seismic resistance of masonry buildings in historic urban and rural nuclei: lessons learned in...

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Seismic Resistance of Masonry Buildingsin Historic Urban and Rural Nuclei:Lessons Learned in SloveniaMiha Tomaževič aa Slovenian National Building and Civil Engineering Institute,Ljubljana, SloveniaPublished online: 09 Jun 2011.

To cite this article: Miha Tomaževič (2011) Seismic Resistance of Masonry Buildings in Historic Urbanand Rural Nuclei: Lessons Learned in Slovenia, International Journal of Architectural Heritage:Conservation, Analysis, and Restoration, 5:4-5, 436-465, DOI: 10.1080/15583051003792898

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International Journal of Architectural Heritage, 5: 436–465, 2011Copyright © Taylor & Francis Group, LLCISSN: 1558-3058 print / 1558-3066 onlineDOI: 10.1080/15583051003792898

SEISMIC RESISTANCE OF MASONRY BUILDINGSIN HISTORIC URBAN AND RURAL NUCLEI: LESSONSLEARNED IN SLOVENIA

Miha TomaževicSlovenian National Building and Civil Engineering Institute, Ljubljana, Slovenia

In the past three decades, significant research has been conducted on seismic behavior ofmasonry buildings in historic urban and rural nuclei. On the basis of the earthquake dam-age observations and subsequent experimental simulations of the observed mechanisms,technical measures for the improvement of the seismic resistance have been proposed andmethods for seismic resistance verification developed. By in situ and laboratory testing, thebasic mechanical properties of the existing and strengthened masonry have been determined.The research results have been used to analyze the requirements of Eurocode regardingthe design methods and parameters for structural assessment and retrofitting of buildings.Modifications have been proposed that would lead to more realistic assessment of seismicresistance in the case of historic masonry buildings. For this purpose, the seismic behaviorof buildings, subjected to design-level earthquakes for the second time in just a few decades,has been analyzed and correlated with the actually observed earthquake damage.

KEY WORDS: heritage masonry buildings, seismic resistance, assessment, redesign, stren-gthening methods, code requirements

1. INTRODUCTION

In the region of Abruzzo, Italy, the April 6, 2009, earthquake (magnitude [M] = 6.3,intensity IX–X by EMS scale) was certainly not the last seismic event to confirm thatmasonry buildings in historic urban and rural nuclei are vulnerable to earthquakes. Theearthquake repeatedly showed that, when reconstructing and renewing the historic build-ings, it is of relevant importance that the basic requirements for improving the seismicresistance be taken into consideration seriously and the works be executed professionally.

On the basis of the earthquake damage observations and subsequent experimen-tal laboratory and in-field investigations, various technological solutions to improve theseismic resistance have been proposed and numerical models for seismic resistance ver-ification of heritage masonry buildings have been developed. It is not the aim of thisarticle to present a state-of-the-art report regarding the recent developments in the field.The discussion will be limited to the contribution of Slovenian National Building and CivilEngineering Institute, ZAG, in Ljubljana, Slovenia, aimed at understanding and improvingthe seismic behavior of this kind of building.

Received 22 October 2009; accepted 17 March 2010.Address correspondence to Miha Tomaževic, Slovenian National Building and Civil Engineering Institute,

Dimiceva 12, 1000 Ljubljana, Slovenia. E-mail: [email protected]

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SEISMIC RESISTANCE 437

The buildings in urban and rural areas, considered today as architectural cultural her-itage, are masonry buildings, which for centuries have been subjected to continuous processof modification and change. Additional stories have been added to original buildings andnew annexes built in the backyards and spare spaces between the individual houses alongthe streets. The buildings have been continuously adapted and reconstructed to follow theneeds of inhabitants. Therefore, if the historic parts of urban settlements are to remain thevital parts of the urban structure, the improvements and accommodations of buildings tocontemporary living and safety standards should not be hindered by simply classifyingthem into the category of cultural heritage. Despite the fact that engineering demands fre-quently lead to conflicts with preservation and restauration requirements, especially if thebuildings are located in seismic-prone areas and technical interventions in their structuralsystems are needed to improve their seismic resistance. Consequently, a compromise isneeded between the engineering demands, code requirements, the needs of inhabitants,technical possibilities, costs, and other issues, in one regard, and the requirements ofconservators and restauratorsin another.

In this regard, Slovenia has learned a lesson in 1976, when a series of earthquakeswith epicenters in Friuli, Italy, severely damaged the region of Posocje (Soca River Valley),the western-most part of the country. After the first earthquake in May of 1976 (M = 6.5,local intensity VIII by EMS scale), no such compromise could have been found regardingthe structural repair and strengthening of buildings of the nearest-to-epicenter village ofBreginj, one of the most known examples of Slovenian rural architecture. Being severelydamaged, but still repairable after the first earthquake, most buildings in the village col-lapsed or have been damaged beyond repair when subjected to subsequent two earthquakesof the same intensity in September of the same year (M = 5.6 and M = 6.1, local inten-sities VII–VIII by EMS scale) (Figure 1). Important part of Slovenian rural architecturalheritage has been lost forever.

Figure 1. Photograph of an important part of Slovenian rural architectural heritage in Breginj and vicinity hasbeen lost during Friuli earthquakes of 1976 (photo by M. Tomaževic) (color figure available online).

INTERNATIONAL JOURNAL OF ARCHITECTURAL HERITAGE 5(4–5): 436–465

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438 M. TOMAŽEVIC

Namely, simple technical measures, whose timely application would have preventedheavy damage and collapse of historic buildings in Breginj, have already been known inSlovenia. In 1974, 2 years before the Friuli earthquake, a local earthquake of moderateintensity (M = 4.8, epicentral intensity VII by EMS scale) damaged several hundreds ofstone masonry houses in Kozjansko, a poor rural region in northeastern part of Slovenia.During post-earthquake rehabilitation of the region, a simple method of retrofitting thistype of buildings by means of the tying the walls with steel ties and strengthening thewalls with cement injections has been proposed and its efficiency experimentally verified(Tercelj et al. 1976; Boštjancic et al.1976; Vugrinec 1977).

Subsequently, 2 years after the 4.8-M local earthquake in 1974, the simple-to-executeand cost-effective method proved to be efficient not only in the laboratory, but also inthe case of a real building subjected to a real earthquake. The building, located in theepicentral area of Friuli earthquakes of 1976 in Bardo-Lusevera, Italy, has been severelydamaged during the earthquake in May 1976, but was strengthened before the September1976 events (Figure 2). The strengthened building survived the earthquakes in Septemberwithout structural damage. If timely application had occurred for damaged buildings in thevillage of Breginj, the method would have most probably prevented severe losses.

In the years that followed, the research carried out in Slovenia has been aimed at pro-viding the practical guidelines and information needed for the efficient and cost-effectiveredesign of this particular type of building. Having the opportunity to correlate the actualearthquake damage with the predictions of calculations, practical proposals and recom-mendations have been prepared to ensure adequate seismic behavior of such building ifproperly applied. Some of these proposals and recommendations will be briefly presentedand discussed here.

Figure 2. Photograph of a house in Bardo-Lusevera, Italy, was severely damaged during May, 1976 earthquake.Strengthened in August, it survived the September shocks of the same intensity without any additional damage(photo by E. Vugrinec) (color figure available online).


