Bull Earthquake EngDOI 10.1007/s10518-008-9086-1

ORIGINAL RESEARCH PAPER

Seismic upgrading of old masonry buildings by seismicisolation and CFRP laminates: a shaking-table studyof reduced scale models

Miha Tomaževic · Iztok Klemenc · Polona Weiss

Received: 10 December 2007 / Accepted: 21 August 2008© Springer Science+Business Media B.V. 2008

Abstract The efficiency of improving the seismic resistance of old masonry buildings bymeans of seismic isolation and confining the structure with CFRP laminate strips has beeninvestigated. Five models of a simple two-story brick masonry building with wooden floorswithout wall ties have been tested on the shaking table. The control model has been builtdirectly on the foundation slab. The second model has been separated from it by a damp-proofcourse in the form of a PVC sheet placed in the bed-joint between the second and the thirdcourse, whereas the third model has been isolated by rubber isolators placed between thefoundation slab and structural walls. Models four and five have been confined with CFRPlaminate strips, simulating the wall ties placed horizontally and vertically at floor levelsand corners of the building, respectively. One of the CFRP strengthened models has beenplaced on seismic isolators. Tests have shown that a simple PVC sheet damp-proof coursecannot be considered as seismic isolator unless adequately designed. Tests have also shownthat the isolators alone did not prevent the separation of the walls. However, both modelsconfined with CFRP strips exhibited significantly improved seismic behavior. The modelsdid not collapse even when subjected to significantly stronger shaking table motion than thatresisted by the control model without wall ties.

Keywords Old masonry buildings · Models · CFRP laminate strips · Confinement ·Seismic isolation · Seismic resistance · Shaking table tests · Upgrading

1 Introduction

High seismic vulnerability of old masonry buildings in historic urban and rural nuclei has beenone of the main reasons of structural damage and collapse, as well as human loss occurredduring earthquakes in Europe in the past several decades. Old masonry buildings representarchitectural heritage of greatest importance, giving additional value to many towns and

M. Tomaževic (B) · I. Klemenc · P. WeissSlovenian National Building and Civil Engineering Institute, Dimiceva 12, 1000 Ljubljana, Sloveniae-mail: miha.tomazevic@zag.si

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cities. Therefore, substantial research has been already carried out to understand the seismicbehavior and develop methods and technologies to improve the seismic resistance and, hence,preserve the buildings for future generations. The effects of many basic structural interven-tions, developed in accordance with the principles and requirements of restoration and con-servation of cultural heritage, have been tested in the laboratories and in-situ. Moreover, theeffects of some methods, applied to the buildings, have been also verified by the repeatedearthquakes in some region. Substantial amount of publications, reporting the success ofapplication of different methods and technologies, is available. The authors of this paper alsogave contribution (Tomaževic 1989, Tomaževic and Apih 1993, Tomaževic et al. 1993, 1994,1996).

Besides traditional technologies, such as the tying of the walls with steel ties, the strength-ening of the walls by injecting cementitious grouts and applying reinforced cement coating,which have been developed decades ago, the methods based on new materials and technolo-gies have been also proposed for upgrading the seismic resistance of old masonry buildings.Although the requirements of preservation of cultural heritage limit the application of suchmaterials and technologies, modern technologies often require minimum intervention in theexisting structural system by providing substantial improvement in seismic behavior at thesame time.

Seismic isolation has not been frequently applied to old masonry buildings, although theidea to separate the upper structure from the foundation system and reduce its response toseismic ground motion, is not new. For example, the “skyscraper” of Ljubljana, a 60 m tallr.c. frame building, constructed in 1930s, is maybe one of the first buildings world-wide,where a special feature has been designed to separate the upper structure from foundation inorder to reduce the seismic effects (Fajfar 1995). Besides seismic isolation, modern structuralprotective systems include passive energy dissipation devices and high-tech active motioncontrol systems (Skinner et al. 1993, Soong and Dargush 1997). Unfortunately, since the lattertwo require large deformations to be efficient, they are not easily applicable to rigid masonrystructures. The idea to use lead reinforcing mesh and lead energy absorbing devices has beenrecently proposed for seismic retrofitting of stone masonry buildings (Benedetti 2004).

It seems that in the case of masonry buildings the simplest idea to separate the rigidupper structure from foundations with soft isolators represents the best solution, though themethod is rarely used (Sarrazin et al. 1996, Zhou and Miao 1996). Seismic isolation is evenless frequently used for heritage buildings (Salt Lake City and County Building; Bailey andAllen 1988).

The requirements of preservation of cultural heritage are not in favor of the use of typicalcontemporary construction materials, such as concrete and steel. Masonry friendly materialsand technologies compatible with the original ones should be used for seismic strengtheningof heritage buildings. Although the same limitations should be taken into consideration inthe case of application of modern synthetic materials, the possibility of using compositematerials, such as carbon or glass fiber reinforced polymer (CFRP or GFRP, respectively)laminates, glued in different forms on the surface of brick and stone masonry walls, has beenstudied in the last decade. Some results of tests, carried out on masonry walls, encourage theapplication, the others, however, pointed out problems resulting from bond and anchorageof CFRP strips, glued on the walls’ surface (Schwegler 1995, Hamilton and Dolan 1998,Triantafillou and Fardis 1997, Triantafillou 2001, Gayevoy and Lissel 2004).

