Seismic Behaviour of the Historical Structural System of the Island of Lefkada, Greece

Structural Studies, Repairs and Maintenance of Heritage Architecture IX 291

Seismic behaviour of the historical structural system of the island of Lefkada, Greece

E. Vintzileou1 & P. Touliatos2 1Faculty of Civil Engineering, National Technical University of Athens, Greece 2Faculty of Architectural Engineering, National Technical University of Athens, Greece

Abstract

In this paper, the main characteristics of the historical structural system of the island of Lefkada, Greece, are presented. The typical damage caused by a recent strong earthquake (August 2003) are briefly described, since they allow for the seismic behaviour of the system to be studied. The pathology of the structural system seems to confirm that the structural system was conceived to sustain earthquakes. Keywords: earthquake, stone masonry, timber framed masonry, damages.

1 Introduction

The island of Lefkada, one of the Ionian Islands, is situated in one of the most earthquake prone regions of Greece. A local structural system was developed before the 19th century in Lefkada. The strong earthquake 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 the local structural system. The 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. In addition, minimum distance is required between adjacent buildings, to allow for better protection against fire and earthquakes. A considerable number of buildings, built according to this structural system are still in use in the city of Lefkada.

In August 14th, 2003, a strong earthquake (Mw 6,2) occurred in Lefkada. A thorough investigation of damages observed in structures built according to the historical structural system, together with a detailed survey of the typical characteristics of the system allowed for the identification of the main features of its seismic behaviour (Vintzileou et al. [1]). The results of this investigation are briefly presented and discussed in this paper. 2 Description of the structural system

2.1 Generalities The historical part of the city of Lefkada, situated by the sea, is developed both sides of a central street that separates the city into two parts of approximately equal areas. Lateral narrow streets start from the central street and are directed to the sea (N-S or S-N direction). This arrangement allows for drainage of rainwater towards the sea. In addition, the-most frequent-north, northwest winds offer favourable conditions for reduction of humidity in the timber parts of the houses. Typical buildings (one- to maximum three-storey buildings, fig. 1) consist of a stone masonry ground floor plus one or two timber framed brick masonry storeys. Intermediate floors and roof are made of timber. Openings (typically rectangular, approx. 1m wide) are symmetrically arranged along the facades of the buildings. Few decorative elements (mainly, in entrances, in the corners of the building, as well as around the openings) characterize the typical building. The roof is covered with tiles. To protect timber framed masonry from humidity, the upper storeys used to be covered along their perimeter by timber planks (fig. 2a). The high cost of replacement of this cover (decayed with time) led to their replacement by plane or corrugated metal sheets (fig. 2b). Covered walkways (fig. 3) constitute a typical morphological and functional element in a city with frequent rainfalls.

Figure 1: Typical building.

Figure 2: Protection of timber elements, (a) by timber planks, (b) by metal sheets.

Figure 3: Covered walkway. Figure 4: Grid of trunks (under the foundation).

2.2 Foundation

The city of Lefkada is founded on low strength alluvia. Therefore, special care was given to the foundation of buildings. Although it is not practicable to explore the footings of existing buildings, according to descriptions (based on information provided by craftsmen that worked in the construction of similar buildings), the buildings are founded on a grid of trunks (fig. 4), at a depth of 0.6 to 1.0m. The space between trunks is filled with sand, rubble stones and hydraulic mortar.

2.3 Ground floor Bearing walls in the ground floor (typically, perimeter walls) are max. 3.0m high. They are 0.6m to 1.0m thick and they are made of stone masonry. The external leaf is made with roughly cut stones, whereas cut stones are used in the corners of the building (fig. 2b), as well as along the perimeter of openings (fig. 2a). 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. Natural pozzolan was used in the buildings constructed up to the end of 19th century. Later, lime was used together with straw (to improve mechanical properties of mortar), whereas in poorer structures, clay mortar was used. In addition to masonry walls, a secondary (timber) bearing system is present in the ground floor: It consists of timber columns arranged close to masonry walls. The typical distance between consecutive timber columns is equal to 2.0÷3.0m. The function of this secondary system is discussed in §3. 2.4 Upper storey timber framed masonry

In fig. 5, the typical arrangement of timber elements in the timber-framed walls is shown. The timber frame of the perimeter walls is connected to the ground floor masonry, through timber beams arranged along the perimeter of stone masonry walls. Metal ties (of various geometry and size, fig. 6) are used to connect the timber elements of the floor with the stone masonry and/or with the timber frame of the upper storeys.