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2. REASONS FOR SEISMIC VULNERABILITY AND IMPROVEMENT

OF SEISMIC RESISTANCE

Since the masonry structures have generally not been conceived to resist earthquakeloads, masonry buildings in historic urban and rural centres in Slovenia are vulnerable toearthquakes. The basic structural elements, masonry walls, arches, vaults, and floors areaimed at carrying the vertical loads. They are built in materials and systems that resistthe compression, but not bending and shear. In this regard, multi-leaf stone masonry isespecially critical, because the lack of bond between the layers frequently results into thedelamination and disintegration and subsequent collapse of the walls when subjected toin-plane and out-of-plane seismic loads. The connection between the structural elementsof historic buildings is adequate for gravity loads, but the elements that would ensure themonolithic behavior of the structure when subjected to seismic loading, such as wall tiesand rigid floors, are frequently missing. In addition to those missing elements, the buildingsare sometimes badly maintained and their structural materials subject to rapid deteriora-tion due to the effects of time and environment. This poor maintainance and deteriorationinevitably results into severe degradation of resistance to seismic loads.

Masonry buildings are typical box-type structures, so that structural integrity shouldbe ensured to utilize the available resistance of structural walls. If the walls are not wellconnected, they separate at vertical joints and the walls orthogonal to the direction of earth-quake motion start vibrating out of their planes. In some cases, vertical cracks in the wallsdevelop due to out-of-plane bending as a result of large spans between the bracing walls. Inthe other cases, however, parts of such walls or the walls as a whole overturn and collapsebecause of the loss of stability (Figure 3).

Structural regularity, such as uniform distribution of structural walls in both direc-tions and along the height of the building, is also needed to achieve adequate seismicbehavior. In the case of historic houses, the wall/floor area ratio is relatively high and thedistribution of load-bearing walls in both orthogonal directions is uniform. In this regard,historic houses generally fulfill the requirements for structural regularity. However, theoriginal adequate layout has been many times modified during the reconstructions in therecent past. Especially in the cities, new stories have been added to buildings and largeparts of structural masonry walls have been removed in the ground floor to make placefor shops and arcades along the streets. Since the removed parts have not been replacedwith load-bearing elements, compatible in terms of resistance and deformability with theremoved parts of masonry, and adequately connected with the remaining structural system,these buildings are extremely vulnerable to earthquakes. Heavy damage and even collapseof such buildings when subjected to design level earthquakes is almost inevitable.

Besides adequate structural integrity and layout, strong, sufficiently resisting wallsare required that are able to carry the seismic loads induced in the building during theearthquake and transfer them to the foundation system and soil. As a result of mechani-cal properties of masonry and characteristics of structural system, shear forces and shearresistance govern the seismic behavior of old masonry buildings. Typically, diagonally ori-ented shear cracks develop in structural walls (Figure 4). In the case of the multi leaf stonemasonry, walls may delaminate and disintegrate; especially if the duration of the earth-quake is long (Figure 5). Although such buildings are usually built without any specificfoundation (Figure 6), the damage that might be attributed to foundation failure is rare.However, the loss of stability of foundation soil, such as land-sliding or soil liquefaction,frequently resulted into heavy damage of the upper structure.


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440 M. TOMAŽEVIC

Figure 3. Photograph of the partial collapse of a gable-end wall due to out-of-plane bending in Bovec, Slovenia,1998 (photo by M. Tomaževic) (color figure available online).

Figure 4. Photograph of diagonally oriented shear cracks in a stone-masonry house in Umbria, Italy, 1997 (photoby M. Tomaževic) (color figure available online).


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Figure 5. Photograph of disintegration of stone-masonry walls in Posocje, Italy, 1998 (photo by M. Tomaževic)(color figure available online).

Figure 6. Photograph of stone foundation of a rural church before strengthening (photo by M. Tomaževic) (colorfigure available online).


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To improve the resistance and reduce seismic vulnerability, the observed deficienciesshould be remedied. To ensure the integrity and box-type seismic response of the building,structural walls should be adequately connected with wall ties, whereas the floors shouldbe strengthened to ensure floor diaphragm action and anchored into the walls for uniformdistribution of seismic loads onto resisting walls. If necessary, structural layout is improvedby filling the openings in the existing walls, or building new walls on new foundations,adequately located in plan of the building and connected with existing floors. Masonry,compatible with the existing one as regards the mechanical properties, should be used, andnew masonry walls or infills should be adequately bonded with the existing masonry.

Various technologies are available for strengthening the walls, depending on therequired level of improvement and masonry type. Load-bearing capacity of foundationsand foundation soil should be verified especially in the cases where the resistance capacityof the strengthened upper structure is significantly improved with regard to the original one.The increased level of ultimate loads induced in the strengthened upper structure requiresimproved load-bearing capacity of the foundation system.

3. REDESIGN

Usually, historic buildings are retrofitted within the framework of post-earthquakereconstruction campaigns, after being already damaged by earthquakes. Only seldom suchbuildings are systematically strengthened before the earthquakes, in order to avoid possi-ble future damage. Technically, there is little difference between pre- and post-earthquakestrengthening. Maybe the post-earthquake strengthening is more reliable, since the earth-quake already indicated the critical points in the structure, which need to be strengthened.In the case of the pre-earthquake strengthening, however, critical points are determined bystructural assessment.

Obviously, redesign means repeated design of the structure, which had alreadybeen designed according to technical standards and codes of the time of construction,to meet the standards and requirements of the present. The interventions to improve theload-bearing capacity of the existing structure are designed and the resistance of thestrengthened structure is verified by calculation. Usually, when redesigning the structure,the resistance is calculated twice: before the intervention, in the original state to verifythe necessity of strengthening, and then after the strengthening to verify the success ofintervention.

In most cases, cultural heritage buildings have not been designed. They have beenbuilt by considering handicraft rules, obtained on the basis of tradition and experience,without any calculations to verify their resistance and stability. This does not modifythe procedure of redesign, which is the same as in the case of the previously designedbuildings. However, because no technical drawings and calculations are available, all datashould be obtained by measurements and structural assessment; whereas in the case of thealready designed structures, the existing data should only be verified by a reduced numberof control measurements.

3.1. Structural Assessment and Determination of Mechanical Properties

of Materials

To reliably assess the seismic resistance of historic buildings, site investigationsare inevitable, including visual inspection and in situ and laboratory tests of structural


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materials. By correlating information, obtained by different testing methods, uncertain-ties can be limited. In addition to inadequate seismic resistance, structural repair andstrengthening may be needed due other reasons. In the case of uncertainty, the structure issometimes monitored for a longer period of time before the final decision is made regardingthe most adequate type of structural intervention.

Depending on the information obtained by inspection and testing, in Eurocode 8-3,which provides the requirements for the assesment and retrofitting of buildings (Eurocode8 2005), three knowledge levels are defined, which determine the admissible method tobe used for structural analysis and the reliability of values of mechanical properties ofmaterials, used as the input in the calculations. The latter is determined by the so calledconfidence factors, CF, which are used to reduce the mean values of mechanical propertiesof materials, taken into account in the redesign:

� Knowledge level KL1: limited knowledge, CF = 1.35;� Knowledge level KL2: normal knowledge, CF = 1.20; and� Knowledge level KL3: complete structural knowledge, CF = 1.00.

A detailed definition of each knowledge level is given in the code.When assessing the structure, besides traditional techniques to open the walls with

both hand tools and modern machines, various non-destructive methods are available bymeans of which the state and structure of the masonry can be assessed and the valuesof a number of physical parameters determined. Using modern non-destructive techniques,humidity, presence and type of salts and other unfavourable materials in masonry, corrosionpotentials, presence of metal parts, such as ties, can be detected, among other elements(Suprenant 1994; Onsiteformasonry 2006).