Recently, experiments to investigate some aspects of seismic isolation and possibility oftying the walls of old masonry buildings with CFRP laminate strips instead of steel ties,have been also carried out at Slovenian National Building and Civil Engineering Institute inLjubljana. Experiments and test results will be presented and discussed in this contribution.

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2 Research program and description of tests

2.1 Structural typology, brick-masonry materials and objectives of research

Typical height of old urban brick-masonry residential houses in Slovenia does not exceedthree to four stories, with story height limited to 3 m. The thickness of structural walls variesfrom 38 to 72 cm, depending on the height of the building. The distance between structuralwalls does not exceed 5.5 m. Floors are usually wooden with timber joists freely supportedby load-bearing walls. Sometimes, especially above cellars, ground floors and corridors,the wooden structure is replaced by brick vaults. Solid, 29/14/6 or 25/12/5 cm bricks, withcompressive strength varying between 7.5 and 15 MPa and lime mortar with compressivestrength not exceeding 2.5 MPa are the constituent building materials.

Availability of data on mechanical characteristics of historic brick-masonry in the regionis rare. A small number of load-resistance tests of different types of existing brick-masonrywalls indicate a rather wide range of values of basic mechanical characteristics (Sheppard andTercelj 1985, Sheppard and Tomaževic 1986, Magenes 1992). The range of typical expectedvalues, which have been considered as target values when designing the models, is givenbelow (subscript P indicates prototype masonry):

– Compressive strength: fcP = 1.5–10.0 MPa,– Tensile strength: ftP = 0.10–0.70 MPa,– Modulus of elasticity: EP = 1500–3800 MPa,– Shear modulus: GP = 60–165 MPa,– Specific mass: γP = 1600 kg/m3.

The walls of old masonry buildings are often not tied with wall ties. As a result, theseparation of walls and subsequent out-of-plane collapse typically take place when subjectedto earthquakes. Therefore, when retrofitting such buildings for seismic loads, the tying of thewalls with steel ties represents the basic measure to improve the resistance. Namely, by thetying the walls with wall ties the structural integrity is ensured and the available resistance ofstructural walls is utilized. To investigate the possibility of omitting the installation of wallties by placing the structure on seismic isolators has been one of the objectives of the study.

Old masonry buildings frequently suffer from moisture and damp, propagating from soiland environment into the interior of masonry walls and severely deteriorating the walls’resistance capacity in the course of time. To prevent moisture and damp propagation, differenttypes of damp-proof courses are installed in the bottom part of masonry walls. Not many dataexist regarding the shear capacity of various types of damp-proof courses (see for examplePage 1995) and using the damp-proof course as seismic isolation device (sliding mechanism,friction isolation). Therefore, one of the objectives of this study has been also to investigateto what extent a very simple damp-proof course in the shape of a PVC sheet, placed in thebed joint and not designed to act as a seismic isolator, can actually be regarded as a seismicisolator.

Last but not least, the idea to replace the usual steel ties with CFRP laminate strips, placedboth horizontally at the level of floors and vertically at the corners of the building, has beenverified.

2.2 Structural layout and description of models

Taking into consideration the payload capacity of the simple uniaxial shaking table,installed in the structural laboratory of the institute, experiences and available materials,

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Fig. 1 Scheme of laying the bricks and placing the wooden lintels and joists (measures in cm)

models constructed at 1:4 reduced scale have been tested. Five models with basically thesame structural layout have been constructed and tested, prepared for testing in five differentways. Since the main objective of the study has been to obtain basic information and notproviding recommendations and instructions for practical application of the tested methods,a simple two-story, single room brick masonry house with wooden floors without wall tieshas been tested. The models maintained the basic structural characteristics of typical build-ings, such as story height, span between the structural walls and openings’ size, with outerdimensions adjusted to the size of the platform of the shaking table. The scheme of laying thebricks and placing the wooden joists is shown in Fig. 1, whereas the actual laying of brickscan be seen in Fig. 2.

Model M1 represented the control model with wooden floors without wall ties. It hasbeen constructed directly on the r.c. foundation slab, bolted to the moveable platform of theshaking table, without any specific measures taken to improve the seismic behavior. ModelM2 has been similar. However, a simple PVC sheet has been placed as a damp-proof coursein the bed-joint between the second and third course of masonry units (Fig. 3). Model M3,also similar to model M1, has been isolated with isolators, placed between the slab bolted tothe platform and the foundation slab onto which the model has been constructed.

