Structural Analysis of Historic Construction – D’Ayala & Fodde (eds)© 2008 Taylor & Francis Group, London, ISBN 978-0-415-46872-5

Seismic isolation, strengthening of walls with CFRP strips and heritagemasonry buildings

M. Tomaževic, I. Klemenc & P. WeissSlovenian National Building and Civil Engineering Institute, Ljubljana, Slovenia

ABSTRACT: The efficiency of improving the seismic resistance of heritage masonry buildings by means ofseismic isolation and strengthening of structure with CFRP laminate strips has been investigated. Five models ofa simple two-storey brick masonry building with wooden floors without wall ties have been tested on the shakingtable. Besides control model, two models, isolated by either damp proof course or seismic isolators, have beentested. Models four and five have been strengthened with CFRP laminate strips, simulating the wall ties placedhorizontally and vertically at floor levels and corners of the building, respectively. One of the CFRP strengthenedmodels has been placed on seismic isolators. Tests have shown that the damp proof course, unless adequatelydesigned, cannot be considered as seismic isolation. The isolators have also not improved the behavior in thecase of the building without wall ties. However, both models confined with CFRP strips exhibited significantlyimproved seismic behavior. They did not collapse even when the accelerations of the shaking table exceeded theaccelerations, measured when tested the control models without wall ties, by three times.

1 INTRODUCTION

Besides traditional technologies, such as the tyingof walls with steel ties, strengthening the walls byinjecting cementitious grouts and applying reinforcedcement coating, which have been used in the lastdecades, new materials and technologies have beendeveloped and proposed also for upgrading the seis-mic resistance of heritage buildings. Among them,seismic isolation in different forms (e.g. Bailey andAllen, 1988; Page, 1995; Sarrazin et al., 1996; Zhouand Miao, 1996,) and strengthening the walls withfiber reinforced polymer laminates have been alreadystudied (e.g. Schwegler, 1995;Triantafillou and Fardis,1997; Hamilton and Dolan, 1998; Gayevoy and Lissel,2004; Trantafillolu, 2001). Although the requirementsof preservation of cultural heritage limit the applica-tion of such materials and technologies, the methodsare convenient because they require minimum inter-vention in the existing structural system by providingsubstantial improvement in seismic behavior at thesame time.

Recently, experiments to investigate some aspectsof seismic isolation and possibility of tying the wallsof heritage masonry buildings with CFRP laminatestrips instead of steel ties, have been also carried outat Slovenian National Building and Civil EngineeringInstitute in Ljubljana. Experiments and test results willbe presented and discussed in this contribution.

2 RESEARCH PROGRAM AND DESCRIPTIONOF TESTS

2.1 Research program

Typically, the walls of heritage masonry buildings arenot tied with wall ties, what results into the separationof walls during earthquakes and subsequent out-of-plane collapse. Consequently, when retrofitting thebuilding for seismic loads, the tying of walls with steelties represents the basic measure to ensure the struc-tural integrity and utilize the resistance capacity ofstructural walls. In this regard, one of the objectivesof the study was to investigate the possibility of omit-ting the installation of wall ties by placing the structureon seismic isolators. Besides, the possibility of using asimple PVC damp proof course as isolating device, hasalso been studied. Last but not least, the idea to replacethe usual steel ties with CFRP laminate strips, placedboth horizontally at the level of floors and verticallyat the corners of the building, has been investigated.

2.2 Description of models

Taking into consideration the payload capacity ofthe simple uniaxial shaking table, installed in thestructural laboratory of the institute, experiences andavailable materials, the tests have been carried outon models constructed at 1:4 reduced scale. Fivemodels with basically the same structural layout have

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Figure 1. Scheme of laying the bricks and position ofwooden joists and lintels.

been constructed and tested, prepared for testing infive different ways. Since the main objective of thestudy was limited at obtaining basic information aboutthe possibility of application and efficiency of testedstrengthening method, a single room brick masonryhouse with wooden floors without wall ties has beentested. The models maintained the basic structuralcharacteristics of typical buildings, such as storeyheight, span between the structural walls and open-ings’ size, with outer dimensions adjusted to the sizeof the platform of the shaking table.The scheme of lay-ing the bricks and placing the wooden joists is shown inFigure 1, whereas a typical model under constructionis shown in Figure 2.

