Structural Analysis of Historical Constructions - Modena, Lourenço & Roca (eds) © 2005 Taylor & Francis Group, London, ISBN 04 1536 379 9

Vulnerability study in seismic areas: the role of on-site and archives investigation

A. Anzani, A. Baila, D. Penazzi & L. Binda Department ofStructural Engineering, Po/itecnico di Milano, Milano, lta/y

A8STRACT: The seismic response of old stone masonry buildings in Western Liguria is approached by a multi-disciplinary view. The historical centres of Bajardo, Taggia and Bussana and others, still display signs of damage due to XIX century earthquakes, to which the problems of lack of maintenance and, in many cases, of abandon have overlapped. A method for evaluating their seismic vulnerability is proposed after: (i) a direct survey ofthe buildings (based on a stratigraphical survey and on a survey ofthe crack patterns and ofthe wall sections), Oi) the collection ofindirect information from documentary sources, (iii) the collection ofdata recorded through in-si tu non destructive or low destructive mechanical testing and laboratory investigation aiming at the chemical, physical and mechanical characterization ofmasonry components. The ro le ofhistorical analysis will be shown as a basic source of knowledge of the structures for the subsequent assessment of seismic vulnerability.

INTRODUCTlON

Most of the historic centres in Italy are characterised by the presence not only of monumental buildings like churches and palaces, but also by a complex built environment made of simple houses which itself constitutes an important part of the cultural heritage. This so called "minor" architecture, often poorly con­structed, is mainly composed of stonework buildings with timber roofs and floors . In spite of the use of apparently similar material and construction tech­niques, building characteristics vary according to their typology - from isolated to arrayed buildings - and to their location - from flat to steep mountain sites. This variety is directly connected to the orographical pro­file ofthe site and also to the common seismic history. Urban structures, like those described, have been fre­quently abandoned and not maintained, therefore they are particularly vulnerable to earthquakes. In many other cases, retrofitting and repair techniques have been systematically applied according to the Italian seismic Code, but mostly without any investigation of the characteristics ofmaterials and structures. The fail ­ure of some of these interventions has been basically due to the lack of knowledge and inadequate skills in the application.

After the most recent earthquakes, the need of adjusting the adopted structural models to the typical behaviour of some building typologies and

masonry morphologies and the need of improving the retrofitting techniques applied after the previ­ous earthquake of 1979 was evident. The problem of repairing and retrofitting should be approached in a multidisciplinary way, that is , considering dif­ferent complementary aspects of the problem such as: historical evolution ofthe building, geometry and crack pattern, material characteristics, technology of construction, possible fai lure mechanisms, etc (Binda et aI., 1999), (Penazzi et aI., 2000). To this respect, the participation of architects/"conservators", historians, structural engineers is also important. A compari­son and a correlation between all the collected data will allow a more robust interpretation of the struc­tural response according to the examined masonry and building typology. This in order to support techniques for evaluating the seismic vulnerability and to define reliable retrofitting procedures of stone-masonry buildings.

The paper describes the application of a multi­disciplinary procedure, addressed to an articulated knowledge of three historical centres of the Western Liguria built in stonework masonry, with special atten­tion to the contribution of the historic survey into the understanding of the seismic vulnerability (8inda et aI. , 2003a). Aim of the research is the suggestion of some guidelines for the investigation procedure that may orient the interventions on historic buildings toward the objective of safety and compatibility.

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2 STUDY OF THE HISTORIC SOURCES

A fundamental analysis for studying an urban centre is lhe historical survey. This is articulated into a twofold research line: on the one hand the cataloguing and the analysis of the data collected through documen­tary sources (indirect sources); on the other hand the direct and criticai observation of the evolution of the village and of the manufacts that it is made of. This compared study is fundamental especially when the research objects are minor centres, mainly made of rura l architecture, like in the case ofTaggia, Bajardo and Bussana. In fact, in these cases the documen­tary and historical sources (indirect) are generally not enough to allow the complete reconstruction of the evolution ofthe centre. Therefore, it is necessary that the historical information coming from the paper doc­uments are confirmed in situ and accompanied by the direct analysis of the centre so to define a complete know1edge.

