1 INTRODUCTION

The conservation of historical centres located in seismic areas in South Europe is a very complexproblem due to the wide variety of involved aspects, such as the quality of the masonry, thebuilding structural assemblages, the economical and political implications in retrofitting large his-toric centres. In Italy, an attention to the conservation of old masonry buildings was paid afterthe Friuli (1976) and Irpinia earthquakes (1980). In fact, up to that time, the building culture de-veloped during the post world war reconstruction appeared to be scarcely interested in main-taining existing masonry buildings, when not of high historical or architectonic value.

At the beginning of the '80s, as a consequence of the mentioned seismic events, several stud-ies took place both in Italy and in South Europe, with the aim of knowing better the seismic re-sponse of masonry buildings and the vulnerability of historical centres.

Such studies were based on the methodological approaches of the current structural design, sopaying a limited attention to the characteristics of both the masonry and the structural assem-blage of the building. In fact, the interest to the definition of well defined structural schemes andprocedures of analysis had, as a consequence, that a scarce attention was paid to the severaluncertainties that are typical of old masonry buildings. This approach is reflected in the standardcodes, promulgated both after the Friuli and the Irpinia earthquakes, in which the prescribed me-chanical models for the global analysis to horizontal forces are based on the assumption of rigidfloor diaphragms and of an elastic-plastic behaviour of the shear walls with controlled ductility.Moreover, the wide variety of masonry types is narrowed to few classes, each one character-ised by a conventional strength.

A multilevel approach to the damage assessment and the seismicimprovement of masonry buildings in Italy

L. BindaDIS, Polytechnic of Milan, piazza Leonardo da Vinci 32, 20133 Milano, ItalyL. Gambarotta & S. LagomarsinoDISEG, University of Genoa, via Montallegro 1, 16145 Genova, ItalyC. ModenaDCT, University of Padua, via Marzolo 9, 35131 Padova, Italy

ABSTRACT: The prediction of damages on masonry buildings subjected to seismic actions re-quires a detailed knowledge of the structure from survey and investigations. The current practicein Italy is to take into account only a limited number among the modes of failure, which are theones suggested by the Italian Code for the design of the retrofitting intervention. Some failuremodes are neglected assuming an implicit resistance capacity of certain structural typologies orconsidering that the retrofitting measures are able to prevent from them. On the contrary, the ef-fects of the last earthquake in the central Italy have pointed out the limited efficiency of the mostin use retrofitting techniques. It follows that the possibility of predicting damages is related to theknowledge of all the possible mechanisms of progressive deterioration or failure, to be obtainedby means of an in-field damage assessment after an earthquake. A multilevel approach for theinspection is here proposed, which consists in the survey of the characteristics of the masonry(technological and mechanical) and of the seismic behaviour of the building (observation of thedamage mechanisms activated by the earthquake and check of the connections between struc-tural elements).

Coherently, the methodologies for the vulnerability analyses, aimed to estimate the awaiteddamage scenario in a region or an historical centre for a given earthquake, were based on a ty-pological approach. In the GNDT 1st level form (C.N.R., 1993) the survey of the building is lim-ited to the identification of the typology of the vertical elements (masonry), the floors, the roofand the stairs. The vulnerability of the building results from the insertion in a class, on the basisof the above mentioned typological parameters; for each class the awaited damage is describedin probabilistic terms, by means of damage probability matrices (Braga et al. 1982).

Although these requirements contributed in spreading the culture of conservation of masonrybuildings, through the repair and retrofitting carried out in the '80s and '90s, the basic attitude offitting the real structure to the reference model provided by the standard rules implied heavy ret-rofitting techniques. In many cases the systematic replacement of some existing structural ele-ments (e.g. wooden floors and roof replaced with concrete beams and floors) was carried out,independently on their efficiency, and the nominal strength of shear walls was increased by jack-eting and injecting techniques, without the awareness of the real quality of the original masonry.

More recently, in the '90s, several theoretical and experimental studies have been carried outon the seismic response of masonry buildings (see for example: Gambarotta 1996, C.N.R. 1995,Magenes et al. 1995); moreover, more detailed approaches have been established for the surveyof the characteristics of the masonry (Binda 1998) and for the identification of the real collapsemechanisms activated by the earthquake (Giuffrè 1993).