However, if the mechanical properties need to be determined, non-destructive meth-ods do not provide useful information. In such a case, the destructive or semi-destructivetesting of masonry walls in the laboratory or in situ is needed. Laboratory tests of the con-stituent materials are not enough. Flat jacks are used to obtain information on the localcompressive stresses in the walls and compressive strength of masonry. However, to obtaininformation on the lateral load bearing capacity, which is usually defined by the diagonaltensile strength of the masonry, ft, either diagonal compression or lateral resistance tests ofwalls are needed. In some cases, the specimens of adequate size (such as 1.0/1.0 m) are cutin the building, transported into the laboratory and tested there by either diagonal compres-sion or a combination of constant vertical load and cyclic shear. In other cases, however,the specimens are simply tested in the building. In the latter case, part of the shear wall tobe tested is separated from the surrounding masonry. As the specimen is already bearingthe vertical loads, only the lateral load needs to be applied (Figure 7). Depending on theactual situation and availabity of the testing equipment, cyclic or monotonic static tests canbe carried out.

The parameter, which defines the shear failure of unreinforced masonry walls, char-acterized by diagonally oriented cracks developed in the walls, is called the tensile strengthof masonry. The tensile strength of masonry is a conventionally defined parameter, repre-sented by the idealized principal tensile stress, σt, developed in the wall panel subjected toa combination of vertical and horizontal load, at shear failure. To evaluate the value fromexperiments, elastic, homogeneous and isotropic behavior of the wall panel all the way upto the attained maximum value of horizontal load, Hmax, is assumed. The idealized prin-cipal tensile stress at that instant, σ t, the tensile strength of masonry, f t, is expressed by(Turnšek and Cacovic 1971) in Equation (1):


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Figure 7. Photograph of in situ lateral resistance test of a stone masonry wall—Pišece Castle Workshop,Onsiteformasory (photo by V. Bosiljkov) (color figure available online).

ft = σt =√(σo

2

)2 + (bτmax )2 − σo

2, (1)

where:

– f t is the tensile strength of masonry;– σ o = N/Aw is the average compressive stress in the horizontal section of the walls due

to actual vertical load N;– τmax = Hmax/Aw is the average shear stress in the horizontal section of the wall at

maximum horizontal load Hmax; and– b is the shear stresses distribution coefficient, depending on the geometry of the wall

and lateral load/vertical load ratio at maximum resistance,

In case that the wall is tested as fixed at the bottom and top and the lateral loadis applied at the mid-height, which is the ususal way of how the walls are tested in-situ(Figure 7), only half of the applied load should be taken into account when evaluating thetensile strength. Cutting of sufficiently large specimen out of the wall and safe transporta-tion to the laboratory is usually more complicated that the in situ test. Usually, the wallsare first tested in the original condition, strengthened, and then retested. Typical values ofmechanical characteristics of historic masonry, needed for structural verification, obtained


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Table 1. Mean values of the compressive strength, f , and modulus of elasticity, E, of stone masonry in theexisting (Exist) and cement-grouted state (adapted from Turnšek et al., 1978; Tomaževic et al., 1979; Tomaževicet al., 1980; Sheppard and Tomaževic, 1986; Tomaževic et al., 2000)

Injected Injected

Masonry type State Tests, n f (MPa) Exist. E (MPa) Exist.

Kozjansko: lime-stone; lime mortar with muddysand non-homogeneous wall

Exist. 2 0.51 1.9 1970 4.2Inj. 1 0.97 8250

Montenegro: lime-stone; lime mortar with pure sandnot plastered

Exist. 3 0.33 6.1 390 6.7Inj. 3 2.00 2610

Montenegro: lime-stone; lime mortar with pure sandplastered

Exist. 2 0.77 3.1 3210 ∼1.0Inj. 2 2.33 2940

Bovec: lime-stone; lime mortar with muddy sand;non-homogeneous; residential houses

Exist. 1 0.98 — 2660 —Inj. – — —

Table 2. Mean values of the tensile strength, ft , and shear modulus, G, of stone masonry in the existing (Exist)and cement-grouted state (adapted from Turnšek et al., 1978; Tomaževic et al., 1979; Tomaževic et al., 1980;Sheppard and Tomaževic, 1986; Tomaževic et al., 2000)

Injected Injected

Masonry type State Tests, n ft (MPa) Exist. G (MPa) Exist.

Kozjansko: lime-stone; lime mortar with muddysand non-homogeneous wall

Exist. 2 0.02 4.0 65 1.5Inj. 1 0.08 100

Montenegro: lime-stone; lime mortar with pure sandplastered

Exist. 6 0.10 2.5 87 1.8Inj. 6 0.25 154

Historic Ljubljana: mix of slate and lime-stone; limemortar with muddy sand; relatively homogeneous

Exist. 1 0.13 1.5 40 8.8Inj. 1 0.20 350

Historic Ljubljana: mix of slate, lime-stone andbrick; lime mortar with muddy sand; relativelyhomogeneous

Exist. 1 0.15 1.1 40 13.8Inj. 1 0.17 550

Bovec: lime-stone; lime mortar with muddy sand;non-homogeneous; residential houses

Exist. 1 0.06 1.8 80 2.1Inj. 1 0.11 170

Bovec: lime-stone; lime mortar with muddy sand;non-homogeneous; public buildings

Exist. 2 0.08 2.5 170 2.4Inj. 2 0.20 400

by the in situ and laboratory testing of different types of masonry walls, are summarized inTable 1 and Table 2.

3.2. Numerical Models

The numerical model used for the seismic resistance analysis of historic masonrybuildings should reflect the actual seismic behavior of the structure under consideration. Inthe case of residential buildings with regular structural configuration, the models developedfor earthquake resistance verification of modern masonry structures can be used, providingthat the basic assumptions of such models, such as rigid floor diaphragm action, are fulfilledalso in the case of the analyzed historic building. Otherwise, the models should be modifiedin consideration of the actual structural behavior.

In the case of monumental buildings with complicated structural layout, finite ele-ment methods can be used. However, it should be borne in mind that, by using elastic


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446 M. TOMAŽEVIC

finite element models, only the potential weak points can be identified, where the structureis prone to damage because of concentration of stresses. Elastic models cannot pro-vide reliable information regarding the actual seismic resistance and seismic behavior.Since the use of the non-linear models for the analysis of seismic response of masonrystructures is time consuming and requires specific skills, these models are used onlyexceptionally.

In the case of the heritage masonry buildings with regular structural configuration,where measures have been taken for the tying of the walls and rigid floor diaphragm action,shear mechanism and shear resistance of the walls govern the seismic behavior. Becauseof specific relationships between the moduli of deformation at compression and shear,shear deformations prevail. Consequently, the stifnesses of resisting walls are proportionalto their horizontal section areas and do not significantly depend on the boundary con-ditions. A relatively simple story mechanism model can be used for seismic resistanceanalysis. Based on these assumptions, a pushover method for seismic reistance verifica-tion of such buildings has been developed at the time of the Friuli earthquakes of 1976(Tomaževic 1978). Although besides the shear also the flexural resistance of the walls hasbeen taken into consideration, the assumption that the resisting walls act as fixed-ended ele-ments, has been taken into account. The method, based on the calculation of the resistancecurve of the critical story (Figure 8), has been included into official regional recommen-dations for the repair of earthquake damaged buildings in Friuli (DT 2 1977). In the yearsthat followed, the method has been modified to be used also for modern masonry struc-tures (Tomaževic 1998). On the basis of the idea proposed in 1976, the method has beensubstantially improved by other investigators (Magenes et al. 2000; Galasco et al. 2002).

