Application of a Multidisciplinary Investigation to Study the Vulnerability of Castelluccio (Umbria)

Structural Studies, Repairs and Maintenance of Heritage Architecture IX 311

Application of a multidisciplinary investigation to study the vulnerability of Castelluccio (Umbria)

L. Binda, G. Cardani & A. Saisi DIS – Department of Structural Engineering, Politecnico di Milano, Italy

Abstract

An investigation procedure was proposed by the authors to study the vulnerability of the diffused historic building patrimony in the seismic area. The "minimal" investigation program is to support the designers in their projects. The knowledge of existing buildings is approached by considering different analysis levels: history, materials, structural morphology of the wall section, observed damage mechanisms and effectiveness of retrofitting techniques. This paper extensively describes the investigation carried out on one of the pilot sites, Castelluccio di Norcia. The research was carried out within the frame of a project supported by the Civil Protection Department of the Italian Minister Council aimed at the vulnerability analysis of the historic centres. Keywords: masonry structures, on site and laboratory investigation, vulnerability analysis, historic centres.

1 Introduction

The 1997 earthquake gave the occasion to learn about the effectiveness of the repair and retrofitting techniques used on historic buildings. In fact most retrofitting mainly performed with upgrading interventions (substitutions of timber floors and roofs with r.c., jacketing, etc.), have caused unforeseen and serious out-of-plane effects (large collapses, local expulsions), due to the “hybrid” behaviour activated between the new and the old structures [1]. That effect was not clearly predictable by the existing assessment methods suggested by the Italian standards, which proposed at the time analytical procedures based on hypotheses often not easy to be satisfied in old historic stone masonry buildings, as the effective strong connection among the structural components

312 Structural Studies, Repairs and Maintenance of Heritage Architecture IX and the presence of stiff floors, both characterising the favourable “box” behaviour of buildings under seismic actions. It was also clear that the main cause of inappropriate choice for the intervention techniques was the lack of knowledge on the material and structural behaviour. A three year research was carried out supported by the Civil Protection Department of the Minister Council, involving the Politecnico of Milan (L. Binda), the University of Padua (C. Modena) and the Ministry of Cultural Properties (L. Marchetti) aimed to set up for historic centres a systematic Database storing information useful to prepare rescue plans and to design interventions for the preservation of the cultural heritage. Such information deals with: i) the technological and constructive characteristics of the surveyed buildings; ii) the material and structure properties (with particular reference to the constructive techniques and to materials used for load-bearing masonry); iii) the materials and the techniques used for restoration before the earthquake; iv) the collapse mechanisms of the buildings due to the earthquake, considering also the ones already retrofitted. The objective of the research was focused on four sample areas: Montesanto di Sellano, Roccanolfi di Preci, Campi Alto di Norcia and Castelluccio di Norcia, all located in the province of Perugia. They are characterised by different typologies of buildings and by different levels of damage: simply isolated buildings in Montesanto, row buildings in Campi and complex buildings in Roccanolfi and Castelluccio [2].

2 Description of the site

The little centre of Castelluccio di Norcia is situated on top of a hill and is following a helicoidal structure, 1453 metres above the sea level. The houses are arranged in concentric half-circling patterns on a slope facing south, whereas the northern side is mainly desert due to the unfavourable climatic and orographic conditions. The town-planning development of the village occurred in two areas: the first one, gathered around the Cassero (the highest part of the village, of which only the planimetric plan and the grid of old streets are still preserved) and the second part at the bottom of the hill where the old stables still remain. The first written documents on Castelluccio date back in 1276. In the 1442 in the reform acts and in the banner orders Castelluccio was called Castello de Monte Precino and belonged to the newly born town of Norcia [3]. Until about the half of the twentieth century there was only one centre, the one included within the old town walls, whereas outside and down the hill there were the stables. The topographic structure of the village is subjected to the soil orography. Thus the main streets wind around the hill dividing the village in four sloping rings. The main streets that in the upper part of the village follow the level contour lines joined through short, narrow and steep radial ramps, called “buci”; these are narrow passages whose vaults still bear traces of black old smoke (Fig.1c.).