The relevant damage suffered by masonry buildings in the Umbria-Marche earthquake (1997-98) has confirmed the need for improving the knowledge on the seismic response of old masonrybuildings also with reference to the reliability of retrofitting techniques. In facts, the effects ofsuch events have shown in many cases both the adopted structural models to be not adequateand the retrofitting techniques applied after previous earthquakes in that area (Val Nerina, 1979)not providing the expected effects. Therefore, there is the need of approaching the problem byconsidering different complementary aspects of this kind of buildings (historical data, buildingprocesses, skill of the builder, material data, maintenance), with the aim of getting exhaustivetools for interpreting the structural response, in order to define reliable retrofitting techniques andprocedures to evaluate the seismic vulnerability of masonry buildings.

In this paper the authors propose a multilevel approach which tackles the problem of theknowledge of existing buildings by considering different levels of analysis: history, materials,structural morphology of the wall section, observed damage mechanisms, effectiveness of retro-fitting techniques (if any). As the methodology is not aimed to the monumental heritage but to theminor historical buildings, a detailed diagnostic approach to the single work is not essential; there-fore, the data are collected by the use of proper forms, which allows to manage and interpret theinformation in an efficient way. The procedure is aimed at balancing between rough and in depthvulnerability analyses, and must be considered at the moment as a first attempt in the definitionof tools for the damage assessment of masonry buildings.

A relevant phase consists in the survey procedure to inspect the internal composition of themasonry, which is investigated and classified with reference both to their constructive charac-teristics (i.e., by detecting the morphology of the section) and to chemical, physical and mechani-cal properties, by on site and laboratory tests. The results are collected in a data base, which al-ready contains similar surveys in other seismic regions in Italy. The other critical phase concernsthe qualitative identification of the damage process, through data on cracks, deformations, localor overall collapses, which is preliminary to the identification of a mechanical model. To this enda set of the most frequent collapse mechanisms has been elaborated and clarified by means ofan abacus of outlines. The final aim of the analysis would be the proposal of mechanical modelsable to interpret and forecast the observed damage modes. Even if the complexities and uncer-tainties that characterise the masonry structures could prevent to succeed quantitatively in suchgoal, nevertheless acceptable results could be considered if able to provide the sensitivity of thestructural response to the model parameters.

At present, this methodology has been applied to a population of masonry buildings in Umbria,in particular in the two small historical centres of Montesanto (in the municipality of Sellano) andRoccanolfi (in the municipality of Preci). The first results seem to be promising for the under-standing of the effects of this last event and, due to the homogeneity of the building characteris-tics in the regions of central Italy, some outcomes could be extrapolated.

2 A MULTILEVEL APPROACH TO THE ASSESSMENT OF VULNERABILITY

The analysis of the seismic response of masonry buildings in the small historical centres of It-aly needs: a) a proper knowledge of the traditional constructive techniques; b) the identificationof the damage and collapse mechanisms activated by the earthquake; c) the check of the effec-tiveness of the most applied retrofitting techniques. Only after a deep knowledge is reached, it ispossible to develop theoretical models which are able to simulate the seismic response of ma-sonry constructions and to intervene correctly to reduce the intrinsic vulnerability of the building,rather then transforming it in order to fit an abstract model.

The methodology here proposed, which has been applied to two small centres struck by therecent earthquake in Umbria (Montesanto e Roccanolfi), represents a first attempt to synthesisethe various knowledge of the authors and the available tools, in order to achieve a diagnosticdamage assessment.

The survey with the 1st level GNDT form is, in fact, too rough both in the definition of theconstructive typology and in the evaluation of the damage.

For the first aspect, a classification is made of the vertical elements, the floors, the roof cov-ering and the stairs, that, nevertheless, does not consider the different local characteristics andthe variability of the quality in each typology; for example, a three leaf stone masonry may pres-ent bad or good characteristics according to the texture of the stone elements and the link be-tween the external leafs. The vulnerability is then directly correlated to the typology, while it iswell known that analogous masonry buildings behave differently depending on the quality ofsome details (connection between orthogonal walls) and the presence of some devices (tie rods).

With regard to the damage assessment, the 1st level GNDT form is substantially based on thesurvey of the appearing damage in the elements of the building (vertical and horizontal), that isthe check of cracking or deformation phenomena. This assessment is very useful in the post-earthquake emergency, as it allows a preliminary estimate of the economic damage, essentiallycorrelated to the appearing damage. On the contrary, if the goal is the analysis of the struc-tural damage, that is the loss of functionality of the building with reference to the after shocksand to the self weights, it is necessary to address the survey to the identification of the seismicresponse mechanisms. To this end it is possible to make reference to previous studies in whichthe most recurrent collapse mechanisms have been pointed out and analysed (Giuffrè 1993).This approach should also help to define if the building is fit for the habitation, even if some otheraspects must be taken into account.