Direct analysis is based on lhe observation of the characteristics ofthe centre, giving information about the historical-constructive evolution ofthe manufacts, their dating and their constructive techniques.

Indirect sources include archives and bibliographic information, collected for reconstructing the evolution of the centre, its building and transformation phases and its load history as a history of the earthquakes occurred in the past.

Of great relevance are also the iconographic aspects (views, ancient documents) giving information on the historical housing, allowing the evaluation ofthe trans­formations of a territory and the comparison with its present configuration (Figs. 1 and 4).

Taggia was founded in 641 after raids by the Saracens had made the previous settlement on the coast unsafe (Costa Balenae). The inhabitants built the new centre within the Aulella valley, constituting a medieval fortified village with a castle "Castrum Tabia". The history of the village is connected to the long Benedictine colonization, lasted until the coming

Figure l. Taggia in XIll century (private archive).

of the Clavesana Marquis f irst and of the Genovese Republic later. The different phases of expansion co[­responded to the construction of new fortified rings, as historically documented (Fig. 3).

The medieval historic centre ofBajardo is laying at 900 m above the sea. Feud of the Ventimiglia Earls, the village was occupied in 1259 by the Genoese and remained under their domination until the birth of the Ligure Republic . The upper part of the village, nearby the church and the Castle, was abandoned after the earthquake of 1887. Therefore, the inhabited part expanded toward the valley (Fig. 4).

At present, the ancient part of the village consists of vaulted passages and radial paths, whereas many medieval ruins still remain.

"" L t

Figure 2. Taggia: evolution of the historical defence walls.

Figure 3. Bajardo, evolution of the ancient centre.

Figure 4. Old Bussana in a view or 1770.

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Bussana is built on 10p of a hill dominating the Armea valley. The first paper referring to Bussana is a document of 979 indicating that its territory was a property of the Bishop of Genova, Teodolfo. Later, his authority was substituted by that ofthe Ven­timiglia Earls, Lords ofthe territory placed west ofthe Armea River. They were responsible of the construc­tion of a castle, dominating the area of the successive Bussana (fig. 4). After the earthquake of 1887, a new centre was built, whereas the old village was aban­doned without any institutional declaration about the property of the ruins. Since the fifties of the XX century, the area was occupied by a community of artists who autonomously carried out repair and con­solidation interventions without any official project, and organized artistic performances, attracting a new diffused attention to the centre.

3 CHRONOLOGY OF SEISMIC EVENTS

The approach to the vulnerability study of historical centres implies the acquisition of data about the geo­morphology ofthe site, the chronology and "history" of seismic events. When it is known, the seismic his­tory ofthe centre and the single building is particularly relevant with respect to the safety aspects and to the intervention implications. This data series contributes to define, on a probabilistic base, the seismic risk which characterises the area under study. The western

a)

b)

Figure 5. Bajardo: a) afterlhe earthquake of 1887; b) 2002.

Riviera since 951 until 1890 had been subjected to a series of heavy and destructive earthquakes.

Taggia, Bajardo and Bussana registered four rele­vant events: 1818, 1831 , 1854 and 1887, as reported by the R. D. of 26th June 1887 n. 106, Province of Porto Maurizio, Lis! 0/ most/y damaged centres (Figs. 5-7). Since 1831 only, detailed and precise descriptions of earthquakes and their consequences are available.

4 DIRECT SURVEY

Direct sources, which are the sole available when his­toric documents have been lost, include various meth­ods for approaching the knowledge ofthe building, of its chronological phases, construction techniques and materiaIs. In particular they are: a) geometrical sur­vey; b) photographic survey; c) typological analysis of the buildings; d) stratigraphic survey, when possible, for gaining chronological information and the building

Figure 6. Ruins of the sacristy and annexed buildings. (Historie picture, Municipality of Bajardo)

a) b)

Figure 7. a) Bussana, ruins ofthe Oralory after the earth­quake of 1887 (Various authors, 1987) b) Bussana, ruins of the Oratory, 2003.