For seismic vulnerability studies of existing buildings, where the walls are not tiedwith steel ties and connected with floor diaphragms, out-of-plane vibrations are critical,which cause separation and subsequent local failure of walls, located orthogonal to the

Figure 8. Illustrations of the construction of story resistance curve—outline of the procedure. Adapted withpermission from Tomaževic (1998).


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Figure 9. Illustrations of the typical mechanisms of partial out-of-plane collapse. Adapted with permission fromD’Ayala and Speranza (2002).

main seismic motion. Typical possible mechanisms of partial collapse of historic buildingsare shown in Figure 9. To estimate the resistance of the building, acceleration values thatcause the separation of the assumed portion of the building, are calculated (Giuffrè 1989;D’Ayala 2002). In the calculations, actual mechanical properties of the masonry are takeninto consideration. Several possible mechanisms are verified: the critical elelement, whichdetermines the seismic resistance of the building as a whole, is the mechanism in which theratio between the acceleration, causing the mechanism, and acceleration of gravity attainsthe minimal value.

4. SEISMIC RESISTANCE: CODE DEMANDS AND POSSIBILITIES

The level of strengthening of existing buildings depends on the acceptable level ofseismic risk. European standard for the assessment and retrofitting of buildings, EN 1998-3(Eurocode 8 2005) requires that the existing buildings be strengthened to achieve the samelevel of seismic safety as the new construction. However, this requirement is many timesnot acceptable without rough violation of the basic principles of preservation and restora-tion of architectural cultural heritage. Because of numerous architectural specifics andartistic value of buildings (e.g., the type and materials of construction, paintings and deco-rations on the walls) the selection of the type of intervention, which is in accordance withthe standards for preservation and restoration of architectural cultural heritage, but wouldensure the stability and resistance of the monument in the case of an earthquake, at thesame time, is extremely limited.


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Anyway, as in the case of the newly designed buildings, two requirements should berespected regarding the seismic resistance of the existing strengthened buildings:

� no collapse requirement, and� damage limitation requirement.

Taking these two requirements into consideration, two limit states need to be verified.Namely, ultimate limit state and damage limitation limit state.

4.1. Design Seismic Loads

According to European standard for earthquake resistant design of structures,Eurocode 8 (Eurocode 8 2004), the structure should be designed to withstand the earth-quake with return period 475 years and 10% probability of exceedance in 50 years,“without local or global collapse, thus retaining its structural integrity and a residualload bearing capacity after the seismic events” (no collapse requirement). However, thestructure shall be also designed to withstand an earthquake having a larger probability ofoccurrence than the design earthquake, i.e. earthquake with return period 95 years with10% probability of exceedance in 10 years, “without the occurrence of damage and limi-tation of use, the costs of which would be disproportionately high in comparison with thecosts of the structure itself” (damage limitation requirement). Although the definition ofthe design earthquake is slightly different, the requirements, specified in the standard forthe assessment and retrofitting of buildings, EN 1998-3, are similar.

Depending on the structural characteristics and importance of the building, differentmethods can be used for the verification of limit states. Non-linear dynamic response (timehistory) analysis is used in specific cases of the most important structures. In the usualdesign practice and regular structures, however, either linear-elastic methods, such as lat-eral force method and modal response spectrum analysis, or non-linear static pushover typemethods are used. In the case where the linear-elastic methods are used, the ductility andenergy dissipation of the structure are taken into account implicitly by reducing the elas-tic seismic loads with the so called structural behavior factor, q. In such a case, only thedesign resistance of the structure and structural elements is compared with the design seis-mic shear and action effects, respectively. In the case of masonry structures, very limitedredistribution of action effects from more to less loaded elements is permitted.

In the case of the push-over methods, where the resistance curve of the structure iscalculated on the basis of the mechanism models and by taking into account the redistri-bution of lateral loads to structural walls, the first step of verification is the same as in theprevious case: the calculated resistance of the structure is compared with the design seis-mic shear. However, in addition to that, ductility and displacement capacity of the structureshould be verified and compared with the code demands.

Displacement (ductility) and energy dissipation capacity of the structure representthe critical parameters when deciding upon the level of the design loads in the case of theseismic resistance verification. To define the level of displacement capacity, which can beutilized in the design, damage limitation requirement should be taken into consideration.The structure should not be designed for the design seismic loads, which would causeexcessive damage to structural system.

On the basis of a number of tests of masonry walls and models of masonry buildingssubjected to simulated seismic loads, the correlation between the damage to structuralwalls and limit states has been analyzed (Tomaževic 2007). Damage classification,


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recommended by the European Macroseismic Scale (EMS-98), has been used (EuropeanSeismological Commission 1998). According to this classification, grade 1–2 damage(negligible to slight structural damage) and grade 5 damage (collapse, very heavystructural damage) determine the limit states of the initiation of damage (crack limit)and collapse, respectively. Grade 3 damage (moderate structural damage) can be definedas repairable, hence representing the acceptable damage limit, whereas grade 4 damage(heavy structural damage) can be assumed to be beyond repair. By correlating the amountof damage, limit states, and corresponding interstory drifts, it can be seen that grade3 damage occurs at the attained maximum resistance, or soon after that. It can be alsoseen that the values of interstory drift at the attained crack limit and maximum resistanceare relatively close together. The study indicated, that in the case of the prevailing shearbehavior, typical for most masonry construction systems when subjected to seismic loads,interstory drift corresponding to damage level between grade 3 and grade 4 representsthe adequate measure of damage limitation to be considered in the design as well as inthe redesign. If the design displacement capacity of the structure, subjected to the designearthquake, is limited with interstory drift where the acceptable damage occurs, there is noneed that the structure be checked for the damage limitation limit state. The requirementsfor damage limitation will be fulfilled automatically.

The relationship between the seismic resistance of the critical story, R, and interstorydrift, which is defined as the ratio between the relative story displacement, d, and storyheight, h: Φ = d/h (usually, it is expressed in % of the story height), is schematicallypresented in Figure 10. For the purpose of design and evaluation of design parameters,such as ductility capacity, μ, and structural behavior factor, q, the actual resistance curveis idealized with a bi-linear relationship.

Figure 10. Graph of the story resistance–interstory drift relationship with indicated limit states. Adapted withpermission from Tomaževic (2007) (color figure available online).


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According to Eurocode 8, design ultimate state is defined by the point on theresistance curve where resistance degrades to 80% of the maximum. However, in thecase of masonry structures, damage to structural walls at this point is already beyond theacceptable limit. Tests and experiments show that in the redesign, displacement (ductility)capacity of masonry structures should not be utilized beyond the interstory drift, equal tothree times the drift at the occurrence of the first cracks in structural walls. Consideringthe resistance curve, the design ultimate state may be therefore defined by either interstorydrift value where the resistance degrades to 80% of the maximum, or interstory drift value,equal to three times the value of interstory drift at the crack limit, whichever is less inEquation (2):

Φdu = min{Φ0,8Rmax; 3Φcr

}, (2)

where:

� Φdu is interstory drift (rotation) at design ultimate limit state;� Φ0,8Rmax is interstory drift (rotation), where the resistance degrades to 80% of the

maximum; and� Φcr is interstory drift (rotation) at the occurrence of the first cracks, crack limit.

Experiments and tests have shown, that the order of magnitude of the interstory drift,attributed to characteristic limit states, does not depend significantly on the type of masonryand construction system. Average measured values and standard deviations are given inTable 3. As can be seen, there is significant scattering of values of interstory drift at col-lapse. However, as only part of the displacement capacity of the structures is taken intoaccount in the design, the scattering of these values is not relevant.