The research in the two small towns in Umbria (Montesanto and Roccanolfi) has been carriedout by a set of forms and tests at different levels of deepness, both with reference to the materi-als, the constructive techniques, the suffered damage and the seismic response.

For the materials and the technological aspects, the methodology, developed in the ambit of aresearch supported by the Tuscany Region (Binda 1998), is so arranged:I level: collection of general data on the building; survey of the different masonry textures in the

building; survey (if possible) of the composition of the masonry in the transversal section,with the analysis of the mortar and void rates;

II level: chemical and physical tests on the constituent materials of the masonry (mortar, stones,bricks); in situ tests for the evaluation of the mechanical properties (flat jack tests); labo-ratory mechanical tests.

The assessment of the seismic damage and the vulnerability analysis has been carried out bymeans of a new approach, derived by the methodology used in Umbria and the Marches for thedamage assessment to the churches (Lagomarsino 1998). Also in this case the analysis consid-ers two levels, with regards to the reconstruction of the history of the suffered damage and tothe information on previous interventions:I level: damage assessment in terms of collapse mechanisms in macroelements individualised in

the building or in the block, in the case of historical centres where there is an interactionamong buildings;

II level: detailed survey of the cracks pattern; collection of information on the recent retrofittinginterventions, with the acquisition of the design tables (plants, sections).

2.1 Survey of the masonry: wall sections and material properties

The first important aspect for the prediction of the seismic vulnerability of an historical buildingconcerns the knowledge of the quality and the intrinsic characteristics of the masonry.

Given the great number of existing cross sections, a systematic study on the mechanical be-haviour of multiple leaf stone-masonry should begin from an extensive knowledge of the differ-ent geometry and constructive techniques of the walls. Therefore a survey has to be carried outon: the texture of the wall, the number of leaves in its section, the connections which may or maynot be present between the leaves, the shape and dimension of the stones, the shape and dimen-sion of the joints.

Together with that survey, information should be given on the material and structural proper-ties, by carrying out a minimum number of tests on site and in laboratory.

2.1.1 Section morphologyThe survey proposed by the authors consists of a photographic and graphic procedure which in-cludes (Figure 1): (a) a picture taken on site, with a camera with the lens of 50 mm and using atripod, which ensures the parallelism between the plane of the photograph and that of the wall;(b) a metric survey of the section elaborating the photograph automatically or manually; (c) thecompilation of a special form, containing the main information, and the storage in a Data Base.

The elaboration of these information allows for: (i) detection of number of leaves and of theirdimension, which is useful for structural modelling, (ii) rate, size and distribution of voids, usefulfor the preliminary study of the wall in the case of repair by injection of grouts.

The cataloguing limited to the section morphology, was already proposed in previous researchand carried out on buildings of different Italian seismic areas (Binda 1998); in the case of seriousseismic damage the wall section could be easily surveyed.

A first elaboration of those data allowed to define the existence of at least four typologies ofmasonry as presented in Figure 2.

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Figure 1. Masonry wall section and voids calculation: a) photograph; b) survey of the section; c) hysto-gram of the components (stone, mortar and voids); d) cumulative distribution of the voids size in a planesection.

Figure 2. Some examples of stone masonry wall sections.

2.1.2 Material propertiesMore information are needed before deciding the technique for repair, in order to give right pa-rameter as input of the structural analysis and the material properties, for the use of compatiblematerials in the intervention. Successive experiences developed in Lunigiana and Garfagnana(within a Research Contract supported by the Toscana Region Building Department to the DIS-Politecnico of Milan, the University of Padua and the University of Florence) allowed to improvethe survey form. A minimum set of tests were chosen for the qualification of the masonry andthe quantification of some parameters (chemical, petrographical and mineralogical analyses onmortars and stone/bricks, physical and mechanical tests on stones/brick and mortars, in-situ flat-jack tests and NDT, as sonic). After that experience the forms were organised in the followingway (Binda 1998): 1) building general description with historical notes, 2a) wall description: ex-ternal surfaces, 2b) section description and elaboration, 3) laboratory and in situ tests (Figure 3).

Chemical, mineralogical-petrographical and physical analyses are useful to define the actualcomposition of the mortars together with the grain size and distribution of the aggregate. Me-chanical tests as compressive or tensile tests are very seldom possible on mortar due to the lowthickness of the joint. Mechanical and physical tests on stones and bricks help in defining theirproperties in the case of substitution or new insertions of materials to be compatible with the ex-isting ones (Figures 3a,b).