Figure 5: Typical timber-framed Figure 6: Metal ties wall. connecting the timber floor with ground floor masonry. 2.5 Floors and roof

Fig. 7 shows the typical construction of intermediate floors and roof. It should be reminded here that intermediate floors do not carry only their own (dead and

WIT Transactions on The Built Environment, Vol 83, © 2005 WIT Press www.witpress.com, ISSN 1743-3509 (on-line) Structural Studies, Repairs and Maintenance of Heritage Architecture IX 295 live) loads. They also transfer the loads of the upper storey(s) perimeter masonry to that of ground floor, as well as to the secondary timber bearing system of the ground floor.

2.6 Connection between timber elements

As shown in fig. 8, 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. Taking into account that Lefkada has always been an island with limited resources, one may assume that the purpose of the constructors was to enhance the overall stiffness of buildings.

Figure 7: Typical construction of (a) intermediate floors and (b) roofs.

Figure 8: Typical connections between timber elements.

3 Seismic behaviour of the structural system

On the basis of the description presented in the previous section, a working hypothesis can be adopted regarding the conceptual seismic design of the historical structural system of Lefkada. It should be mentioned that this working hypothesis was confirmed by the typical damages observed after the 2003 earthquake (see Section 4), as well as by parameter analyses carried out within a research project (Vintzileou et al. [1]), presented also in Vintzileou et al. [2]: (a) Stone masonry walls in the ground floor are thick enough (given their limited height), well interconnected in the corners of the buildings, and pierced by a limited number of openings. Thus, they are conceived to provide the stiffness that is necessary for the building to be subjected to rather small deformations during seismic events. This favourable behaviour is enhanced by the floor, consisting of closely spaced timber beams and timber roofing, carefully connected among them, as well as with stone masonry. Thus, the box action of the ground floor is ensured. (b) The upper storey(s) are made of thin (typically, 10-12cm) timber framed masonry. In this way, a-favourable for the seismic behaviour-reduction of the total weight of the building is achieved. On the other hand, the stiffness of timber framed walls is ensured thanks to the closely spaced horizontal, vertical and diagonal timber elements, as well as by their connections (fig. 8). Furthermore, the limited dimensions of brick masonry panels (fig. 5) well confined by the timber elements, ensure their satisfactory in-plane and out-of- plane behaviour. In addition, the negative effect of failure (or even, collapse) of a number of brick masonry panels is substantially reduced. Timber framed walls are connected with the ground floor masonry through the timber floor, whereas the box action of the upper floor is completed by the rather stiff roof system (fig. 7b). It should be mentioned that the diaphragm action of both floors and roof was also proven analytically in Vintzileou et al. [2]. (c) As in all structural systems conceived to resist earthquakes, the system of Lefkada is also characterized by a symmetric layout of openings. In addition, openings are repeated in all storeys in the same locations, thus allowing for a smooth transfer of loads down to foundation. (d) Taking into account paragraphs (a), (b) and (c) above, the secondary timber system may seem to be superfluous. In fact, it is so flexible (compared to the stiff main system) that it cannot contribute to the seismic response of the building. Nevertheless, (i) its presence in the totality of buildings of the historical part of Lefkada, (ii) its typically careful connection with the timber floor, as well as (iii) the fact that (as shown in fig. 9) the timber framed walls are supported by the ground floor masonry, as well as by the secondary bearing system suggest that this secondary system was conceived to play a structural role within the system. The hypothesis made by the second author [3] regarding the function of the secondary system is the following: It is well known that during a seismic event, the interstorey drift is larger in the ground floor. Therefore, the brittle by nature stone masonry may be cracked or even partially collapsed. In such a case, the

WIT Transactions on The Built Environment, Vol 83, © 2005 WIT Press www.witpress.com, ISSN 1743-3509 (on-line) Structural Studies, Repairs and Maintenance of Heritage Architecture IX 297 secondary system (unable to resist lateral actions) is able to safely carry the vertical loads (mainly, the self weight of the building). Thus, after the earthquake, stone masonry in the ground floor can be reconstructed or repaired and the building will then be able to sustain the next earthquake to occur. This working hypothesis was fully confirmed by the observed pathology of the buildings, as well as by the analytical work presented elsewhere (Vintzileou et al. [2].

Figure 9: The role of the secondary timber system (Touliatos et al. [3]).