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Figure 1a: View of Castelluccio. Figure 1b: View from Castelluccio.

Figure 1c: Narrow deep vaulted ramps.

stables cassero

Figure 2a: Plan of Castelluccio. Figure 2b: Old town wall still included in the complex civil buildings.

There are no isolated houses apart from two new buildings. The village is clustered in a compact building structure (Fig.2a) and part of the old town wall and towers are still included in the most complex buildings (Fig.2b). It is to be remembered that in the case of complex buildings or aggregates, the study of the seismic behaviour is much more difficult and in general less clear than those having a regular structure.

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3 Building typologies Representative typologies of a centre can be recognisable through similar features (number of stores, exposure, type of facade, material and structural elements, etc.). By means of an accurate geometrical survey it is also possible to recognize that buildings may have had an evolution along the time: born as an isolated building, they could have become row or complex buildings, after the addition of several volumes. The more complex the building is, the more difficult the detection of its vulnerability is; therefore, its structural evolution should be known as much as possible. The geometrical survey is not enough then, and efforts should be made to find through historic documents and also observations on site the modification it was subjected to, along the time.

Figure 3a: The building typology of Castelluccio: simple rural buildings.

PIANTA PIANO SECONDO U.M.I.5 U.M.I.6

Figure 3b: The building typology of Castelluccio: row residential buildings.

Figure 3c: The building typology of Castelluccio: complex residential buildings.

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The development of Castelluccio occurred in two distinct areas: (i) the first one, gathered around the highest part of the village, of which only the plan and the grid of old streets are still preserved, (ii) the second at the bottom of the hill, where are still standing the old stables (Fig.3a). The building typology of the top part is very complicated made of residential buildings distributed in rows (Fig.3b) and complex aggregations (Fig.3c) that can reach also five stores. All the data concerning Castelluccio were systematically collected through a survey form or template, used since 1997 after the seismic event in the Umbria region [4]. The vulnerability analysis of building is based on the prediction of failure mechanisms which can occur during an earthquake. The interpretation of failure or damage mechanisms in the case of large aggregates of buildings, attached together to form a sort of curtain and/or built on steep slopes, is particularly complex. Blocks or parts of buildings may be identified and surveyed, also adopting axonometric representations which can better show the different levels of the ground, in order to single out their typical failure mechanisms. The stratigraphical method [5] was adopted to subdivide the complex buildings into homogeneous blocks characterised by relative chronological relationships (Fig. 4). Every block corresponds to a unique building phase, recognised from constructive details and different masonry typologies; its relationship with the other blocks may be “preceding” or “subsequent”, even if there is no possibility of an absolute dating. Critical connections between blocks have been investigated, so to clarify the phases of expansion and transformation of the complex. In Fig. 4 two examples of the evolution of building along the time are presented.

Figure 4: Stratigraphical analysis made on volumes for the chronological phases of the complex building of Castelluccio. 4 Materials characterization When working on an urban scale (even if small centre), a “minimal” investigation program can be helpful and essential to have significant

316 Structural Studies, Repairs and Maintenance of Heritage Architecture IX information by sampling from buildings representative of the whole. The aim is to identify the different materials used for the masonry walls and their mechanical and physical properties and their behaviour. This investigation is also useful to detect compatible materials and techniques for representative prevention and repair measures. An important aspect of the investigation consists in defining the masonry texture and the geometry and composition of the masonry section. A procedure for the investigation of the texture is shortly described in [6]. This investigation is useful both for the definition of the geometry and mechanical behaviour necessary to the modelling phase and for the design of possible strengthening intervention (e.g. grout injection). Figure 5 shows some example of masonry texture with the corresponding cross section, which was possible to survey on damaged buildings after the seismic event. It is worth to remark that textures regular on façade often do not correspond to regular morphology in the section. Therefore, a correct analysis of section can inform on the mechanical behaviour of existing masonry. Especially when multi-leaf walls are present, a proper investigation of the arrangement of materials in the thickness of the wall cannot be disregarded.