The masonry characteristics in most cases cannot, at the moment, be obtained with reliabilityindirectly from the ones of the single materials as it is suggested by the Italian Code (D.M. 1987)in the case of brick and stone masonry made with regular elements. The difficulty is due to thelack of appropriate constitutive laws in the case of highly inhomogeneous multiple leaf masonry.Experimental tests carried out on site with single and double flat-jack are at the moment the onlyavailable way to have quantitative values of some parameter as local compressive stress compo-nent, modulus of elasticity and Poisson ratio (Figure 3c).

Non Destructive Tests (NDT), such as sonic test, can be applied before and after the inter-vention (e.g. injection of grouts, deep repointing) to detect its effectiveness (Figure 3d).

2.2 The damage assessment in terms of collapse mechanisms

The analysis of the seismic behaviour of masonry buildings is not simple, both for the difficultyin modelling the materials (three leaves stone masonry) and for the complexity of this kind ofstructures, particularly when located in historical town centres, where buildings are connectedeach other and result from growths and transformations over the centuries. Even if the researchachieved in the last decade significant results (Gambarotta 1996, Gambarotta & Lagomarsino1997, Brencich et al. 1998), the knowledge of all the possible damage and collapse mechanismsmust start from a direct observation and interpretation of the effects of a real earthquake.

The reading of the damage mechanisms may be always carried out in a historical centre lo-cated in a seismic area, as the marks of the past earthquakes are never totally wipe out. In factpermanent deformations remain (walls that are out of plumb), the traditional techniques for therepair of masonry cracks are usually evident (unstitch and stitch) and the old improvement meth-ods are well identifiable with respect to the other structural elements (tie rods, buttresses).

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Figure 3. Forms for the collection of the properties of the masonry and its components. Analysis of themortar: a) mineralogical, chemical and granulometric analysis; b) physical and mechanical tests. In situtests on masonry: c) flat jack test; d) sonic test.

It is evident that the assessment of the damage in a seismic emergency represents a preciousopportunity for learning from earthquakes, as it is possible to acquire information that afterwardsmay be lost. Nevertheless, in this context it is difficult to carry out an in-depth study (Giuffrè1993), as the accessibility to the buildings is limited by the risk of further collapses and, anyway,in the seismic emergency it is necessary to survey many town centres in a short time. An exam-ple in which the diagnostic approach to the assessment of the structural damage has been ap-plied in a survey methodology applicable at a territorial level is that of churches (Lagomarsino1998). For this type of constructions it is possible to apply a macroelement approach, that is thesubdivision of the building in parts, which are almost independent from the structural point ofview, in the case of an earthquake; for each macroelement the possible collapse mechanisms areconsidered.

In this paper an analogous methodology is proposed for the ordinary buildings, both isolatedand inserted in the urban context. In the case of buildings the concept of macroelement losesmeaning, as the great variability of this structures (shape of the plant, number of floors, etc.)does not allow the singling out of recurrent typologies. Nevertheless, it is possible to recognise aset of local and global collapse mechanisms, traceable to in-plane or out-of-plane seismic actions.The damage assessment follows the same layout used for the churches; in fact for each mecha-nism it must be indicated: a) the presence of element in which the mechanism should occur; b)the damage level (1 - negligible to slight damage; 2 - moderate damage; 3 - substantial to heavydamage; 4 - very heavy damage; 5 - destruction); c) the intrinsic vulnerability of that part of thebuilding to that mechanism, through indicators linked to specific construction weaknesses. Figure4 shows an example of the form, related to the mechanism of overturning of a facade of thebuilding.

1. OVERTURNING OF THE FACADE •

DETACHMENT OF THE FACADE FROM THE ORTHOGONAL WALLS • • • • •

• Poor clamping between the facade and the orthogonal walls • Lack of chains or efficient buttresses

Figure 4. An example of damage assessment in terms of collapse mechanisms.

The outcome of the assessment made in terms of mechanisms is not fully objective, as it doesnot consist in the survey of the cracks but in the interpretation of the seismic structural behav-iour, so evaluating the severity of the damage. However, the damages have been framed in awell defined set of possible mechanisms and this limits the scattering of judgement. Particularlyhelpful is the schematic illustration of the mechanisms by means of an abacus, with the aim tolead the technician in the survey; in fact sometimes the cracks which are less evident result to bevery important, as even a light damage warns of the vulnerability of the building.