Model M1 represented the control model withwooden floors without wall ties. It has been con-structed directly on the r.c. foundation slab, fixed tothe moveable platform of the shaking table, withoutany specific measures to improve the seismic behavior.Model M2 was similar, however, a simple PVC sheethas been placed as a damp-proof course in the bed-joint between the second and third course of masonryunits. Model M3, also similar to model M1, has beenisolated with isolators, placed between the foundationslab and the model’s upper structure.

The effect of tying the walls with CFRP laminatestrips has been studied on Models M4 and M5. Thestrips which simulated horizontal and vertical ties(confining elements) have been bonded to masonry onthe outer side of the walls. Horizontal strips have beenplaced at the level of wooden floors. They have beenconnected together at the corners of the building withsteel anchor plates. Vertical strips, placed at the cor-ners, have been glued at the bottom of the walls on steelangle profiles, bolted into the model’s r.c. foundation

Figure 2. Typical model during construction.

Figure 3. Position of CFRP laminate strips used to confinethe models and strengthening of walls. Transverse section.

slab. In addition, the piers between the openings havebeen strengthened with diagonally placed CFRP lami-nate strips without any special provision for anchoringat the ends. The position and dimensions of CFRPstrips are shown in Figures 3 and 4. In the same figure,the main dimensions of the models are also indicated.Whereas model M4 has been built directly on the foun-dation slab as was the case of control model M1, modelM5 has been placed on seismic isolators as was the caseof model M3. Whereas model M3 has been placed on

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Figure 4. Position of CFRP laminate strips used to confinethe models and strengthening of walls. Longitudinal section.

six isolators, only four isolators have been used in thecase of model M5 in order to further shift the naturalfrequency of vibration of the isolated model from thepredominant frequency of the model earthquake.

Since physical modeling of CFRP laminates andbonding properties turned out to be rather compli-cated if impossible, the technique of simple modelinghas been used so that the strength characteristics ofbrick masonry remained close to the prototype values.Tests of model masonry walls indicated that similar-ity in non-linear behavior and failure mechanism atthe simulated conditions of prototype loading has beenfulfilled just as well. At the same time, the distributionof lateral stiffnesses and masses along the height of themodels was in the proportions usual for typical proto-type buildings, hence fulfilling also the conditions ofsimilarity of dynamic behavior. Since all models havebeen tested in equal loading conditions, their seismicbehavior can be directly compared. However, whencalculating the values of physical quantities measuredon the models to prototype, model scale factors for thecase of the simple model similarity should be takeninto consideration.

The mechanical properties of model masonry, deter-mined by compression tests (3 specimens) as well ascyclic lateral resistance tests at different levels of pre-compression (7 specimens) indicate that the modelshave been constructed with model masonry, which

Figure 5. Calibration test of seismic isolator.

had mechanical characteristic similar to the prototypeones. The following mean values have been obtained:

– compressive strength: f = 6,1 MPa,– modulus of elasticity: E = 1864 MPa,– tensile strength: ft = 0,23 MPa,– shear modulus: G = 678 MPa, and– ductility factor: µ = 3,9.

As a damp-proofing element, a commercially avail-able PVC sheet, 2 mm thick and cut to fit the dimen-sions of the cross section of the walls, has been used.The PVC sheet has been placed in the mortar bed jointbetween the second and the third course of units of thewalls in the ground floor of model M2. The slidingmechanism and friction characteristics of the damp-proof course in relation to vertical stresses in the wallshave been determined by testing.

The isolators were 92 mm in diameter and 100 mmhigh. They have been manufactured of vulcanizedrubber, 8 mm thick. In order to keep adequate stiff-ness in vertical direction, ten pieces of 2 mm thicksteel sheets have been uniformly distributed alongthe height of each isolator. Before placing and fix-ing the isolators in the position, the deformabilitycharacteristics of each isolator have been determinedby calibration tests (Figure 5). The measured aver-age lateral stiffness at lateral displacement d = 10 mmwas KH = 0,0325 kN/mm, at displacement d = 30 mm,KH = 0,0284 kN/mm, whereas the average stiffness invertical direction amounted to Kv = 1,64 kN/mm.