The relationships between the resistance of the critical story and interstory drift,measured during the shaking table tests of a series of models of historic stone and brickmasonry buildings with and without wall ties, are shown in Figure 11 (Tomaževic et al.1993; Tomaževic et al. 1996). The resistance is given in a non-dimensional form of theseismic resistance coefficient SRC = R/W (ratio between the measured base shear, R, andweight of the building, W). Similar relationships have been observed by other investigators(Benedetti and Pezzoli 1996).

In the case of higher buildings (three to four stories high) in historic urban centresin high seismicity zones, the requirements of contemporary Eurocodes are difficult to befulfilled by applying the methods, acceptable from the point of view of preservation ofarchitectural cultural heritage. For such buildings, especially monumental buildings, codedemands would require unacceptable structual alterations. Although explicit reduction ofthe seismic action for design purposes is not possible, the standard recommends that, if

Table 3. Mean values of interstory drift at limit states (adapted from Tomaževic, 2007)

Interstory drift Standard deviationLimit state Damage level Φ = d/h (in %) σΦ (in %)

Crack limit Grade 2 0.30 0.15Maximum resistance Grade 3 0.61 0.41Design ultimate state Grade 3–4 1.00–1.20 —Collapse Grade 5 3.29 2.09


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Figure 11. Graph of the base shear–interstory drift relationships measured during the shaking table tests ofmodels of historic masonry buildings. Adapted with permission from Tomaževic et al. (1993) and Tomaževicet al. (1996) (color figure available online).

necessary, the resistance of the structure be evaluated on the basis of its displacementcapacity. In other words, the increase in values of the behavior factor q is possible, howeverwithout allowing excessive damage to structural elements.

By analyzing the seismic behavior of historic stone masonry buildings in townsand villages, which suffered from earthquakes in the past few decades (Tomaževic et al.1979; Tomaževic et al. 1980; Lutmanet al.; Klemenc 2000), it can be seen that they sur-vided the earthquakes with only moderate damage, although their calculated resistancedid not meet the code demands. Taking into consideration fact that relatively high dis-placement capacity of such buildings has been observed experimentally (Figure 11), itcan be concluded that the elastic seismic loads can be reduced by taking into consid-eration values of the behavior factor, q, at the upper limit of the recommended range(q = 1,5−2,5).

Such reduction of elastic seismic loads does not influence the safety of the build-ing against collapse. As a consequence, only a slight increase of damage when subjectedto design earthquakes can be expected, which, however, will remain within the limitsof the acceptable damage. The conclusion has been verified by analyzing the non-linearseismic response of several stone masonry buildings to recorded ground accelerationtime history of the Bovec earthquake of 2004 (Tomaževic et al. 2005; Tomaževic andLutman 2007). The comparison between the design seismic shear, required by Eurocode8 (BSCd), and the proposed reduction (BSCd,R), is presented in Table 4. The designseismic shear is expressed in the non-dimensional form of the design base shear coeffi-cient, i.e. the ratio between the design base shear, BSd, and the weight of the building,W (BSCd = BSd/W).


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Table 4. Design seismic load in dependence on allowable displacement and ductility capacity (factor q) fordifferent seismic zones

Based on: Design ground acceleration ag 0.05 0.10 0.20 0.25

EC8-3 Behavior factor q 1.5 1.5 1.5 1.5BSCd 0.08 0.17 0.33 0.42

Possible reduction Increased behavior factor qR 1.5 1.5 1.8 2.0BSCd,R 0.08 0.17 0.28 0.31

4.2. Design Values of Mechanical Properties of Materials

As recommended by EN 1998-3, mean values of mechanical properties of materials,determined either on the site or by testing specimens taken from the existing structure, andnot characteristic values as in the case of the newly designed structures, are considered inthe redesign. However, the mean values are reduced with the so called confidence factor,CF, the value of which depends on the thoroughness of inspection of the building andreliability of data needed for structural evaluation. Nevertheless, the standard requires thatbesides confidence factor, CF, partial safety factors for material, γ M, be also taken intoaccount to calculate the design values of material strength as shown in Equation (3):

fd = f

CFγM, (3)

where:

� f d - the design value of material strength;� f - mean value of material strength, determined by testing;� CF - confidence factor, depending on knowledge level (KL); and� γ M - partial material factor for masonry, as specified by Eurocode 6-1.

According to Eurocode 6 (Eurocode 6 2005), the values of partial factor for masonry, γM,depend on the production control and inspection of works on the site. In normal situation,the values within the range from 1.5 (optimum production control and severe inspection onthe site) to 3.0 (no proof regarding the production control and inspection) are considered.In seismic situation, the choosen value can be reduced by one third, however in no case γM

should be smaller than 1.5.There is no reason that besides the confidence factors, CF, the partial safety factors

of materials, γM, be considered in the calculations. Speaking of structural safety, it is notpossible to assess uncertainties regarding the values of mechanical properties of materials,depending on factory control and inspection at the site, as in the case of the new construc-tion. For the purpose of redesign, the actual structural materials are tested and the actualvalues of mechanical properties are determined. Moreover, as there has been no factoryquality control and inspection on the construction site at the time of construction of her-itage buildings, γM = 3.0 should be considered in normal and γM = 2.0 in seismic situation.Consequently, only one half of the mean value of masonry strength, obtained by testing theactual materials, can be considered in redesign—at the best.

Since the in-situ lateral resistance tests of walls are relatively expensive, only a lim-ited number of specimens is usually tested on the site. Although the mean values, obtained


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by testing, are taken into consideration according to EN 1998-3, there is no recommenda-tion regarding the number of specimens to be tested. By taking into consideration the usualscattering of results, obtained by the in-situ testing of the same type of existing masonry(±20%), it can be recommended, that in the case where at least two specimens are tested,the mean value of the test results is taken into account, ft = f t,m. However, if only one testresult is available, the value, reduced by considering the expected scattering of test results,should be considered: f t = f t/1.2.

The value of the confidence factor CF to be considered in the redesign depends on thethoroughness of inspection and the number of tests which have been carried out to assessthe state of the structure and material properties. According to EN 1998-3, CF = 1.35is recommended in the case of the knowledge level KL1, CF = 1.20 in the case of theknowledge level KL2, and no reduction, CF = 1.00 in the case of the knowledge levelKL3, i.e., complete structural knowledge.

Past experiences and recent studies indicate that good correlation between the cal-culated results and observed behavior of heritage buildings can be obtained in the casewhere the resistance of the building is assessed by using the shear mechanism models andaverage, unreduced values of material properties, obtained by tests, as input data in thecalculations. On the basis of the results of these studies, it is proposed that, in the redesign,the requirements of Eurocode regarding the confidence factor CF be modified as follows(Tomaževic and Lutman 2007):

� CF = 1.0: mechanical properties of masonry are determined either by in-situ tests orin the laboratory by testing specimens, taken from the building under consideration. Atleast one specimen of the specific masonry type should be tested in the building andthe composition of the masonry should be verified by removing plaster at least in onelocation in each story. The value is corresponding to knowledge level KL3;

� CF = 1.35: mechanical properties are obtained by testing at least one specimen in thecluster of buildings of the same typology. Identification of a given type of stone-masonryis carried out by removing plaster and opening the walls at least in one location in eachstory of the building under consideration. The value is corresponding to knowledge levelKL2; and

� CF = 1.7: no testing. The values of mechanical properties are taken from the literaturefor masonry type, corresponding to the masonry type under consideration. Identificationinspection only is carried out. The value is corresponding to knowledge level KL1.

