced in modern Bam. 5.2.1.3 Stability-based retrofit Stability-based retrofitting, (Webster, 2006), which has been introduced more recently, is less invasive, and consists in limiting relative displacement between elements of a structure and using gravity as a restoring force. Since the overturning of walls is the first mode of failure common to adobe buildings, (Tolles, 2006), the first step towards stability under earthquake loading is to attach structure´s vertical elements to its horizontal elements. For instance, for gable walls or any walls susceptible to overturning (i.e. of thinner walls between support points), full-height centre core rods can be introduced to provide restraint, prevent out-of- plane failure, and increase wall ductility in both in-plane and out-of-plane directions, (Tolles, 2006) Stability-based measures do not stiffen the structure and do not come into play until old cracks reopen, new cracks have developed, and enough displacement occurs to engage the stabilising measures, which work in two ways, (Webster, 2006): They increase structural damping due to friction hysteresis across the cracks; They lower response frequency caused by the rocking of walls. Webster (2006) provides examples of historical buildings in California where stability-based measures were used between 1992 and 2005. Stability based methods, after Webster (2006), with some additions, are considered at three different levels: Structural continuity at floor/roof; Out-of-plane overturning stability; Containment of wall material.
Structural continuity at floor/roof level Existing (or introducing a) timber bond beam interconnecting all walls; Top-of-wall continuity (stainless steel straps, cables), through-wall tied; Connecting discontinuous existing bond beam elements, incl. previously reinforced beams introduced; Addition to a): introduction of wooden pegs as shear connectors. References (Navarro Grau, 2006);
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(Sikka and Chaudhy, 2006). Nako preservation project; (Hurd, 2006); (Webster, 2006), Shafter Courthouse, USA.
Out-of-plane overturning stability (Anchoring together roof, walls, and floors)
Top-of-wall pins (steel or fibreglass) (grouted in place with a fly-ash/soil mixture (Roselund, 1990); Whereas (Tolles, 2006) mentions epoxy grout being used with stainless steel tie rods; Vertical center core rods (steel or fibreglass). Design of the fibre-rods based on 0.8* weight of the gable-wall section above bond beam (Webster, 2006). Advantages of earthen grouts (Barrow et al., 2006) are compatibility with historic adobe and reversibility (Barrow, 2006): the consist of adobe soil, sand, a small amount of Portland cement, ground additive (Sika ground aid) to minimise shrinking during curing; Diaphragm (partial or full, i.e. plywood); Top-of-wall anchorage; Through-wall floor anchorage (in the case of Tolle 2006, with viscous dampers). References (Tolles, 2006), testing model.
Containment of wall material Horizontal and/or vertical straps or cables, through-wall tied; Horizontal and/or vertical center core rods; Surface mesh, through-wall tied (polymer with mud plaster or welded wire mesh); Top-of-wall continuity hardware, through-wall tied, in conjunction with top of wall pins. References Torrealva et al. (Torrealva, 2006) tested Polymer mesh geogrid completely covering the walls on both sides. The mesh was connected with plastic threads through a whole previously drilled in the wall. Spacing every 40.0 cm (Torrealva, 2006). Covering the wall with mud plaster greatly increases the initial shear strength and stiffness of wall, controlling the lateral displacements and presenting the cracking of the wall to great extent. The cost of the mesh technique can be reduced by placing mesh only in part of the wall. The amount of mesh is more important than the quality of the mesh (Torrealva, 2006) specifies that in order to maintain the integrity of the adobe walls, both horizontal and vertical reinforcement are necessary. Use of organic textile fibres (jute or coir) as natural, organic instead of polymer fibres - under plaster (suggested by Sikka and Chaudry, 2006) - yet to be tested.
Seismic retrofit is believed to have an effect only once ground shaking > 0.3 g (Webster, 2006). In- plane diagonal and X-Cracking at corners and doorways result from PGA levels as low as 0.1 to 0.2g. The relationship between damage and earthquake severity in relation to strength-based, stability-based and lack of retrofit as derived is presented in (Tolles, 2002). (Ginell, 1995) assesses the effectiveness of some of these retrofits in terms of earthquake severity as shown in Figure 5.20.
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Figure 5.20 - Effectiveness of Stability-Based Retrofitting.
Damage related to the use of the aforementioned retrofitting techniques includes cracking damage propagating from structural anchorage and cross-ties. Due to the weakness of earth as a structural material, low stress concentrations at these locations, which can hardly be avoided, generally lead to cracks and crushing of material. Anchorage can pull into wall thus being ineffective in adequately restraining out-of-plane motion or initiated cracks. Wall anchors (or tie rods) retrofitted with the intention of holding walls together with perpendicular walls or diaphragms are claimed in (Tolles, 1996) to be difficult to attach to adobe successfully because of the material´s weakness in shear and tension. In order to make the use of anchorage effectively, it is important to understand the behaviour of earth around the anchors. 5.2.1.3.1 Additional repair techniques Webster (2006) describes the seismic strengthening of a wall at O´Hara Adobe which was initially thought to be 90.0 cm thick, but was later found to consist of two separate leaves, each 30.0 cm thick. The wall such was filled with urethane-type foam of 48.0 kg/m³ between walls and super- lightweight (1121.0 kg/m³) concrete under and beside the already existing bond beam, (Webster, 2006). Alternative measures should be researched and tested.
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