To confine the structure and tie the walls with hori-zontally and vertically placed CFRP laminate strips, aswell as to strengthen the walls with diagonally placedstrips, readily available CFRP laminate, 1,2 mm thick,has been cut to 2 and 3 cm wide strips. The stripshave been glued on the masonry according to theinstructions provided by the manufacturer. The tensile

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Table 1. Maximum accelerations and displacements of theshaking table, measured during individual test runs (averagevalues for the groups of models).

Models M1, M2 Models M4 andand M3 M5

dmax dmaxDuration Test run amax (g) (mm) amax (g) (mm)

12 s R005 0,028 0,795 0,027 0,600R025 0,131 3,480 0,109 2,854R050 0,386 7,116 0,239 5,660R075 0,505 10,827 0,380 8,509R100 0,688 14,620 0,483 11,322R150 – – 0,727 17,034R200 – – 1,015 22,721R300 – – 2,682 34,002R350 – – 3,555 39,492

strength of material, Sika® CarboDur S, in the direc-tion of fibers amounts to 3000 MPa, and the modulusof elasticity to 165000 MPa. Before gluing the strips,the surface of masonry has been thoroughly cleanedand penetrated with primer. Original epoxy adhesivematerial, SikaDur, has been used to glue the strips onthe masonry.

2.3 Seismic load and instrumentation of models

The shape of the ground acceleration time history, usedto control the shaking table motion, corresponded tothe 24 seconds long strong phase of the N-S com-ponent of the ground acceleration record, recorded atPetrovac during the Montenegro earthquake of April15, 1979. Maximum measured ground accelerationwas 0,43 g. The actual model earthquake, prepared fortesting the so called complete models, was 12 secondslong, and had the same maximum ground accelera-tion as was the case of the actual acceleration record.The shaking table motion during test run R100 repre-sented such an earthquake (100% of intensity). Sincethe models have been made of materials with strengthcharacteristics similar to the prototype (simple mod-els), the actual model earthquake represented 48 seclong earthquake (St = SL = 4) with maximum groundacceleration 0,11 g (Sa = 1/SL = 0, 25).

Shaking table displacements in each successive testrun have been scaled from 5% to 350% of thoseof the model earthquake (test runs R005 to R350,respectively). All models have been tested with thesame sequence of seismic excitations with increasedintensitiy of motion in each successive test run, thecharacteristics of the model earthquake did not influ-ence the observations. Maximum accelerations anddisplacements of the shaking table motion obtained in

Figure 6. Typical response spectra of shaking table motion.

Figure 7. Instrumentation of models.

each test run are given in Table 1, whereas the typicalresponse spectra are shown in Figure 6.

All models have been instrumented with a set of dis-placement transducers and accelerometers (Figure 7),fixed to the models at the level of floors. The miss-ing live load at the levels of floors has been modelledby means of concrete blocks of adequate mass, whichhave been fixed to wooden joists with steel bolts so thatthe in-plane rigidity of floors has not been significantlyaffected. In order to prevent damage to instrumentsand shaking table at the moment of collapse, concreteblocks have been loosely hanged on the crane.All mod-els have been oriented so that the direction of shakingtable motion coincided with longer dimension of the

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Figure 8. Mechanism of collapse of non-strengthened con-trol model M1.

model. In other words, seismic loads acted in the direc-tion of load-bearing walls, pierced with window anddoor openings.

3 TEST RESULTS

3.1 Failure mechanism

The control model M1 exhibited typical behaviour ofold masonry buildings with wooden floors withoutwall ties: in the beginning of tests when subjected tolow intensity earthquake ground motion, the behaviourwas monolithic. However, with increased intensity ofshaking, vertical cracks developed in the upper part ofthe model. As a result of separation of walls, the upperstorey of the model disintegrated in the subsequent testruns and collapsed (Figure 8).

The tests of model M2 have shown that the damp-proof course in the form of a simple PVC sheet placedin the mortar in the bed joint cannot be consideredas seismic isolating device. Although the compressivestresses in the walls with installed damp-proof coursewere low, the measurements have indicated that nei-ther sliding along the damp-proof course took placenor rocking motion of the upper part of the buildinghas been observed. The walls in the upper storey disin-tegrated and the storey collapsed at the same intensityof excitation as was the case of control model M1(Figure 9).