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evolution; e) survey ofthe masonry section and surface texture; f) survey ofthe crack pattem; g) analysis ofthe principal structural elements including load-bearing walls, roofs, floors and vaults, staircases, oftheir con­nections, damages, effectiveness ofpast repair; and h) laboratory characterization of material samples, and on site tests (flat jack tests, son ic tests, etc.). Among these, some aspects play a fundamental role fo r ana­lyzing the structural behaviour, as described in the fo llowing.

4. 1 Sludy of lhe building typologies

The analysis of the whole building structure is a first key for reading the behaviour of masonry structures in their complex, and constitutes also a direct dating source. A historic centre is subjected to continuous modifications, often connected with the evolution of the building typology.

Aim ofthe analysis ofthe building typologies is to understand the rules ofthe formation and the growing of the built types, starti ng from the first construction unti l the fusion and complex additions so to judge which interventions are compatible with their voca­tion to being modified, and which are detrimental and must be considered superfetations.

Observing the dwellings wi thin the most ancient nucleus of the studied historic centres, it is possible to find out some of lhe morphological aspects that influence the architecture from Liguria.

The conformation ofthe natural environrnent, char­acterised by a steep soil, sometimes simply a slope, sometimes organised by degrading belts, originates the so called "house on the slope". The village develops by concentric bel ts, according to the ground contours, so to form the typical "pinecone" pattern (fig. 5). Gen­erally, the strips with minimum radius are on top of the hill which is often dominated by the main church anel/or the castle (Fig. 4).

The importance of recognizing diffe rent building typologies, in view of the definition of their seismic vulnerability, depends on the correspondence between construction typologies and mechanisms of damage. Considering complex cases as for the geometry and original destination, it has to be pointed out that any typology, for instance palaces, religious buildings, towers, castles, fo rtifications, churches, etc., display specif ic problems, also connected to the function of the building itself (Figs. 8- 10).

Such differences, that have influenced the original constructive solutions and the wall masonry qual­ity, similar!y have to influence the safety assessment and the techl1iques of intervention. Therefore, for any typology, the matter should be to individuate and manage specific leveI of investigation, modelling, ver­ification and specific repair techniques, satisfying also the function requi rements.

o l' tower house typology

isolated tower connecting

tissue

isolated tower

Figure 8. Taggia, exam ple of tower-house.

connecting tissue

91JO.- 1100 1200--1300 1300-- 1500 1400 141JO.- 1500 1600

Figure 9. Historical building typologies in the centre of Taggia.

Figure 10. Taggia, example of wall-houses.

4. 2 Study ofthe masonry typologies: prospecls and sections

With reference to stonework masonry, and parti cu­lar!y to the multiple leaf ones, the prediction of the seismic vulnerability can only be performed provided the intrinsic characteristics of the masonry have been accurately detected.

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Figure I I. Study of masonry typologies in Bussana.

The behaviour of masonry highly depends on the construction technique and this is shown from the sec­tion layout. [n fact, the outer face frequently does not reveal how the masonry section is built; this is due to the fact that apparently similar facing textures (e.g. regular bonding) may correspond to different types of sections, as one or multiple leaves (Hendry 1994; Fig.II).

A systematic study ofthe mechanical behaviour of stonework masonry begins from an extensive investi­gation taking into account the different leaves consti­tuting the wall and the kind of connections which may or may not exist between the leaves themselves.

Some cri teria have to be singled out in order to identify homogeneous groups of walls, not on ly on a geometric basis but also on a mechanical one, in order to set up appropriate constitutive laws. Giuffre (199 1), for instance, proposed a classification based on the ratio of the distance between two subsequent dialons (stone blocks laid transversally and having a length equal to the who le section of the wall) to the thickness ofthe masonry wall.

Nevertheless, this method ignores the sections with­out transversal connections which represent the high­est percentage of the stone patrimony. Binda et aI. have proposed a classification of multiple leaf walls based on the number of different leaves and on the type of connections between them (Binda et aI. , 1999). The survey has been carried out by photographs taken on site. A 2D graphic restitution is carried out, care­fully representing the stones, the mortar and the voids between them (Fig. 12); subsequently the surface areas

Figure 12. Mechanical behaviour ofthe masonry typologies identified in Bajardo.

