methodologies for the vulnerability analysis of historic

Methodologies for the vulnerability analysis of historic centres in Italy

L. Binda Department of Structural Engineering

Abstract

During the last few decades several approaches to the study of the vulnerability of historic centres have been proposed by different research groups in the Mediterranean countries under the seismic risk. The aim is to (i) provide the responsible authorities with information and parameters for the most appropriate vulnerability analysis methods, (ii) support the designers in their repair and prevention projects. As an example to clarify the results reached by the research, a methodology proposed within a research supported by the Protection Department of the Italian Minister Council is presented. The approach is proposing an investigation on site and in a laboratory based on a “minimal” low cost program. The knowledge of existing buildings is approached by considering different levels of analysis. The methodology, calibrated on four historic centres situated in Umbria (Italy), allowed us to define an abacus of the typical collapse mechanisms which can be used for analytical methods based on the application of single or combined kinematic models involving the equilibrium of macro-elements. Keywords: masonry structures, on site and laboratory investigation, vulnerability analysis.

1 Introduction

The earthquake which struck Umbria and Marche regions in 1997 only occurred 18 years after the one which in 1979 hit Val Nerina (part of Umbria). In this part of Umbria some historic centres were retrofitted and again damaged in 1997. The 1997 earthquake gave the occasion to learn about the effectiveness of the repair and retrofitting techniques. In fact most retrofitting mainly performed with upgrading interventions (substitutions of timber floors and roofs with r.c.,

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Structural Studies, Repairs and Maintenance of Heritage Architecture IX 279

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 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 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. From 1997 some of the National research was directed to collect knowledge on the construction techniques and materials of buildings and dwellings of the numerous still preserved historic centres of Italy in different Regions. The object of the research was not the single building, but the whole historic centres. The strategic aim was also, beside collecting information on the effectiveness of the repair techniques adopted in the distant and recent past, to define a methodology for the vulnerability study of a building patrimony previously considered as minor, but with meaningful testimonies of cultural heritage. 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 Property (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] (Fig.1a, b, c, d). The centres of Montesanto and Roccanolfi were more seriously damaged than Campi and Castelluccio, where only few buildings showed some evidence of a starting collapse. The extensive survey of the buildings was useful to produce an abacus of the typical damages occurring in constructive typologies, as already previously done for churches [3], a critical evaluation of the repair techniques and suggestions for future interventions [4, 5]. The better knowledge of damages led to the consequent systematisation of the mechanical models able to describe their specific behaviour by kinematics models, both for in-plane and out-of-plane mechanisms.


280 Structural Studies, Repairs and Maintenance of Heritage Architecture IX

Recently, procedures for the evaluation of the seismic vulnerability, based on the application of single or combined kinematics models involving the equilibrium of macro-elements [6, 7], have been developed by direct comparison with the real damage occurred [8, 9].

Figure 1a: Plan of Montesanto. Figure 1b: Plan of Roccanolfi.

Figure 1c: Plan of Campi di

Norcia. Figure 1d: Plan of Castelluccio.

Automatized procedures able to combine the different possible mechanisms have been implemented at the University of Padova, both concerning global and local analyses (VULNUS, [6] and C-SISMA, respectively). They have been recently updated in Visual Basic ambient, in order to allow vulnerability analyses in a more large scale, more quickly and easily in comparison to the first applications. The research gave a useful contribution to the draft of the new National Code published at first in March 2003, for the design, execution and effectiveness evaluations of non-invasive repair and strengthening interventions, based on “improvement” rather than on heavy upgrading of historic masonry buildings belonging to the minor patrimony.

2 On site and laboratory investigation at the scale of the historic center

The selection of the centres on which the approach was calibrated was very accurate in order to limit the sample population to the most significant buildings.