Figure 5: Constructive masonry typologies of a stable badly damaged.

a) b)

Figure 6: Masonry materials sampled from Castelluccio buildings: (a) sampling, (b) stone cores.

A diagnostic investigation was possible on some private houses, stables and religious buildings. Samples of mortar and of some of the most recurrent stones have been sent to the laboratory (Fig.6, 7). Mechanical, physical tests and

Structural Studies, Repairs and Maintenance of Heritage Architecture IX 317 chemical analyses were carried out. The mortars sampled from private and religious buildings have revealed a high presence of lime pebbles (Fig. 7a) that (as the chemical analysis have confirmed) means putty lime as binder. The aggregate is mainly calcareous and the ratio binder-aggregate could be stated around 1:2 1:2.5. The grain-size distribution is similar for the entire sample (Fig. 7b). Three litho types were recognized as the most used in stone masonry and sampled for physical and mechanical characterization: Scaglia, Pietra maiolica and Breccia, which are all calcareous stones. Cylindrical specimens were cored from the stones to be tested mechanically, where possible, in dry and saturated conditions and in two directions. Figures 8 and 9 give some results obtained for the different typologies.

Grain-size distribution - Castelluccio di Norcia Lime pebbles 100 90

h sieve h 80 70 60 50 40 30 20 CL Church S.Maria Assunta-M1

% material passing through eac through passing material % 10 CL Stable-M1 0 0.01 0.10 1.00 10.00 100.00 a) [m m] b)

Figure 7: Mortars sampled from Castelluccio buildings: (a) lime pebbles in the mortar, and (b) grain-size distribution of the aggregates.

Bulk Water absorption Porosity I.R.S. Water absorption Density by tot. Immersion by capillary rise Kg/m3 % (peso) % (volume) (Kg/m2)/min (g/cm2)/h1/2 PIETRA (dry) ( 48 h ) ( 48 h ) (from 1' to 24h) White scaglia 2590 1.20 3.08 0.32 0.010 Breccia 2387 3.33 7.69 0.96 0.042 Pietra Maiolica 2683 0.31 0.83 0.07 0.004

Figure 8: Physical characterisation of the Castelluccio stones.

5 In situ tests

On the basis of the geometric and material surveys of the single buildings and of the surveys of the crack patterns, the following in situ and laboratory tests were carried out on strategic points as a minimum level of investigation: 1) flat jacks tests; 2) sonic tests [7]. The results of the tests with simple and double flat jack carried out on some sample buildings of Castelluccio di Norcia (Fig.10) pointed out the differences in behaviour of masonry belonging to important buildings (church or the bell

318 Structural Studies, Repairs and Maintenance of Heritage Architecture IX tower) or complex structures in comparison to simple buildings, as the stable. In particular, it is possible to see that both sonic velocity and flat jack results are in agreement, assuming higher values for more important constructions.

UMBRIA STONES - CASTELLUCCIO DI NORCIA (PG) Uniaxial Compression Tests on cylinders of stone sampled Water absorption by capillary rise from Castelluccio [D=5 cm H=10 cm] 1 160 Calcareous stone (Breccia) 150 Calcareous stone (Pietra Maiolica) 0.9 140 Calcareous stone (Pietra Maiolica) Calcareous stone (Scaglia Bianca) 130 0.8

) Calcareous stone (Scaglia Bianca) 120 2

0.7 ] 110 2 100 0.6 90 80 0.5 70 0.4 60 Stress [N/mm 50 0.3 40 Water absorption (g/cm absorption Water 30 0.2 20 0.1 10 0 0 0246810 -10 -8 -6 -4 -2 0 2 4 6 8 10 Time (h 1/2) a) Strain m/mm b) Figure 9: Some results on stones sampled in Castelluccio: water absorption by capillary rise (a) and compressive strength (b).