Although improved behaviour of model M3, placedon rubber seimic isolators, has been expected, modelM3 exhibited practically the same poor behaviour as

Figure 9. Mechanism of collapse of non-strengthenedmodel M2 with damp proof course.

Figure 10. Mechanism of collapse of non-strengthened andisolated model M3.

non-isolated models M1 and M2. However, a slightdifference in the sequence of damage propagation hasbeen observed. Whereas damage propagated graduallyin dependence on intensity of motion in subsequenttest runs in the case of models M1 and M2, the collapseof model M3 was sudden, without cracks occuringduring the previous test runs (Figure 10).

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Figure 11. Damage to CFRP laminate strengthened, non-isolated model M4 at the end of shaking table tests.

Figure 12. Unsignificant damage to CFRP laminatestrengthened, isolated model M5 at the end of shaking tabletest.

The seismic behaviour of both models strengthenedwith CFRP laminate strips, however, was signifi-cantly improved. They did not suffer severe damageor collapse even when subjected to ground motionwith accelerations, which by more than three timesexceeded the accelerations measured during the test-ing of non-strengthened models (Figures 11 and 12).Since the capacity of shaking table has been reached

Figure 13. Detached anchor plate caused rocking of modelM4. Vertical strip buckled at the bottom of the model.

and the output motion already distorted, tests had tobe terminated at that point. In the case of non-isolatedmodel M4 the anchor bolts, by means of which steelanchor angles of vertical strips have been fixed tothe foundation slab, pulled out (Figure 13) and themodel started rocking on the foundation slab. Con-sequently, masonry crushed at the corners and severecracks occurred in the lintel parts of the walls.

In the case of model M5 on seismic isolators, oneof the isolators detached (Figure 14). However, almostno damage has been observed in the model’s walls. Ithas to be noted, that also in the case of model M4 nostructural damage has been observed before the pullingout of anchor bolts. By comparing the results of testsof CFRP laminate strengthened and non-strengthenedmodel walls, it seems that, in the particular case stud-ied, this is the result of confining the model structurewith horizontal and vertical CFRP strips, and not theresult of diagonally placed strips on the wall piers.However, additional measurements should have beencarried out in order to confirm this observation.

The changes in dynamic characteristic of the testedmodels, measured before the tests and after each sub-sequent test run, are presented in Table 2. The valuesof the first natural frequency of vibration f and coef-ficient of equivalent viscous damping ζ (in % ofcritical damping), have been determined by hittingthe model with impact hammer and analyzing the

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Figure 14. Detachment of isolator at the end of tests ofmodel M5.

Table 2. First natural frequency of vibration f (in s−1) andcoefficient of equivalent viscous damping ζ (in % of criticaldamping) measured on the models before the beginning ofshaking table tests and after characteristic test runs.

BeforeModel test R50 R75 R150 R300

M1 f (s−1) 15,6 12,3 12,3 – –ζ(%) 13,5 16,7 15,5 – –

M2 f (s−1) 19,0 15,3 13,9 – –ζ(%) 5,5 13,4 13,8 – –

M4 f (s−1) 21,2 20,6 19,5 18,9 12,6ζ(%) 3,8 5,0 9,5 10,2 8,9

M5 f (s−1) 2,2 2,1 2,1 2,1 2,1ζ(%) 11,6 10,2 10,1 10,0 10,3

measured response. Fourier analysis of accelerationand displacement records has been used to obtainethese data. Unfortunately, dynamic characteristics ofisolated, non-strengthened model M3 have not beenmeasured.

As expected, a trend of degradation of the firstnatural frequency of vibration and increase in val-ues of coefficient of equivalent viscous damping canbe observed with increased intensity of excitation inall cases, except in the case of isolated model M5,which has not suffered damage during testing. Thedifferences in initial values, especially in the case ofmodels M1 and M2 can be mainly attributed to rela-tively large scattering of deformability characteristicof used model masonry materials.The increase in stiff-ness in the case of model M4, confined with CFRPlaminate strips, however, can be attributed to the effectof confining the model structure horizontally and ver-tically with rigid elements. No changes in the firstnatural frequency of vibration and damping in the caseof isolated model M5 are the result of seismic isolators

Figure 15. Response of control model M1 during test runR075.

which reduced the response and prevented structuraldamage.