The methodology is based on a preliminary subdivision of the building aggregate into elemen-tary cells, characterised by homogeneity in the geometry and the construction; in the case of anisolated building, it is considered as a single cell, unless constructive discontinuities or planimetricirregularity are clearly present.

The following damage mechanisms are considered for the single cell:1. overturning of the front walls2. local overturning in the upper part of external walls3. overturning of a corner;4. hammering of beams or sliding between r.c. tie beams and the masonry;5. collapse of the outer leaf of the masonry;6. widespread shear failure of external walls;7. shear failure in the piers (weak floor mechanism);8. failure of the lintels (global overturning mechanism of the piers);9. cracks in presence of discontinuity in the masonry (closed openings, chimney pipes);10. shear failure mechanisms in the internal walls;11. failure of architraves in the internal walls;12. cracks and failure mechanisms in masonry vaults;13. damage and collapse of the staircases;

14. unthreading of the beams and other damages in the floors;15. damage in the roof covering;16. overturning of standing out or overhang elements (balcony, eaves, chimney pot).

Once the survey of all the cells has been carried out, if a complex building aggregate is con-sidered, the interaction mechanisms between the adjacent cells must be analysed, consisting of:1. hammering between adjoining buildings;2. mechanisms due to planimetric irregularity;3. mechanisms due to altimetric irregularity;4. hammering due to the offset between the levels of adjacent floors.

Likewise what has been done for churches, it is possible to define a mean damage index in thecell, weighting the contribution of the different mechanisms on the base of the number of dam-aged elements with respect to the whole; for example, in the case of four front walls, the over-turning of each one weighs 1/4 in the evaluation of the index. By considering the interactionmechanisms, it is possible to define a mean damage index in the building aggregate.

The vulnerability is also defined by the check of some particular devices or constructive de-tails, which are identified as decisive for each single damage mechanisms.

Figure 5 shows the schematic drawings which are representative of some of the proposeddamage mechanisms; they are part of the abacus which helps the technician in the survey. Somecomments are needed, as the considered damage mechanisms are typical of stone masonrybuildings and have been observed in Umbria and in the Marche. In mechanism 4 cracks appearat the interface between the tie beam and the masonry, both at the floor and the roof levels. Inmechanism 9, the walled up windows represent a lack of continuity in the masonry, in whichcracks appear. In the interaction mechanism 2, the torsional effect due to planimetric irregularityof the plant induces larger shear deformation in the end walls, the ones that are far from the ro-tation centre. Finally, in the interaction mechanism 3, the different dynamic behaviour of thelower and the higher parts of the building leads to cracks due to overturning of the upper part orsliding, if there is a plane discontinuity in the masonry (r.c. tie beam).

2.3 Survey of the previous retrofitting interventions

Most of the masonry buildings in the area stricken by the 1997 earthquake were already dam-aged in 1979, under the effects of a seismic motion of similar intensity, and were successivelysubjected to repair/strengthening interventions (in some cases not yet finished when the lastearthquake occurred). This makes even more difficult, but interesting too, the classification andinterpretation of the failure mechanisms, which are always very complex in ancient masonrystructures.

In order to understand the effectiveness or the lack of the most in use retrofitting techniques,a qualitative assessment in terms of mechanisms is not sufficient but it is necessary to survey thecrack patterns and to collect information on the intervention. To this end, simple forms have beenused, which are similar to the ones presented in Figure 3.

3. Overturning of a corner 4. Sliding between r.c. tie beams and the masonry

5. Collapse of the outer leaf of the masonry 6. Widespread shear failure of external walls

7. Shear failure in the piers8. Failure of the lintels

9. Cracks in presence of discontinuity1. Hammering between adjoining buildings

2. Mechanisms due to planimetric irregularity 3. Mechanisms due to altimetric irregularity

Figure 5. Some of the sketches in the abacus of the collapse mechanisms considered for the buildings.

Figure 6. Crack patterns in the plants (indicated by two parallel segments) and the front views of thebuilding, after the Val Nerina earthquake (1979): the hatched areas are those that collapsed.

Figure 7. The design of the intervention after the Val Nerina earthquake (1979), consisting in r.c. ringbeams (hatched areas), r.c. floor slabs and grout injections: a) ground floor; b) first floor; c) second floor.