As will be shown, the requirements of EN 1998-3 regarding the design values ofmaterial properties do not reflect the actual situtation. Whereas they are too conserva-tive when introducing partial factors for materials, the requirements are too optimisticas regards the recommended values of confidence factor. As the results of the in-situtests indicate, the mechanical properties of historic masonry, especially stone masonry,significantly depend on the local tradition, local materials and the way of construction(Tables 1 and 2). If the mechanical properties of the particular building under considera-tion are not determined by testing, its seismic resistance can be either underestimated oroverestimated.

In the case where the seismic resistance of the building is determined by diagonaltension failure of the walls, the resistance of the building, calculated by taking into accountthe confidence factor CF = 1.2, will be approximately 10% lower than the resistance,calculated without any additional safety (CF = 1, 0). It will be approximately 25%–30%lower, if calculated by considering confidence factor CF = 1.7. In the case of the diagonal


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tension failure, the resistance of the walls and buildings is not directly proportional withthe reduction factors, but is depending also on the ratio between the compressivs stressesin the walls and the tensile strength of masonry.

4.3. Design Resistance: Code Requirements and Reality

In the case of historic buildings with regular structural layout and where the out-of-plane mechanisms are hindered by the wall ties and floor diaphragms, shear mechanismprevails. In the case where the diagonal tension failure of the walls determines the seismicresistance of the building, the resistance of an individual wall can be calculated by Turnšekand Cacovic (1971) in Equation (4a):

Rs,w = Awftb

√σd

ft+ 1, (4a)

or, by introducing the design parameters according to Eurocodes, as shown inEquation (4b):

Rds,w = Awft

CFγM

1

b

√CFγM

ftσd + 1 (4b)

where:

� Rds,w is the design shear resistance of the wall;� Aw = t l is the cross-sectional area of the wall;� ft is the tensile strength of masonry (Eq. 1); and� σ d is the design compressive stress in the horizontal section of the walls due to design

vertical load Nd.

To compare the requirements of the code with the observations after earthquakes andthe results of experimental investigations, the seismic resistance of a number of heritagestone masonry buildings, damaged by the earthquake of Bovec of 1998 and strengthenedafterwards by tying the walls with wall ties and injecting the walls with cementitious grout,has been analyzed (Lutmanet and Klemenc 2000). Seismic resistance of buildings has beencalculated by using the push-over method, mentioned in section 3.2. Shear mechanism,Equation (4) and mean values of material properties (Tables 1 and 2) without reductionhave been considered as the main parameters in the original analysis.

The results of this analysis are summarized in Table 5. The values are given in thenon-dimensional form in terms of the seismic resistance coefficient SRC, i.e. the ratiobetween the resistance, R, and the weight of the building, W. To correlate the actual situa-tion with the requirements of the code, the calculations have been repeated by consideringthe design values of the tensile strength of the cement-grouted masonry, reduced by par-tial safety factor for masonry (γM = 2.0). Since the tensile strength of masonry has beendetermined by testing, confidence factor CF = 1.0 has been taken into account also in therepeated analysis. By comparing the actual situation with the requirements of the code, itcan be seen, that the calculations, where the code values of safety factors are taken into con-sideration, yield significantly lower values of the seismic resistance, than the calculationswith unreduced values of the tensile strength of masonry.


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Table 5. Seismic resistance of typical strengthened stone masonry buildings in Bovec in terms of theseismic resistance coefficient, calculated without and with reduction of the tensile strength of masonry(SRC = R/W)

Wall area(% of floor) f t,d = f t ft,d = ft

CFγM

Bldg. Stories, n x-dir. y-dir. f t (MPa) SRCu-x SRCu-y SRCu-x SRCu-y

1 2 12.0 9.1 0.19 0.25 0.26 0.13 0.142 2 10.9 6.4 0.19 0.28 0.23 0.15 0.123 2 9.7 12.0 0.11 0.34 0.47 0.22 0.314 2 6.9 8.6 0.11 0.25 0.33 0.16 0.215 2 12.1 11.1 0.11 0.42 0.38 0.27 0.256 2 4.7 14.6 0.11 0.19 0.47 0.12 0.317 2 7.2 14.3 0.11 0.21 0.47 0.14 0.318 2 15.1 13.7 0.11 0.40 0.33 0.26 0.219 2 10.5 9.5 0.11 0.39 0.29 0.25 0.19

10 2 10.5 9.9 0.11 0.31 0.34 0.20 0.2211 2 10.3 10.2 0.11 0.28 0.35 0.18 0.2312 2 11.9 10.3 0.11 0.29 0.34 0.19 0.2213 2 9.8 10.9 0.11 0.32 0.34 0.21 0.2214 2 8.8 8.33 0.11 0.31 0.33 0.20 0.2115 2 10.6 12.0 0.11 0.35 0.36 0.23 0.2316 2 7.9 4.2 0.11 0.35 0.21 0.23 0.14

Average 9.93 10.32 0.31 0.34 0.20 0.22σ 2.47 2.76 0.07 0.08 0.05 0.06C.o.V. 0.25 0.27 0.21 0.23 0.24 0.26

Since the shear behavior is predominant, the seismic resistance of the analyzed his-toric stone masonry houses is almost proportional to their wall areas in both orthogonaldirections. The deviations of resistance values are almost identical with the deviations ofthe wall areas.

In Table 6, the design seismic resistance of the average building, analized in Table 4,is compared with the design seismic loads. First, the average values of the design resistanceand design seismic shear have been calculated by strictly following the requirements ofEN 1998-3. Then, the average value of the design resistance has been calculated by con-sidering the unreduced mean value of the tensile strength of masonry, determined by

Table 6. Seismic resistance of an average strengthened heritage building in Bovec, calculated in accordance withthe requirements of EN 1998-3 and practical recommendations

Based on: Design ground acceleration ag 0.10 0.20 0.225∗ 0.25

EN 1988-3 Behavior factor q 1.5 1.5 1.5 1.5BSCd-EC 0.17 0.33 0.38 0.42SRCd-EC 0.21 0.21 0.21 0.21SRCd-EC/BSCd-EC 1.24 0.64 0.55 0.50

Recommendations Behavior factor qR 1.5 1.8 1.9 2.0BSCd,R 0.17 0.28 0.30 0.31SRCd 0.33 0.33 0.33 0.33SRCd/BSCd.R 1.94 1.18 1.10 1.06

Note: ∗Design ground acceleration in Bovec.


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testing, whereas the design seismic shear has been calculated by considering the valuesof behavior factor q, which are recommended in Table 4. It has to be stressed again, thatthe increased values of factor q are still within the range of values recommended by EN1998 for unreinforced masonry.

As can be seen, unrealistic conclusions regarding the effectiveness of strengtheninginterventions are obtained in the case where the resistance of the strengthened buildingis verified by taking into account the requirements of EN 1998-3. According to suchanalysis, the analyzed buildings in Bovec, damaged during the earthquake of 1998(M = 5.6, epicentral intensity VII–VIII by EMS scale) and strengthened afterwards,should be damaged again when subjected to repeated earthquake in 2004, where peakground acceleration of 0.47 g has been recorded close to the epicenter (M = 4.9, epicentralintensity VI–VII by EMS scale). However, the adequately strengthened buildings survivedthe earthquake undamaged or only slightly damaged. Non-linear dynamic response (timehistory) analysis of a number buildings to the recorded ground acceleration time history,which has been conducted after the earthquake, has also indicated (Tomaževic et al. 2005;Tomaževic and Lutman 2007), that the recommendations proposed in this paper are basedon a realistic ground.