Figure 13. Location of masonry sections in Bajardo.

o S1- single leal

14% 21 % O S2- two connected leaves

• S3- two unconnected leaves

• S4-threeleaves

Figure 14. Distribution of masonry typologies in Bajardo.

occupied respectively by the different materiais are measured, then working out the percentage areas and the voids dimension and distribution.

In the studied centres, different masonry typolo­gies have been observed which have been subse­quently classified and statistically worked out (Figs. 13 and 14).

From the analysis of this inforrnation constitutive laws for modelling the masonry behaviour and for designing possible repair interventions (e.g. injection of grout) can be selected (Binda and Saisi, 2002). Input

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1200-1300 1600

1700-1720 1800-1900

Figure 15 . Historical evolution of the Monastery of the Sisters of Visitation in Taggia; fourth levei of the complex.

data for the structural analysis with numerical meth­ods are the section survey and the type of connection between the layers. These features can also be taken into account when modelling the in-plane or out of plane failure mechanisms. To this respect work has already been done to study, on an experimental and a numerical basis, the stress distribution between the dif­ferent layers of muItiple-leaf masonry walls (Giuffré, 199 1; Binda et aI. , 2003).

4.3 Stratigraphica/ ana/ysis

The stratigraph ical method (Francovich, 1982; Parenti, 1992; Mannoni, 1994) allowed the subdivision of the building into homogeneous blocks characterized by relative (as opposite of absolute) chronological re la­tionships. Any block corresponds to a unique building phase, recognized by the observation of constructive details; its relationship with the other blocks may be "preceding" or "subsequent", often with no possibility of an absolute dating. The application of lhe strati­graphical analysis to the Monastery of the Sisters of Visi tation in Taggia is reported as an example. The Monastery is a vast architectura l complex, result of numerous transformation and subsequent built inter­ventions. The whole building comprises a XV- XVI part with traces ofa civil building ofXIV cent. , a cen­trai nuc leus dating XVII- XVIII cent., and aggregated parts of other minor buildings, fo rming a unique organ­ism oflarge dimensions in the historic centre of Taggia (Fig. 15). Criticai connections between blocks have

Figure 16. Stratigraphica l analysis: outer front with chrono­logical phases.

a) _.. ______ --'

Figure 17. Comer between orthogonal walls: a) absence of connection; b) presence of connection.

been investigated, so to clarify the phases of expansion and transformation ofthe complexo

The study has then been comp1eted by the inves­tigation of dated elements like the brick type and dimensions and by the chronological characterization of the construction techniques and masonry details, beyond the survey and characterization ofthe different masonry typologies.

In (Fig. 16), the case of the most criticai area in the westem part of the complex is represented. The presence of many openings causes the possibility of separation of the wall leaves in case of out of plane mechanisms and/or the occurrence of shear mech­anisms under in plane actions; the presence of an externai lower volume can cause hammering between the lower and the higher parts.

In (Fig. 17) two examples of lack and pres­ence of connections between orthogonal walls in the Monastery is shown.

In Bajardo buildings corresponding to different typologies have been examined (church, castle, civil building). Completing the scarce documentary sources by the direct observation of the masonry typologies,

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Figure 18. Bajardo: reconstruction of the historical evolu­tion of the village.

~.;.~~ • I ~. -~ ~ .. : ... ~ ..

' . . ~ .. , .

! •

First levei: plan Second levei : plan

Third levei: plan Fourth levei : plan

Fifth levei: plan Sixth levei: plan

Figure 19. a) Bajardo, civil building. Stratigraphical anal­ysis of the volume: plans.

the criticai connections between homogeneous blocks, and the construction techniques, the historical evolution of the village could be reconstructed (Fig. 18). The stratigraphical analysis carried out on a civil building is reported as an example in (Fig. 19). It is interesting to notice that, though the walls are mainly made of masonry built with a simple technique, the building in its global aspect is a very complex result of different aggregation phases.

North front West front

South front East front

Section A-A Section B-B

Figure 19. b) Bajardo, civil building. Stratigraphical anal­ysis ofthe vo lume: fronts and sections.