The next figures show, as an example, an isolated masonry building in Montesanto (Sellano -PG), which was damaged by the earthquake in 1979 (Figure 6), then repaired (Figure 7) anddamaged again by the last earthquake (Figure 8). The retrofitting consisted in reinforced con-crete ring beams at the foundation level and at the floor levels, executed with a breach in themasonry; the vault at the first floor was strengthened with a concrete cape and the originalwooden floors were substituted by r.c. slabs. Finally, grout injections were made in the masonry,with a deep repointing of the outer leaf. It is interesting to note that the damage mechanismshave been completely different in the two cases, so proving that the interventions changed theseismic behaviour but were not able to prevent from damage.

Figure 8. Crack patterns in the assonometric and the front views of the building, after the last earthquake(1997): the main horizontal cracks correspond to a twisting rotation of the raised part of the building.

3 SOME CONSIDERATIONS ON THE OBSERVED FAILURE MECHANISMS

One of the main characteristics of this earthquake, with reference to the damages in masonrybuildings, is that some new failure mechanisms have been observed, besides those that were re-peatedly recorded after previous earthquakes (namely in Friuli, in 1976; in Irpinia-Basilicata, in1980). Moreover, the damage assessment after this earthquake is particularly interesting as itrepresents the first occasion to check the actual efficiency of the modern intervention tech-niques, proposed in the last two decades.

The interventions were made in order to “retrofit” all (damaged and undamaged) the existingbuildings, i.e. to make them as safe as the new buildings designed and constructed according tothe current seismic code. Such scope was intended to be attained essentially by:a) substituting the original timber floors with reinforced concrete ones;b) constructing r.c. beam ties in the wall thickness at every floor level;c) jacketing or injecting the walls in order to improve their shear strength.

As it is well known, the first two type of interventions are intended to improve the structuralresponse of the building: type a) by ensuring the “rigid floor” action and type b) by connectingintersecting walls in order to prevent out of plane failures; the r.c. tie beams are also intended toincrease the overall strength of shear walls, as they can increase the strength of the existing ma-sonry spandrel beams and ensure the equilibrium even after shear cracks appear in the masonrypiers and spandrel beams. Interventions of type c) can obviously increase both the in-plane shearstrength and the out of plane flexural strength of masonry walls. The term “can” is intentionallyused referring to the effects of the interventions, as no real definitive proofs exist till now of theiractual efficiency in the real, in field, conditions they are in the practice applied. Neverthelesssome research have already been carried out in the past on the effectiveness of grout injections(Modena et al. 1997, Binda et al. 1997) and of jacketing (Modena & Bettio 1994, Bettio et al.1996, Modena et al. 1998).

The debate is still open in fact on such crucial issues like:§ How to select the proper technique taking into account the actual properties of the structure?§ How to determine the relevant properties in order to select the intervention technique?§ How to control the effectiveness of the interventions?

In this occasion particular attention was paid to some of the less known and/or less investi-gated mechanisms. It is very clear in fact that even an advanced knowledge of the fundamentalfailure mechanisms – i.e. the in-plane shear and the out of plane flexural failures, on which theresearch efforts of the last twenty years were substantially concentrated – is a too poor tool forinterpreting the actual behaviour of the structures under considerations when subjected to signifi-cant (i.e. capable to produce damages) seismic actions.

The proposed methodology can actually contribute to a more appropriate, even if qualitative, atleast in this phase, evaluation of the possible occurrence of major damages other then in-planeshear and out-of-plane flexure of the walls. These are for example:§ the collapse of the outer leaf of the multiple leaf stone masonry walls;§ local collapses, due to local geometrical of mechanical inhomogeneities;§ failures due to inadequate application of strengthening techniques, which can be do to inade-

quate choice and/or inadequate execution.Typical examples of such types of failures are as in the following. In Figure 9, the out of plane

collapse of the outer leaf of multiple leaf stone masonry walls is represented. This can happen inboth unstrengthened (Figure 9a) and strengthened walls, in this second case both when r.c. tiebeams were inserted into the wall (Figure 9b) and when the injections were used (Figure 9c).Without entering into much details, and being aware that the phenomenon must be studied be-sides the simplifications which can be used at this stage of research, it seems to be very clearthat no knowledge exists. Consequently no efficient ways has been found to predict and coun-teract such an apparently very secondary aspect of the seismic response of the examined build-ings. In any case, no study and design decisions can be carried on without a precise knowledgeof the inner composition of the masonry. What seems for the moment useful for the strengthen-ing interventions it is not to avoid the future occurrence of this local failure mechanism, butmaybe its evolution into much more severe effects. The phenomenon is in some cases very lo-calised, but in some others it seems capable to trigger progressive collapse mechanism leading toeven possible global failures.