stables

cassero



The villages of Montesanto and Roccanolfi, seriously damaged by the 1979 and 1997 earthquakes, but also by several earthquakes in the past centuries, showed an interesting scenario for the study of the behaviour of repaired and consolidated masonry historic buildings. Some of them presented earthquake-proof interventions made before the 1979 seismic event. These buildings showed various repair techniques as the use of: timber rods with steel connections to the walls, steel rods, buttresses or buttressed walls. The above mentioned techniques can be defined as “traditional” and were commonly and efficiently applied in the past before the use of reinforced concrete. It can be observed that if these techniques were successful, then they can be used also in future repairs. Furthermore, buildings repaired according to the regional plan (P.d.R.) were identified. The following repair techniques were applied: stiffening or substitution of timber floors with r.c. beams or plates, tie concrete beam insertion in the walls, cementitious grout injections, reinforced injections, roof substitution, jacketing, local replacement in the walls, joint re-pointing, etc. The survey allowed us to detect many failures of the more recent repair due to incompatibility of the new materials or techniques with the existing ones. No doubts that, in order to study the vulnerability of the historic centres the above information is important, but not exhaustive. At the large scale of urban level, in order to suggest the best strategies for repair and prevention, an articulated procedure of investigation is needed. This investigation can take advantage from two sources of information: (i) indirect (archives and bibliographic information, collected for reconstructing the evolution of the building from origin and its load history, also through the study of the earthquakes occurred in the past), (ii) direct as: geometrical and photographic survey, typological analysis of the building, stratigraphic survey aimed at understanding the process of formation and growing of the built types, survey of the masonry section and texture, characterisation of material samples and of the crack pattern, analysis of the principal structural elements including load-bearing walls, roofs, floors and vaults, staircases, of their connections, damages, effectiveness of past repair. The identification of the most typical failure mechanisms activated by the earthquake added more information to choose appropriate analytical models, which can be used both for prediction analyses and for design of proper interventions.

2.1 Building typology

The detection of the building typology is the first step to be carried out starting from an accurate geometrical survey. It is well known that representative typologies of a centre can be easily recognisable through similar features (number of stores, exposure, type of facade, material and structural elements, etc.). Furthermore buildings may have had an evolution along the time: born as an isolated building (Fig.2a), it could have become a row building (Fig.2b) or a complex one (Fig.2c), 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 effort should be made to find through historic documents and also observation on site the modification it was subjected to, along the time. Sometimes due to the ground slope (e.g. more than 100 meters from the base to the top of the village in the case of Campi), the buildings develop following a row typology (Fig.1c), generally with three storeys: 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 collapsed buildings.

A'

A

a) UI 197

UI 198UI 199 b)

c)

Figure 2: Example of: a) simple isolated building; b) of row building; c) of complex building.

In the case of Roccanolfi and Castelluccio, the plan of the site was much more complex, with wind up narrow streets and rather complicated volumes of the buildings (Fig. 3a,b). The town-planning development of Castelluccio is shown in two parts: (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. The building typology of the top part is very complicated as shown in Figure 3a, while the stable typology is very simple (Fig.3b). Complex buildings- The interpretation of failure or damage mechanisms in the case of large complexes 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.

a) b) Figure 3: The building typology of Castelluccio: a) complex civil buildings

and b) simple rural buildings. The stratigraphical method [10] was adopted to subdivide the complex buildings into homogeneous blocks characterised by relative chronological relationships. Any block corresponds to a unique building phase, recognised from constructive details (Fig. 4); its relationship with the other blocks may be “preceding” or “subsequent”, often with 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. The study has then been completed by the investigation of dated elements like the brick type and dimensions and by the chronological characterisation of the construction techniques and masonry details, beyond the survey and characterisation of the different masonry typologies.

Figure 4: Stratigraphical analysis made on volumes for the chronological

phases of the complex building.

2.2 Masonry morphology

The second step 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 the following.

The masonry is surveyed by pictures, obtained as parallel as possible to the masonry surface, and by placing close to the section or to the texture a graduated stick in order to know the wall dimension. The dimensions are then verified by



the archaeological survey method (scale 1:1). The 2D graphic plotting is realised with a special care in the representation of stones, joints and voids. Successively, the surface occupied by the different materials, which compose the masonry, is measured, evaluating the stone, mortar and voids percentages, the dimension and the distribution of voids. This information 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 the mechanical behaviour of existing masonry structures, especially when multi-leaf walls are present, can not disregard the proper investigation of the arrangement of materials in the thickness.

Regular texture

Irregular courses

Figure 5: Constructive masonry typologies.

2.3 Material characterization: laboratory and on site tests

When working on an urban scale (even if small centre), a “minimal” investigation program is suggested to carry out as essential to have significant 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. On the basis of the geometric and material surveys of the single buildings, the in situ tests should be carried out on strategic points. Investigations can be performed through: 1) flat jacks tests; 2) sonic tests;



3) sampling of materials for their chemical-physical-mechanical characterisation [11]. A diagnostic investigation was possible on some private (houses and stables) and religious buildings of the four centres. Samples of mortar and of some of the most recurrent stone materials have been taken. Mechanical, physical tests and chemical analyses have been carried out. The mortars sampled from private and religious buildings have revealed a high presence of lime pebbles that (as the chemical analysis have confirmed) means putty lime as binder. The aggregate is mainly calcareous and the ratio binder-aggregate may be stated around 1:2 1:2,5. Cylindrical specimens were cored from the stones to be tested mechanically in dry and saturated conditions in two directions. The results of the tests with simple and double flat jack carried out on some sample buildings of Campi di Norcia pointed out the differences in behaviour of masonry belonging to important buildings or complex structures (church or the bell tower) in comparison with private buildings. 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.