5 MATERIAL CHARACTERIZATION

Beyond studying the morphology of the masonry walls, the material characterization has to be car­ried also in view of designing repair interventions safeguarding the compatibility with the existing built patrimony. A few testing procedures may be suffi­cient for defining the properties of masonry. Through chemical, mineralogical-petrographical and physical analyses the composition of the mortars (type of binder, type of aggregate, ratio binder/aggregate, levei of carbonation and the grain size distribution of the aggregate) can be defined. Compressive ortensile tests are seldom possible on mortar, due to the small thick­ness ofthe joints, whereas they can suitably be applied to stones and bricks, together with physical tests. In most cases, the characteristics of masonry cannot be reliably obtained from the characteristics of its compo­nents - as suggested by the Italian Code (D.M. 1978) for brick and stone masonry made with regular ele­ments - basically because of the lack of constitutive laws appropriate to highly non-homogeneous multi pie leafmasonry. Single and double flat-jack tests carried out on site are therefore the only available method to obtain quantitative results on local compressive stress state, modulus of elasticity and Poisson 's ratio (Binda et aI. , 1999), and indirectly (by empirical correlation) on shear modulus and shear strength. NDT techniques Iike sonic tests can be applied for masonry qualifi­cation, to eventually estimate its elastic modulus in

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5000

~ 4000

~ ~ 3000 '5 "ti ~ 2000

E rl 1000 111 VI

o

)( X

• •

O

. )( Taggia

• Bajardo

I I

I

I

• I I

/ l7 I •

, /'~

L>4 v. ... ..- '. • 500 1000 1500

I I

J I

,

/ /

2000 2500

sonic velocity (mlsec)

Figure 20. Elastic modulus (by flat jack test) vs. sonic velocity measured in Taggia (grey line) and Bajardo (black li ne).

larger portion of the wall (provided a correlation has been found between fiat jack and sonic tests) and to detect the effectiveness of a repair intervention (e.g. injection of grouts, deep repointing, etc.).

Sonic tests and fiat jack tests were then carried out, together with the sampling of morta r and stones for laboratory characterization. In (Fig. 20), the results of sonic and fiat jack testing in two areas are shown as an example. A good correlation can be observed between the data and a substantial difference between the data corresponding to the two villages is evident. In particular, apart from the two upper crosses corre­sponding to the church, collected in Bajardo indicate a lower quality of the masonry examinecJ, which in fact correspond to civil bui ldings. Vice-versa, the data collected in Taggia, corresponding to a noble build­ing typology (a monastery), indicate a better quality ofthe masonry consisting ofa regular texture of stone blocks of various dimensions, with numerous wedges and big rounded stones that cross almost the whole wall thickness.

The mechanical characteristics of the materiaIs, together with the information collected on the con­struction techniques of the vertical and horizontal load-bearing elements and their connections can sub­sequently be used as a contribution for the vulnerabil­ity study.

6 VULNERABILITY STUOY

In addition to a detailed knowledge of the structure 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 ltaly is to take account of only a lim­ited number of modes of failure; some modes of fai lure are neglected implicitly assuming a strength capac­ity of certain structural typologies, after appropriate retrofitting measures. On the contrary, the possibility

I I I

Figure 2 1. Bajardo, vul nerabil ity analysis of lhe civil building.

of damage prediction is related to the knowledge ofthe highest number of possible mechanisms of progressive deterioration or sudden failure. 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 procedure has been already applied to the evaluation of the damage of churches (Ooglioni et aI. , 1994; Lagomarsino et aI. , 2001) and to the study ofsmall scale urban contexts in historic centres (Giuffre, 1993). The modelling of the structure behaviour and its safety assessment may be improved by an articulated knowledge ofthe building typology, the constructive techniques, the stratigraph­ical composition of the building, the characteristics of the materiais and the structure.

Starting from this information and from the recog­nition of different homogeneous blocks, a further de-composition of the construction needs being car­ried out, simplifying the complexity of the blocks resulted from the stratigraphical analysis ofthe volume into macro-elements, generally characterized by a ver­tical regularity (Fig. 21). The response of these parts ofthe building may be considered independent under the seismic actions, and characterized by individual collapse mechanisms.