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Figure 9. Out of plane collapse of the outer leaf of multiple leaf stone masonry walls: a) collapse in an un-strengthened masonry (Sellano); b) collapse of the outer leaf under to the r.c. tie beam (Cesi); c) failure ofan injected wall (Montesanto).

Examples of what local inhomogeneity can cause in a masonry building are shown in Figure10. Masonry piers failed due to inappropriate use of “modern” units, to unintentional reduction ofthe resting section of the inclined strut or hidden weakness of a horizontal section of the wall. InFigure 10a a pier has beam substantially weakened by inappropriately laying, with horizontal per-foration, non structural clay units; by a metal case was inserted where lifelines (in addition, elec-tricity, gas, etc.) enter into the building. In Figure 10b a sliding of a pier was caused by inappro-priate dimensioning of the pier itself combined with the very poor capacity of hollow concreteunits laid down without any grouting. Finally, the change in the door and windows distributionsmakes it difficult to avoid substantial weakness caused by lack of mechanical continuity betweeninfilling and existing masonry (Figure 10c). Again, nothing can be predicted on the real responseof the building without precise checks of the geometry and of the composition of masonry walls.

The failures due to inadequate application of the interventions are the most difficult to inter-pret, what actually caused and how developed the failures themselves and how serious weretheir consequences on the overall response of the building structure. In Figure 11 it is shownhow, unfortunately most frequently, rigid r.c. slabs and tie beams inserted into the wall thicknesswere not capable to prevent the out of plane collapse of the masonry. Some details visible in theupper part of the picture seem to suggest that the intervention contributed to reduce the alreadyweak cooperation between the leaves of the masonry in the very critical section where the wallsare connected to the floor. In fact in those connections the confining actions of the floors are ap-plied and most probably less uniformly distributed. Even the contribution of the internal wall, per-pendicular to the collapsed facade was completely missing in the collapse, possibly due to therestoration interventions.

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Figure 10. Failure due to local inhomogeneity in the masonry: a) inappropriate use of perforated, non struc-tural clay units (Isola); b) sliding of a pier with very poor hollow concrete units (Isola); c) lack of mechani-cal continuity between infilled and existing masonry (Sellano).

Figure 11. Out-of-plane collapse of a wall, in presence of a r.c. tie beam and a rigid floor (Sellano).

However, how serious is to be considered the, complete failure of a single wall, while the restis substantially kept in shape? Similar question (i.e. how grave are the consequences?) is to beraised considering the very frequent local failures of jacketed walls, almost always very clearlyconnected to poor detailing. Examples are shown in Figure 12, representing failures respectivelydue to insufficient steel mesh overlapping (Figure 12a) and insufficient transversal ties confiningaction (Figure 12b) (Modena & Bettio 1994, Modena et al. 1998).

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Figure 12. Failure in masonry with jacketing (Isola): a) insufficient steel mesh overlapping; b) insufficienttransversal tie configuration.

4 CONCLUSIONS

A new methodology is presented to understand the seismic behaviour and to assess the damagein historical masonry buildings, by means of in field inspections after an earthquake. It consists ofa multilevel approach aimed to acquire information on the technological, chemical, physical andmechanical properties of the masonry, on the collapse mechanisms activated by the earthquakeand on the previous strengthening interventions. All the information is collected into properforms, in order to apply the analysis in a town centre or at territorial level.

In order to make a reliable prediction of the response under strong earthquakes and to appro-priately select the proper intervention for the seismic improvement, precise and sophisticatedtools are needed to inspect, test and analyse the existing masonry types.

An important aspect is to learn from the earthquake, which is to correctly interpret the seismicbehaviour of the building, by means of a diagnostic damage assessment. To this end a new pro-cedure is proposed, which is based on a set of damage and collapse mechanisms, typical of thebuildings.

The application of the proposed method to some buildings in Umbria, after the earthquake of1997, has pointed out some new failure mechanisms, which appears strictly related to the char-acteristics of the masonry building of the central Italy and to the retrofitting interventions of thelast two decades.

For this fact, particular attention has been devoted to the acquisition of data on those interven-tions. The collected data form the basis for further analysis, in order to achieve a deep knowl-edge on the seismic behaviour of masonry buildings and on the actual effectiveness of the mostin use interventions in seismic areas.

The proposed methodology is only a first step in the definition of reliable and effective set offorms for the damage assessment in the emergency after an earthquake and for the analysis ofthe vulnerability of historical masonry buildings.