5. SELECTION OF ADEQUATE STRENGTHENING METHODS

Seismic retrofit of buildings of historic value is part of a complex, multidisciplinaryprocess of preservation of architectural cultural heritage. As a result of their intended use,old masonry buildings in historic urban and rural nuclei represent a specific case, sincethe same level of living standards and safety should be ensured for their inhabitants andusers as in the case of the new construction. However, the interventions needed to achievesuch requirement are not always in line with the requirements of preservation. They rep-resent a compromise between engineering demands and available technologies, economicconsiderations and principles of preservation. As a result of such compromise, the require-ments of seismic codes cannot be always strictly fulfilled. Therefore, the principle “bettersomething than nothing” is many times followed when deciding about which technicalsolution to apply for strengthening the structure. Experiences indicate that even partialimprovements in most cases prevent the worse.

Numerous criteria should be considered when deciding if and how to retrofit thebuilding. The basic criteria are of technical nature. The type, location and amount of inter-ventions depend on the resistance of the building in the existing state, evaluated by the siteassessment and calculations. Structural type and quality of materials represent the mainparameters, upon which the decision is made regarding the method and technology ofintervention. Any information regarding the effectiveness of the selected method is of rel-evant importance in the decision-making process. In addition to technical, some generalcriteria, like costs of interventions with regard to importance of the building, availability oftechnology and skilled workmanship, duration of works and usability of building, amongothers, should also be considered. Last but not least, efficient quality control system andinspection should be available.

5.1. Structural Integrity

Integrity of structural response is needed to fully utilize the availabe resistance ofmasonry walls to dynamic seismic loads. Therefore, the tying of the walls of historic


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Figure 12. Illustration of the typical distribution of wall ties in plan of a rural stone masonry house. Adaptedwith permission from Tomaževic (1998).

buildings with wall ties represents the basic step of interventions in the structure forimproving the seismic resistance (Figure 12). In most cases, the tying of the walls issufficient for providing structural integrity, in the others, however, existing wooden floorstructures should be strengthened, anchored and connected with masonry walls.

The analysis of earthquake damage and experimental research already confirmed theeffectiveness of the simple tying the wall with steel ties, where the reinforcing steel barsare placed at the floor levels on each side of the walls, and anchored at the ends on steelplates. The bars are threaded at the ends so that they can be prestressed after placing andfixed with nuts. By a series of shaking table tests of models of stone and brick masonryhouses (Figure 13), the mechanism of action of the ties has been studied and ties designprocedures proposed (Tomaževic et al. 1993; Tomaževic et al. 1996).

Recent experiments indicated the possibility of replacing the steel ties with rein-forced polymer laminates. If confining the building with vertically and horizontally placedstrips, the resistance and displacement capacity of the models have been significantlyimproved (Figure 14) (Tomaževic et al. 2009). However, because of the great differencein strength and deformability characteristics in the case of traditional masonry and mod-ern polymer laminates, technological problems regarding bond and anchoring carbon fiberreinforced polymer (CFRP) strips need to be resolved before practical application.

Theoretically, the replacement of wooden floors with massive reinforced concreteslabs represents the optimum solution for improving the structural integrity. Horizontallyrigid slabs ensure good connection of the walls and prevent excessive out-of-plane vibra-tions. However, experimental investigations and post-earthquake observations indicate, thatthe replacement of wooden floors with rigid reinforced concrete slabs sometimes adverselyaffects the seismic response. Numerous cases have been observed where the inadequatelyanchored and connected new slabs caused severe damage to existing structural walls, byshearing the walls and causing the delamination of masonry (Figure 15a). The same kind ofmechanism has been observed also in the laboratory (Figure 15b). To prevent the negative


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Figure 13. Photograph of tie design recommendations have been prepared on the basis of the results of shakingtable tests of a series of: (a) stone and (b) brick masonry building models. The mechanism of action of wall tiescan be clearly seen (photo by M. Tomaževic) (color figure available online).

effect, new floor slabs should be sufficiently supported and adequately anchored into thewalls. This prevention is especially important in the case of the multi-leaf stone masonry,where the impact of the rigid slab might shear the wall in the horizontal direction. Asa result, the delamination of the wall and falling out of the outer layer of masonry, notsupporting the floor loads, can take place.

Observations and tests show that the replacement of wooden floors with rigid mono-lithic floors is not necessarily the optimum solution. The available resistance of the wallscan be fully utilized also in the case where the wooden floors remain in the building, how-ever, adequately anchored into the walls and stiffened horizontally, if necessary. Of course,if left in the building, the quality of timber should be adequate and the floors should meet allrequirements regarding the load-bearing capacity nad deformability for the intended use.

5.2. Strengthening of Stone Masonry Walls

Whereas several options are available for strengthening the brick masonry walls,like coating and repointing, the efficient method of strengthening of multi-leaf rubblestone masonry remains the injecting of cementitious grout into the voids in the wall. Theearthquakes, which repeatedly affected the regions, where the buildings have been alreadystrengthened after the previously occurred earthquakes, indicated, that the methods, effec-tive in the case of the brick masonry, do not have much effect when applied to the stonemasonry walls. Whereas the effect of injections has been tested, the effects of other meth-ods, such as repointing or coating the stone masonry walls with either reinforced cementcoating of fiber reinforced polymer (FRP) laminates, have not yet been systematically


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Figure 14. Photograph of the effect of confining the walls of brick masonry buildings with carbon fiber rein-forced polymer (CFRP) laminate strips: a) The model without wall ties disintegrated when subjected to moderateearthquake, whereas the model, confined with CFRP laminate strips, withstood the shaking of much strongerintensity without collapse (b). Adapted with permission from Tomaževic et al. (2009; photo from Archives ZAG)(color figure available online).

investigated. A comprehensive experimental program to study the effect of CFRP wrap-ping on the seismic resistance of stone and brick masonry walls is underway in Ljubljana.It is expected that the results of the study will be available in the near future.

Stone masonry walls of traditionally built houses in historic settlements in the coun-try are usually three-leaf masonry, with two outer layers of irregularly sized, rubble stoneswith an infill consisting of smaller pieces of stone bound together with lime mortar. Cutstone, or partly cut stone is used in the case of public and monumental buildings. Locallyavailable material, such as limestone and slate, can be found. Connecting stones that con-nect the load-bearing layers at regular intervals and improve the homogeneity of stonemasonry, are not frequently used. Regular cut stones are used for better connections of thewalls at the corners and wall intersections. Generally, mortar quality is poor, with muddysand in the countryside, and limited amount of lime added. In historic urban nuclei, stonemasonry is relatively homogeneous, mixed sometimes with bricks, without any distinctgap between the two layers. Stone masonry walls are thick, with thickness in residentialbuildings varying between 0.50–0.75 m, but exceeding 1.0 m in the case of monumentalbuildings, like castles, towers and churches. Of course the structure and composition ofthicker walls is different.

Injecting the suspension of bonding and filling materials under pressure into thenumerous voids of stone masonry walls is efficient, because the grout, which has filled


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460 M. TOMAŽEVIC

Figure 15. If not properly supported and/or anchored, rigid reinforced concrete slabs push out the walls.Photographs of: a) the case of a stone masonry building in Umbria, 1997; b) brick masonry building modelwith rigid slabs at the end of shaking table test (photo by M. Tomaževic) (color figure available online).

the voids, bonds the materials into a monolithic structure after hardening. The delamina-tion and disintegration of masonry when subjected to seismic loads are prevented and theintegrity of the walls is ensured, which significantly improves the seismic resistance. InSlovenia, the method has been applied for strengthening of stone masonry houses for thefirst time after the earthquake of Kozjansko in 1974.