4 The pathology of the structural system

After the August 2003 earthquake, detailed survey of damages observed in the buildings of the city of Lefkada was carried out. The main damages are categorized in what follows. It should be noted, however, that in numerous cases, the observed damages are not due only to the recent earthquake. In fact, one could distinguish damages obviously occurred during previous earthquakes that were never repaired and that have expectedly deteriorated. Finally, given the magnitude of the earthquake, the fact that a large number of buildings are not inhabited (and, hence, not maintained) for several decades and taking into account that no design has preceded various modifications made to the initial bearing system of some houses, the observed damages are of a degree that allows the vast majority of the buildings to be preserved. Stone masonry in the ground floor: Typical damages were observed, namely cracks of varying width (ranging from hair cracks to several centimeters wide). The cracks were either inclined (shear cracks) or almost vertical close to the corners of buildings (due to out-of-plane bending). Timber elements in the top of ground floor masonry: In several cases, extensive decay of the timber elements was observed (fig. 10). Obviously, this

WIT Transactions on The Built Environment, Vol 83, © 2005 WIT Press www.witpress.com, ISSN 1743-3509 (on-line) 298 Structural Studies, Repairs and Maintenance of Heritage Architecture IX damage is not due to an earthquake. However, the reduction of the sectional dimensions of the timber beam serving as support to the upper storey(s) masonry led to direct transfer of loads also to the secondary timber system of the ground floor. Thus, during the earthquake, the flexible secondary system was forced to respond. The result was, in some cases, permanent loss of verticality of timber columns (fig. 11), a damage that could not be interpreted initially, since the stone masonry in the ground floor was not damaged at all.

Figure 10: Advanced decay of timber beams transferring vertical loads from timber framed walls to stone masonry.

Figure 11: Permanent loss of verticality of 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. Timber framed masonry: Similar decay was observed in the timber elements of this system as well. In addition, cracks were observed in several cases between

WIT Transactions on The Built Environment, Vol 83, © 2005 WIT Press www.witpress.com, ISSN 1743-3509 (on-line) Structural Studies, Repairs and Maintenance of Heritage Architecture IX 299 brick masonry and surrounding timber elements (fig. 12). In a limited number of cases, out of plane collapse of the filling masonry was observed.

Figure 12: Typical damage observed in timber framed walls.

Figure 13: Permanent distortion of ground floor due to demolition of stone masonry walls.

Covered walkways: Extensive decay of the timber columns supporting the walkway was observed in several cases. As expected, decay is concentrated close to the base of timber columns. As a result, horizontal displacement of the columns at their base was observed (reaching values up to several centimeters). Excessive horizontal displacements of the building as a whole: This is a failure observed in a limited number of buildings (fig. 13) in which the stone masonry was partly demolished in the ground floor, when the use of those buildings was modified from residential to commercial. The demolition of masonry was done without previous design. Thus, the secondary bearing system

WIT Transactions on The Built Environment, Vol 83, © 2005 WIT Press www.witpress.com, ISSN 1743-3509 (on-line) 300 Structural Studies, Repairs and Maintenance of Heritage Architecture IX of the ground floor became primary. However, its stiffness being very limited, this system could not prevent large (permanent) horizontal displacements that led to distortion of the building as a whole.

5 Concluding remarks

The survey of a large number of buildings in the historical part of the city of Lefkada, as well as the thorough study of the behaviour of numerous buildings after a recent strong earthquake, have proved that the historical structural system of Lefkada was conceived to resist earthquakes. It is obvious that the buildings (some of them have survived several strong earthquakes occurred in the course of more than a hundred years) were not designed in the modern sense of the term. Nevertheless, one may observe that several practical rules-known to contribute to a satisfactory seismic behaviour-were applied by the constructors, such as: Box action (ensured by careful connections in corners and crossing of walls, as well as by floors and roof behaving as stiff diaphragms), reduction of self weight of the buildings (use of thin timber framed walls in the upper floors), laborious connections between timber elements (adding to the overall stiffness) and measures to protect timber elements from decay, since their contribution to the seismic behaviour of the buildings was obviously recognized, as well as a secondary bearing system able to carry vertical loads in case of extensive damages to the ground floor stone masonry. It is believed that the historical part of the city of Lefkada can be preserved provided that measures compatible with the structural system as conceived by its inventors are taken.

References

[1] Vintzileou, E., Touliatos, P., Zeris, Ch., Repapis, C., Adamis, Ch., Zagkotsis, A., Palieraki, V., Leonardos, E. Assessment of and Recommendations for Interventions to Buildings in the nucleus of the city of Lefkada, Research Report, National Technical University of Athens [NTUA], 2004 (in Greek). [2] Vintzileou, E., Zagkotsis, A., Repapis, C., Zeris, Ch. Seismic behaviour of the historical structural system of the island of Lefkada, Greece, accepted for publication in Construction and Building Materials. [3] Touliatos, P., Gante, D. Local historic antiseismic constructions; The example of Lefkada, NTUA, Athens 1995 (in Greek).