5 REFERENCES

Bettio, C., Modena, C. & Riva, G. 1996. The Efficacy of Consolidating Historical Masonry by Means of In-jections, Proc. of Seventh North American Masonry Conference, Notre-Dame, Indiana, June 1996.

Binda, L., Modena, C., Baronio, G. & Abbaneo, S. 1997. Repair and Investigation Techniques for StoneMasonry Walls, Construction and Building Materials, Vol. II, n. 3, pp. 133-142.

Binda. L. 1998. Sperimentazione di Tecniche d’Intervento di Miglioramento Strutturale su Edifici in Mura-tura nei Centri Storici: Caratterizzazione Meccanica delle Murature in Pietra della Lunigiana e VerificaSperimentale dell’Efficienza delle Tecniche di Intervento per la Riparazione e il Consolidamento degliEdifici in Muratura , Final Report of a Research Contract with the Tuscany Region.

Binda, L., Palma, D. & Penazzi, D. 1999. Investigation Before and After the Intervention for a Cautious Re-pair of Stone. Masonry Structures in Seismic Areas, Proc. Int. Conf. in Karlsruhe, 1998 (to appear).

Braga, F., Dolce, M. & Liberatore, D. 1982. A statistical study on damaged builidings and ensuing reviewof MSK-76 Scale (in Italian), In: Terremoto in Irpinia del 23 Novembre 1980 (Capitolo 5), ProgettoNazionale Geodinamica (CNR) , Roma: Ed. Scientifiche Associate.

Brencich, A., Gambarotta, L. & Lagomarsino, S. 1998. A macroelement approach to the three-dimensionalseismic analysis of masonry buildings, Proc. of the XI European Conference on Earthquake Engineer-ing, Paris, Sept. 1998, A.A. Balkema (Abstract Volume & CD-ROM): p. 602.

C.N.R. 1993. Assessment of the exposure and the seismic vulnerability of buildings: Instructions for mak-ing out the 1 st level form, Appendix 1 to "Seismic Risk of Public Buildings" (in Italian). Roma: NationalResearch Council - National Group for the Defence from Earthquakes.

C.N.R. 1995. Experimental and numerical investigation on a brick masonry building prototype: Numeri-cal prediction of the experiments. G.N.D.T. Report 3.0, Pavia.

D.M. LL.PP. 1987. Norme tecniche per la progettazione, esecuzione e collaudo degli edifici in muratura eper il loro consolidamento. G.U. n. 285 suppl., 5/12/1987.

Gambarotta L. (Ed.) 1996. The Mechanics of Masonry Between Theory and Design (in Italian), Proc. of aConf. in Messina, Sept. 1996, Bologna: Pitagora.

Gambarotta, L. & Lagomarsino, S. 1997. Damage models for the seismic response of brick masonry shearwalls. Part II: the continuum model and its applications, Earthquake Engineering and Structural Dy-namics, 26: 441-462.

Giuffrè A. 1993. Safety and Conservation of Historical Centres: The Case of Ortigia (in Italian). Bari:Edizioni Laterza.

Lagomarsino S. 1998. A new methodology for the post-earthquake investigation of ancient churches,Proc. of the XI European Conference on Earthquake Engineering, Paris, Sept. 1998, A.A. Balkema(Abstract Vo lume & CD-ROM): p. 67.

Magenes, G., Kingsley, G.R. & Calvi, G.M. 1995. Static testing of a full-scale, two-story masonry building:test procedure and measured experimental response. in Numerical prediction of the experiments, Ex-perimental and numerical investigation on a brick masonry building prototype, CNR-GNDT, Report 3.0,Pavia, pp. 1/1-41.

Modena, C. & Bettio, C. 1994. Experimental Characterization and Modeling of Injected and Jacketed Ma-sonry Walls”, Proc. Italian-French Symposium Strengthening and Repair of Structures in SeismicArea, Nizza, October 1994, pp. 273-282.

Modena, C., Binda, L. & Anzani, A. 1997. Investigation for the Design and Control of the Repair Interven-tion on Historical Stone Masonry Wall, Proc. 7th Int. Conf. and Exhibition, Structural Faults + Repair97, Edinburgh, Vol. 3, pp. 233-242.

Modena, C., Zavarise, G. & Valluzzi, M.R. 1998. Modeling of Stone Masonry Walls Strengthened by RCJackets, in Computer Methods in Structural Masonry-4 , Proc. Intern. Conf., Florence, Sept. 1997,E&FN SPON, pp. 285-292.