In Fig. 11a,b the results of the flat-jack and sonic tests are reported. The difference so high in the results is due to the different masonry typology: the one of the bell tower made with regular horizontal courses of large stones and the other of the stable made with irregular texture made with small stones and lot of mortar. The same difference is observed with the sonic tests made in transparency. Therefore the stables were made with much weaker and more vulnerable masonry.

CL.SM.J1S CL.SM.J2D

CL SM so1

a) b) c)

Figure 10: Localization of the in situ tests: the bell tower (a), a civil building U.I.158 (b), and a stable (U.I.123).

5.1 Collapse mechanisms The crack pattern survey must be carried out in order to interpret the type of damage and its causes. Damages which are frequently attributed to the earthquake can have a different nature and be caused by excessive dead load or soil settlements or simply by lack of maintenance. A complete survey of the structural and physical damages can help in understanding the vulnerable points of the structure and also the possible future mechanisms.

Structural Studies, Repairs and Maintenance of Heritage Architecture IX 319

CL SM J2D CL ST J4D 2.0 2.0

mean LVDT 234 vert. mean LVDT 2345 vert. mean LVDT 6,7 horiz. mean LVDT 6,7 horiz.

1.5 1.5 2] 2]

1.0 1.0 Stress [N/mm Stress Stressi [N/mm Local state of Stress 0.5 0.53 [N/mm2] 0.5

Local State of Stress ε l 0.05 [N/mm2] ε v ε l ε v 0.0 0.0 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Strain [µm/mm] Strain [µm/mm] 2000 2000

1600 1600

1200 1200 c e m/sec s /

800 m 800

400 400 0 Velocità soniche (m/sec) soniche Velocità 0 13 14 15 16 17 18 Velocità soniche (m/sec) 13 14 15 16 17 18 Figure 11a: Plots of the flat-jack tests and pulse sonic tests by transparency on the church bell tower (CL SM J2D) and on the stable (CL ST J4D).

CL SM J2D CL ST J4D Local state of stress 0.53 N/mm2 0.05 N/mm2 Max. reached state of 1.32 N/mm2 0.58 N/mm2 stress Elastic modulus (Young) 1521 N/mm2 205 N/mm2 Transvers. dilatation coeff. 0.31 0.25

CL SM J2D CL ST J4D Mean value of tot. velocity 1050.74 m/sec 611.16 m/sec

Figure 11b: Results of the flat-jack tests and pulse sonic tests by transparency on the church bell tower (CL SM J2D) and on the stable (CL ST J4D).

The possibility of damage prediction is related to the knowledge of the highest possible number of mechanisms of progressive deterioration or sudden failure [8]. The extensive survey carried out by the authors together with other researchers in Umbria after the 1997 earthquake allowed an abacus of failure mechanisms referring to different building typologies to be set up [8,9,10]. The most diffused damage mechanism observed in Castelluccio was the overturning of the façades (Fig. 12a,b) but the vulnerability was often connected to the new repair interventions following the rescue plan of 1991 (Fig. 12c). In those cases the strengthening showed to be incompatible with the existing structure.

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(a) (b) (c) Figure 12: Damage mechanisms: overturning of the façades, in civil buildings (a) and in old (b) and renewed (c) stables.

Figure 13: Example of reconstruction of the stables.

Figure 14: Example of a crack pattern survey on a complex building.

In Castelluccio, where the last earthquake of 1997 did not have a great effect, the crack patterns observed are mainly due to the lack of maintenance following the previous seismic event of 1979 together with the low quality of the masonry typology and to the recent non-compatible repair and retrofitting interventions, which in some cases lead to the complete reconstruction of the old stables in civil houses, as shown in figure 13. Nevertheless defining the mechanism of

Structural Studies, Repairs and Maintenance of Heritage Architecture IX 321 failure for complex building is not an easy matter due to the interference of variously connected volumes (Fig. 14.)