7 DISCUSSION OF THE RESULTS ANO FUTURE OEVELOPMENTS

The frequent failure of retrofitting and prevention measures adopted for historical centres in Italy, mainly ma de of stone masonry buildings, suggested an integrated approach, aimed at better understand­ing the behaviour of constructions built in stone masonry as a composite material. The procedure starts from the historical investigation and the geometrical survey of each bui lding, proceeds with the study of

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the construction technique of the walls and the mate­rial characterization (on site and in laboratory), ending with the detection ofthe probable failure mechanisms ofthe structure, according to its disposition in plan and height (e.g. isolated or arrayed buildings). The multi­levei approach carried out by the authors seems to be useful for collecting important information from the point ofview ofthe structure and materiais behaviour. The results will be used in the future to define the buildings vulnerability, to calculate their safety levei and to choose the most appropriate retrofitting techniques.

REFERENCES

Binda, L. , Anzani, A., Baila, A. & Baronio, G. 2003a. A multi­leveI approach for damage prevention in seismic areas. Application to historie centres of the Western Liguria, in The Masonry Society, Proceedings of the Nintl! Norrh American Masomy Conference, Omnipress, Madison, Wisconsin, ISBN 1-929081-1 6-2, pp. 556-566.

Binda, L. , Anzani, A. & Fontana A. 2003b. Mechanical behaviour of multiple-Ieaf stone masonry: experimen­taI research , Keynote lecture, in MC Forde (ed. by), Proceedings of the International Conference Structural Faults and Repair, CO-ROM Engineering Technics Press, Edinburgh, ISBN 0-947644-53-9.

Binda, L. , Baronio, G. , Penazzi , O. , Palma, M. & Tiraboschi, C. 1999. Caratterizzazione di Murature in Pietra in Zona Sismica: Data-Base sulle Sezioni Murarie e Indagini sui Materiali , Proceed 9th Conv. Naz. Ing. Sismica, Torino, CO-ROM.

Binda, L. & Saisi , A. 2002. Conservazione degli Edifici Storici: Miglioramento Strutturale e Compatibilità con i Materiali e la Struttura Originaria, L'Edi/izia, n. 2, pp. 34-41.

Ooglioni , F., Moretti, A. & Petrini, V 1994. Le chiese e il terremoto. Dalla vulnerabi/ità constatata nel terremoto dei Friu/i ai mig/ioralllento an/isismico nel restauro, verso la po/itica di prevenzione. Trieste, ed. UNT.

Francovich, R. 1982. Restauro architettonico ed archeologia stratigrafica, in G. Pietramellara L. Marino, Contributi sul restauro archeologico, Ed. Alinea, Firenze.

Giuffré, A. 1991. Letture sulla meccanica delle murature antiche, Edizioni Kappa.

Giuffré, A. 1993. Sicurezza ecol1servazione dei cel1tri storici: il caso di Ortigia, Bari , Laterza.

Guerrieri, F. 1999. Manuale per la riabi/itazione e la ricostruzione postsismica deg/i edifici, ed. by, Dei, Roma.

Hendry, W. 1994. Aspects of stability and strength of stone masonry structures, Third International Masomy Confer­ence, Proceedings ofthe British Masomy Society, No. 6, March.

Lagomarsino, S. , Maggiolo, L. & Podestà, S. 2001. Vul­nerabilità sismica delle chiese: proposta di metodologia integrata per la prevenzione ed il rilievo dei danno in emergenza, Atti deI 90 Convegno Nazionale: L'illgegnera sismica in ltalia, Potenza-Matera, 9- 13 settembre, CO-ROM.

Mannoni , T. 1994. Caratteri costrullivi dell 'edi/izia storica, Sage, Genova.

Parenti , R. 1992. Fonti material i e lettura stratigrafica di un centro urbano: i risultati di una sperimentazione " non tradizionale", in Archeologia Medievale IX, 7-62, Ed. L'lnsegna dei Giglio , Firenze.

Penazzi , O. , Valluzzi , M.R. , Cardani , G. , Binda, L. , Baronio G. & Modena, C. 2000. Behavior of historie masonry buildings in seismic areas: lessons learned from the Umbria-Marche earthquake, Proceed. 12 IBMaC, Madrid.

Various Authors 1987. Bussana, rinascita di una cincl morta, Ed. De Agostin i, Novara.

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