As an indication of efficiency of confining thestructures wth CFRP laminate strips, measured topacceleration and relative first storey displacementresponses of two of the tested models are shown inFigures 15 and 16. As a measure of intensity of exci-tation, shaking table acceleration time history is alsoplotted in each figure. In the figures, the responses ofcontrol model M1 and CFRP laminate strengthenedmodel M4 to seismic excitation of the same intensityare compared (test runs R75 and R300, respectively(seeTable 1 for comparison of maximum shaking tableaccelerations).

Maximum values of the base shear evaluated on thebasis of the measured acceleration responses of themodels during each test run are compared in Table 3.Base shear is given in terms of the base shear coef-ficient BSC, i.e. the ratio between the base shearBS developed in the model during shaking and theweight of the model above the base W : BSC = BS/W .Base shear has been calculated as the sum of prod-ucts of masses, concentrated at the levels of floorsmi and measured average maximum values of accel-erations at the same level ai: BS = �mi ai. Sinceall models were equal, the mass concentrated at thesecond floor was taken as m2 = 287 kg, mass at thefirst floor m1 = 448,4 kg, whereas the total mass of

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Figure 16. Response of CFRP laminate strengthened modelM4 during test run R300.

Table 3. Maximum base shear coefficient evaluated onthe basis of the measured response of the models duringcharacteristic test runs BSC = �miamax,i/W .

Model R50 R75 R100 R150 R200 R300

M1 0,245 0,662 0,306 – – –M2 0,225 0,453 0,511 – – –M3 0,305 0,434 0,492 – – –M4 0,228 0,380 0,488 0,739 1,050 2,032M5 0,108 0,322 0,415 0,564 0,702 0,895

the model above the foundation slab, on the basis ofwhich the weight of the model W was calculated, wasmtot = 856,8 kg.

From the results of tests, summarized In Table 3,it can be clearly seen that no basic difference in theresistance has seen obtained in the case of the modelswithout wall ties. Whereas control model M1 attainedits maximum resistance during test run R75, mod-els M2 and M3 attained the maximum of somewhatsmaller value at the same intensity of motion duringtest run R100. Significant improvement in resistance(by four times) has been obtained in the case of CFRPlaminate confined model M4. In the case of CFRPconfined and isolated model M5, noticeable reductionof seismic forces can be observed. As a result of iso-lation, more than two times lower baser shear than in

the case of model M4 without isolators developed inthe model at full intensity of shaking table motion.

4 CONCLUSIONS

Experiments have shown that a simple damp-proofcourse in the form of PVC sheet installed in the mor-tar bed joint cannot be considered as seismic isolation.Experiments have also shown that seismic isolationalone is not enough to improve the seismic behaviourof heritage masonry buildings without wall ties. How-ever, the shaking table tests of models confined withhorizontal and vertical CFRP laminate strips indi-cated significantly improved seismic behaviour. TheCFRP laminate confined models did not collapse evenwhen subjected to ground accelerations which by morethan three-times exceeded accelerations causing thecollapse of the models without wall ties.

The experiments indicated the possibility of replac-ing the commonly used steel ties by CFRP lam-inate strips. Placed also vertically, CFRP laminatestrips additionally strengthen the structure, if prop-erly anchored to foundation system at the ends. Theexperiments also confirmed the long known fact thatseismic isolation of rigid masonry structures repre-sents an efficient way to reduce seismic loads in thecase of the usual short periodic earthquakes. However,the isolation does not permit that the usual meaures ofseismic strengthening, such as the tying of walls withwall ties, be omitted.

The experiments indicated the efficiency of con-temporary technical solutions. However, they alsopointed out that technological problems need to beresolved before wider application of such methodsto heritage masonry buildings. In this regard, effi-cient interaction between the materials with extremelydifferent mechanical characteristics, such as CFRPlaminates and masonry is critical.

ACKNOWLEDGEMENTS

The research presented in the paper has been carriedout within the framework of research project L2-0691and research program P2-0274, financed by the Min-istry of High Education, Science and Technology ofthe Republic of Slovenia and co-financed by rubberindustry, Sava Company Ltd., program Construmatfrom Kranj, Slovenia. CFRP laminates and bondingmaterials have been given at disposition free of costsby Sika AG, Slovenian branch from Trzin, Slovenia.

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