6 Conclusions

The proposed investigation procedure was is aimed to: (i) characterize the masonry buildings forming an homogeneous historic centre; (ii) put in evidence the structural problems; (iii) provide information and parameters for the most appropriate vulnerability analysis method, (iv) support the local designers in the choice of the most suitable repair and intervention projects. A detailed analysis of the constructions supplies not only the geometrical information but also the constructive techniques, characteristics and damages which can lead to kinematics mechanisms. This methodology calibrated on other historic centres of Umbria (and Liguria) region after the seismic event of 1997, allows for the evaluation of the previous retrofitting intervention effectiveness and of the future possible mechanisms.

Acknowledgements

The authors wish to thank M. Antico, M. Cucchi, M. Iscandri, L. Cantini and C. Arcadi for their help during the experimental works and the students M.P. Le Rose e G. Di Ponzio. The research was supported by GNDT and Civil Protection Dept., of Italian Ministry Council.

References

[1] Penazzi, D., Valluzzi, M.R., Cardani, G., Binda, L., Baronio, G., Modena, C., Behaviour of Historic Masonry Buildings in Seismic Areas: Lessons Learned from the Umbria-Marche Earthquake, Proc. 12th Int. Brick/Block Masonry Conf., Madrid, Spain, pp. 217-235, 2000. [2] Binda L., Modena C., Marchetti L., Cardani G., Valluzzi M.R., Indagine sulla consistenza dell’edilizia storica, sul danno pregresso e sull’efficacia degli interventi svolta su quattro centri campione in umbria, XI Cong. Naz. “L’Ingegneria Sismica in Italia”, ANIDIS, Genova, CD-ROM, C1-01, ISBN 88-86281-89-7, 2004. [3] Cordella R. , Lollini P., Castelluccio il tetto dell’Umbria, Perugina Ed. [4] Di Ponzo G., Le Rose M.P., “Calibrazione di una metodologia di indagine per la conoscenza, il rilievo dei danni e della vulnerabilità sismica dei centri storici: Castelluccio di Norcia”, degree thesis, Faculty of Architecture, Politecnico di Milano, 2004. [5] Mannoni, T., Caratteri costruttivi dell’edilizia storica, Sage, Genova, 1994. [6] Binda L., Penazzi D., Saisi A., Historic Masonry Buildings: Necessity of a Classification of Structures and Masonries for the Adequate Choice of Analytical Models, 6th Int. Symp. Computer Methods in Structural

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Masonry (STRUMAS VI), Ed. T.G. Hughes & G.N. Pande, Computers & Geotechnics Ltd, Roma, pp. 168-173, 2003. [7] Cardani, G., La vulnerabilità sismica dei centri storici: il caso di Campi Alto di Norcia. Linee guida per la diagnosi finalizzata alla scelta delle tecniche di intervento per la prevenzione dei danni, Ph.D. thesis, Politecnico di Milano, Italy, 2004. [8] Giuffrè, A., Sicurezza e conservazione dei centri storici: il caso di Ortigia, Bari, Laterza, 1993. [9] Binda L., Cardani G., Saisi A., Modena C., Valluzzi M.R., Marchetti L., Guidelines for Restoration and Improvement of Historical Centers in Seismic Regions: the Umbria Experience, IV Int. Sem. Structural Analysis of Historical Constructions”, Padova, Vol. 2, pp. 1061-1068, 2004. [10] Valluzzi, M.R., Cardani, G., Binda, L., Modena, C., Analysis of the seismic vulnerability of masonry buildings in historical centres and intervention proposals, 6th Int. Symp. on the Conservation of Monuments in the Mediterranean Basin, Lisbon, Portugal, April, pp. 561-565, 2004.