The results of the in-situ tests show that, depending on the quality of masonry, theresistance of stone masonry walls can be significantly improved (Tables 1 and 2). Typicaleffect of improvement, obtained by the in-situ lateral resistance tests of a rural stonemasonry wall in Bovec, is shown in Figure 16. As can be seen, by injecting the cement-based grout, also the rigidity of the wall is increased. It is therefore recommended, that,when strengthening the building, not only the walls which have been indicated as weak bycalculation, but all walls in the critical story be injected. Otherwise, unexpected damagewhen subjected to future earthquake can occur as a result of unexpected distribution ofseismic loads.

Originally, the dry part of the grout mix consisted of 90% of Portland cement and10% of pozzolana to ensure the plasticity and injectability of the grout. Water is added tothe mix so that volumetric ratio between the dry part of the mix and water varies between1:1 and 1:0.9. Unfortunately, the use of cement has serious deficiencies. Foreign materialsthat change the original texture of traditional masonry are introduced by cement. Impuritiesin the cement that dissolve in the water may damage frescoes and other decorations, whichcan be frequently found on the surface of historic stone-masonry walls. Last, but not least,dampness may appear on the walls after injecting the cement grout.

These inconveniences are mainly due to capillary activity of the hardened cementgrout. The cracks and voids in the original masonry act as a barrier preventing the capillary


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Figure 16. Graph of the effect of strengthening of typical stone masonry wall with grouting. Adapted withpermission from Tomaževic et al. (2005) (color figure available online).

water transport, whereas the hardened cement grout, which has filled the voids after grout-ing, represents a capillary active fabric. In order to solve the problem, the capillary active(hydrophilic) cement grout fabric is replaced by a water-repellent (hydrophobic) one. Forthis purpose, water-repellent additives in the form of specially prepared inorganic salts orstearic acids, are added to the mix. During the hardening of cement, the organic moleculesof the additives are incorporated into the capillary system, what causes a high contact anglewith water and hinders water penetration and transport. To further improve the characteris-tics of the grout, part of cement is replaced by inert aggregates in the form of fine-grainedsands (Tomaževic and Apih 1993).

As can be seen, by changing the composition of cement mix by adding hydropho-bic additives and replacing cement with inert aggregates, the compressive strength of theoriginal cement mix is reduced significantly. In order to investigate the influence of thereduction of the grout’s strength on the lateral resistance, the energy dissipation capacityand the ductility of the grouted stone-masonry walls, a series of grouted walls have beentested under earthquake loading conditions (Figure 17). Although the compressive strengthof different grout mixes varied from 7 to 32 MPa, the variation had little influence on thelateral resistance of the grouted walls (Figure 18). The average value of tensile strengthof the set of eight specimens was 0.33 MPa, with standard deviation of 0.07 MPa andcoefficient of variation equal to 21%, which represents the usual dispersion of values ofmechanical properties of the tested type of masonry walls.

It is believed that the potential resistance of the wall is determined by the strengthof the original mortar, which transfers the external loads acting on the wall from stone tostone. As confirmed by visual inspection, the grout does not penetrate into the originalmortar. Consequently, the strength of the original mortar is not improved and, hence, thepotential resistance of the wall does not change, although the voids have been filled withstronger material. What has changed is the connection between the stones. By prevent-ing the separation, splitting and buckling of the load-bearing layers of the wall, the groutactivates the potential load-bearing capacity of the original masonry. The bonding mecha-nism of the injected grout has been recently confirmed and theoretically explained by otherinvestigators (Vintzileou 2006). The analogy with a masonry building, where the tying ofthe walls with steel ties prevents the separation of the walls and activates the potentialseismic resistance of the whole building, is obvious.


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462 M. TOMAŽEVIC

Figure 17. Photographs of the seismic resistance test of a stone masonry wall at ZAG (photo by M. Tomaževic)(color figure available online).

Figure 18. Illustration of the reduced strength of the hardened injection mix did not significantly affect theseismic resistance of stone masonry walls. Adapted with permission from Tomaževic and Apih (1993).

On the basis of the results obtained by this and similar studies, the conclusion canbe made that the composition of the grout mix can be designed for each particular type ofmasonry and for each particular problem to be solved. Locally available materials compat-ible with the original texture of historic walls can be used as a replacement of part of thecement in the grout in order to reduce the undesirable side effects to an acceptable level.In this regard, the use of lime seemed to be an obvious solution from the very beginning.However, the problems of injectability of mixes and hardening of the lime-based grouts,have not yet been resolved. Of course the composition of the mix should ensure injectabil-ity, i.e. capability of the grout to enter into masonry, fill the voids and bond the stones


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after hardening. Even in the case that experienced professionals are executing the works,testing is needed to verify the effectiveness of the procedure each time when a new type ofmasonry or new composition of the grout is to be injected.

6. CONCLUSIONS

Although a conclusion can be made that the basic principles of improving the seis-mic resistance, such as improving the structural integrity and resistance of load-bearingelements, did not change, significant development has been made in the last decades in allphases of the process of redesign and retrofit of historic buildings. New methods have beendeveloped for structural assessment, numerical models and techniques have made possiblereliable structural evaluation, new and improved technologies are available for structuralstrengthening. Moreover, even a special standard for the assessment and retrofitting ofbuildings has been prepared as part of the family of structural Eurocodes.

Observations and analyses of damage to buildings in historic urban and rural nucleiafter strong earthquakes have indicated that, by selecting adequate technical solutions andcarefully executing the works, the required degree of seismic safety can be achieved.The number of cities and villages, where the repeated earthquakes in the last decadeseither confirmed the effectiveness of interventions or indicated the errors made in theretrofitting process, is increasing. Although the seismic resistance verification in accor-dance with the rules of the code may indicate, that the seismic resistance is not sufficient,good seismic behavior of adequately retrofitted heritage buildings has been observed inmost cases. The research results have been used to analyze the requirements of the recentEurocode for structural assessment and retrofitting of buildings, as regards the seismicdemand and structural resistance. It has been found that the requirements of the code wouldsometimes require unacceptable structural interventions. On the basis of the observedseismic behavior, experimental simulation and actual mechanical properties of masonrymaterials, obtained by in-situ and laboratory testing, modifications have been proposed,which would lead to more realistic demands for architectural heritage buildings. The pro-posed modifications do not reduce the generally required safety against collapse, but willslightly increase the level of the expected damage. To confirm the proposal, the seismicbehavior of buildings, subjected to design level earthquakes for the second time in justa few decades, has been analyzed and correlated with the actually observed earthquakedamage.

Unfortunately, the number of settlements in seismic prone regions with historic her-itage buildings, vulnerable to earthquakes, is still large. Many of these buildings have beenalready renewed, however no technical intervention to improve their seismic resistance hasbeen carried out. Despite bad experiences after the earthquakes, despite the fact that theprocedures and technologies for improving the seismic behavior of heritage buildings havebeen developed and verified, and despite the fact that the costs of earthquake damage wouldbe significantly reduced if preventive rehabilitation of seismically vulnerable buildings betimely carried out, the systematic preventive actions are still lacking.

The technologies and procedures for retrofitting are constantly improved and opti-mized. New materials are developed and new lessons are learned after each earthquake.However, to implement the results of research into practice, political readiness andawareness of seismic risk is needed.


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ACKNOWLEDGMENT

The article has been written on the basis of the lecture given at the symposiumin honor of Professor Luigia Binda’s retirement from Milan’s Politecnics. The papersummarizes already published research data and experiences, obtained in the last fewdecades within the framework of numerous research projects and site observations afterearthquakes, carried out at Slovenian National Building and Civil Engineering Institute,formerly Institute for Testing and Research in Materials and Structures in Ljubljana,Slovenia. The details can be found in the referenced publications. This contribution hasbeen prepared within the framework of the research program P2-0273 for the period of2009–2013, financed by Slovenian Research Agency.

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