2.4 Crack pattern and structural damage survey

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 to 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 as it is described in the following section.

2.5 Use of an abacus for the interpretation of the crack pattern

The assessment of seismic vulnerability of masonry buildings requires the identification of the damage and collapse mechanisms activated by the earthquake. The current practice in Italy is to take account of only a limited number of modes of failure; some modes of failure are neglected implicitly assuming a strength capacity of certain structural typologies, after appropriate retrofitting measures. On the contrary, the possibility of damage prediction is related to the knowledge of the highest number of possible mechanisms of progressive deterioration or sudden failure. The extensive survey carried out by the authors together with other researchers in Umbria after the 1997 earthquake allowed to set up an abacus of failure mechanisms referred to different building typologies, and depending upon if the building had been repaired. In Figure 6 some examples are given of different mechanisms [11]. The adopted diagnostic approach is based on the recognition of local and global collapse mechanisms traceable to in-plane or out-of-plane seismic action. The modelling of the structure behavior and its safety assessment by macro-elements can highly benefit of the abacus provided that the characteristics of the materials and the structure are known.



Figure 6: Some examples from the collapse mechanisms abacus.

3 Seismic vulnerability analysis

The methodology proposed for assessment of seismic vulnerability of existing buildings in historical centres concerns the application of simply kinematics models able to describe the mechanical behaviour of structural components and assemblages (macro-models) [3, 7]. Macro-elements are defined by single or combined structural components (walls, floors and roof), considering their mutual bond and restraints (e.g. the presence of steel ties or tie concrete beam insertions), the constructive deficiencies and the characteristics of the constitutive materials. Once the critical structural configuration is defined, the subsequent step is the identification of the most probable collapse mechanism/s characterizing each macro-element. The analysis is performed at global level by using the program “Vulnus”, set-up at the University of Padova in the last eighties [6, 12] and, locally, by selecting the most significant elements in the building, by the application of the models describing the single mechanisms, both for in-plane and out-of-plane collapses. Automatic procedures have been implemented recently in Visual Basic ambient at the University of Padova (Vulnus VB release and C-Sisma program), which allow to execute vulnerability assessment for whole centres more quickly in comparison to the first applications. The Vulnus methodology is able to define two indexes, I1 and I2, concerning the in-plane shear resistance and out-of-plane collapse mechanisms, respectively. It can combine different mechanisms for global vulnerability analyses of buildings with sufficient regularity (both in plane and in elevation) and limited height (three storeys or less), and that take into account the type of connection among the structural elements. The significant parameter describing the



kinematics models is the collapse coefficient c=a/g, which corresponds to the seismic masses multiplier characterising the limit of the equilibrium conditions for the considered element. Preliminarily, if the seismic degree of the zone is given, it is possible to execute safety assessments of the buildings in seismic conditions according to the current standards prescriptions (e.g. cmin=0.28 for Umbria region). Some examples of kinematic models and related c coefficients are given in Figure 7. The proposed procedures, applied to different typologies of buildings (isolated, rows and complexes), showed their reliability in comparison with systematic adoption of typical assessment methods based on the “box behavior” of the structure, which take into account only the in-plane shear strength of the masonry panels composing the walls, as also suggested by Italian standards until now [8]. Isolated buildings are well described in their structural conditions by the simplified macromodelling [8]; for buildings characterized by more adjacent constructions (rows, complex) the general procedure for the vulnerability assessment is to perform first of all the global analysis and to control some local aspect by using the single kinematics models [13]. In particular, for row buildings and particularly irregular configurations, both in plan and in section, as found both in Roccanolfi and Castelluccio villages, the subdivision in homogeneous units (respect to dating, transformations, etc. and regardless the possible different private properties), namely units of minimum intervention (UMI), is necessary. However, the critical analysis of the results obtained at general level is essential [14].

a) b)

α

Ψ

δ =ψδ =α ψ

c)

hNhP

dNbPc

×+×

×+×=

2

2

lwhs

c wallc

×××

=2'

28.1σ

QPN

LHqPN

HLc

++

++×=

32

3α

Figure 7: Examples of simple kinematics models for out-of-plane (a: overturning of a solid wall) (b: crushing of the masonry) and in-plane mechanisms (c: effect of in-plane overturning actions).

The assessment procedures, calibrated on the real damaged sites, can be usefully applied both for analyses of vulnerability for centres under seismic hazard, in order to examine the current condition and to prevent their future



damage, and to simulate possible interventions, both in damaged and undamaged conditions, evaluating their impact with the pre-existing situation [2, 14].

4 Conclusions

The wide investigation program applied in the Umbria region after the last seismic event has allowed to gather a great amount of data on: - history and evolution of buildings and centres under investigation; - characteristic building typologies (isolated, row and complex buildings); - characterisation of structures and materials; - damages and interventions after the 1979 and 1997 earthquakes; - effectiveness of the restorations after 1979; - possible damage mechanisms of the restored and non restored buildings in

the future; - vulnerability analysis of repaired and non repaired buildings.

The study allowed to calibrate analytical procedures able to assess the seismic vulnerability of historical centres, to be used for prediction analyses in not damaged sites and/or to design suitable interventions. The results are now being reported on a Database which will in a near future appear on the Website of the Italian Department of Civil Protection.

The methodology applied to the four centres is now calibrated and can be used for other similar cases.

Acknowledgements

The author thanks M. Antico, M. Cucchi, M. Iscandri, l’arch. L. Cantini and C. Arcadi for their help during the experimental works. The research was supported by GNDT, Italy.

References

[1] Penazzi, D., Valluzzi, M.R., Saisi, A., Binda, L., Modena, C., Repair and strengthening of historic masonry building in seismic area, Proc. Int. Millennium Congress ‘More than two thousand years in the history of architecture safeguarding the structure of our architectural heritage’, Bethlehem, Palestine. Vol. 2, Section V (7 pp.), 2001.

[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, 25-29 Gennaio 2004, CD-ROM, C1-01, ISBN 88-86281-89-7, 2004.

[3] Doglioni, F., Moretti, A., Petrini, V., Le chiese e il terremoto. Dalla vulnerabilità constatata nel terremoto del Friuli al miglioramento antisismico nel restauro, verso la politica di prevenzione, Trieste, ed. LINT, 1994.



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[5] Binda, L., Cardani, G., Penazzi, D., Saisi, A., Performance of some repair and strengthening techniques applied to historical stone masonries is seismic areas, Proc. ICPCM a New Era of Building, Cairo, Egypt, 18-20/2/2003, 2, pp. 1195-1204, 2003.

[6] Bernardini, A., Gori, R., Modena, C., Valutazioni di resistenza di nuclei di edifici in muratura per analisi di vulnerabilità sismica. R.I. 2/88, University of Padova, Istituto di Scienza e Tecnica delle Costruzioni, 1988.

[7] Giuffrè, A., 1993. “Sicurezza e conservazione dei centri storici: il caso di Ortigia”, Bari, Laterza, 1993.

[8] Valluzzi, M.R., Michielon, E., Binda, L., Modena, C., Modellazione del comportamento di edifici in muratura sotto azioni sismiche: l’esperienza Umbria –Marche, 10th Conf. ANIDIS “L’ingegneria sismica in Italia”, Potenza, September 2001 (12 pp., on CD-ROM), 2001.

[9] Avorio, A., Borri, A., Corradi, M., Ricerche per la ricostruzione. Iniziative di carattere tecnico e scientifico a supporto della ricostruzione, Regione Umbria, DEI, Rome, 2002.

[10] Mannoni, T., Caratteri costruttivi dell’edilizia storica, Sage, Genova, 1994.

[11] 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.

[12] Bernardini, A., Gori, R., Modena, C., Application of coupled analytical models and experiential knowledge to seismic vulnerability analyses of masonry buildings. On: Earthquake Damage Evaluation and Vulnerability Analysis of Buildings Structures, A. Kortize Ed., INEEC, Omega Scientific. 1990.

[13] Valluzzi, M.R., Cardani, G., Binda, L., Modena, C., Analysis of the seismic vulnerability of masonry buildings in historical centres and intervention proposals, 6th International Symposium on the Conservation of Monuments in the Mediterranean Basin, Lisbon, Portugal, 7-10 April 2004, pp. 561-565 (on CD-ROM), 2004.

[14] Valluzzi, M.R., Cardani, G., Binda, L., Modena, C., Seismic vulnerability methods for masonry buildings in historical centres: validation and application for prediction analyses and intervention proposals, 13th World Conference on Earthquake Engineering, August 1-6, 2004, Vancouver, B.C., Canada, 2004.



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