The effect of tying the walls with CFRP laminate strips has been studied on Models M4and M5. The strips which simulated horizontal and vertical ties (confining elements) have

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Fig. 2 Typical model under construction

Fig. 3 PVC sheet placed in the mortar joint of model M2

been bonded to masonry on the outer side of the walls. Horizontal strips have been placedat the level of wooden floors. At the corners of the building the horizontally placed CFRPlaminate strips have been epoxy glued on steel anchor plates. Vertical strips, placed at thecorners, have been anchored at the bottom of the walls by steel angle profiles, epoxy gluedand bolted into the r.c. foundation slabs. In addition, the piers between the openings have beenstrengthened with diagonally placed CFRP laminate strips without any special provision foranchoring at the ends. The position and dimensions of CFRP strips are shown in Fig. 4. Inthe same figure, the main dimensions of the models are also indicated. Whereas model M4has been built directly on the foundation slab as has been the case of the control model M1,model M5 has been placed on seismic isolators as has been the case of model M3. There hasbeen a difference between models M3 and M5: whereas model M3 has been placed on sixisolators, only four isolators have been used in the case of model M5 in order to further shiftthe natural frequency of vibration of the isolated model from the predominant frequency ofthe model earthquake.

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Fig. 4 Position of CFRP laminate strips used to confine the models and strengthening of walls in (a)longitudinal and transverse direction (b). The position of isolators in the case of model M5 as well as thegeneral dimensions of the models are indicated (measures in cm)

2.3 Physical modeling and model masonry materials

It has been originally planned that the so called complete models, built with materials whichhave the strength characteristics reduced in the geometrical scale but strain characteristics thesame as the prototype, will be tested. However, since the physical modeling of CFRP lami-nates and adhesive materials’ properties turned out to be rather complicated, if impossible,the strength characteristics of model brick-masonry remained within the range of possibleprototype values. It is believed that the benefits resulting from reliable information regard-ing the interaction between masonry and CFRP laminates prevail against the drawbacks insomewhat incorrect physical modeling of linear dynamic response.

Model bricks, 63/30/30 mm in size, have been made by casting a special mortar, composedof crushed brick aggregate instead of sand, lime, and cement in the proportion of 9:2:0.75,and water, into especially shaped steel form. The use of crushed brick aggregate ensured thatspecific mass of model bricks was practically the same as that of prototype bricks. Crushedbrick aggregate also ensured that strain characteristics of model masonry did not differ toomuch from prototype values. Cement-lime-sand mortar in the proportion of 0.4:1:11 has beenused for laying the bricks.

Samples have been taken from each batch of mortar mix in order to determine the compres-sive strength of units. By testing more than 200 specimens, the mean value of compressive

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strength of model bricks fb = 8.4 MPa has been obtained, with standard deviation of 3.2 MPa.The mean value of mortar strength, used for the construction of models, determined on 189specimens, has been estimated to fm = 0.5 MPa (standard deviation 0.12 MPa).

The mechanical properties of model masonry, determined by compression tests (3 speci-mens) as well as cyclic lateral resistance tests at different levels of precompression (7 spec-imens) indicate that the models have been constructed with model masonry, which hadthe same mechanical characteristic as the prototype. The following mean values have beenobtained;

– Compressive strength: fcM = 6.1 MPa,– Tensile strength: ftM = 0.23 MPa,– Modulus of elasticity: EM = 1864 MPa,– Shear modulus: GM = 68 MPa,– Specific mass: γM = 1685 kg/m3 and– Ductility indicator: µ = 3.9.

As can be seen, the values are close to the lower limit of the range of the prototype values.In order to investigate the efficiency of diagonally placed CFRP laminate strips, glued

on the walls’ surface with epoxy bonding material in the case of models M4 and M5, 7model walls, strengthened with CFRP strips have been also tested. Test results confirmed theconclusions of researchers who investigated the efficiency of strengthening masonry wallswith composite fiber laminates. It has been shown that adequate anchoring of strips at theends is of relevant importance, and not the efficiency of bonding between the laminatesand masonry (Schwegler 1995, Triantafillou and Fardis 1997). Namely, in all cases thediagonally placed CFRP laminate strips delaminated from the masonry as soon as the firstdiagonal cracks occurred in the walls. Failure occurred in the bricks and did not pass thebonding material. Namely, because of rigidity of CFRP laminate and great difference indeformability characteristics of masonry and CFRP laminate (modulus of elasticity of thelaminate is thousand times greater than modulus of elasticity of masonry), the elongationsof the laminate strips could not follow the deformations of masonry in the non-linear range.Since the adhesive material proved to be effective, surface part of the bricks along the stripspulled out.

Typical model walls in original and strengthened state after cyclic lateral resistance testsare shown in Fig. 5, whereas the lateral load—displacement relationships of the same walls,measured during the tests, are shown in Fig. 6. In Fig. 5b, the delamination and pulling outof masonry along CFRP laminate strips is clearly visible.

If the seismic behavior of masonry buildings is studied by testing their physical models onearthquake simulators, the similitude between the damage patterns and failure mechanismsobserved during model tests and those observed on the prototype buildings after earthquakesis regarded as the most important indicator of the accuracy of physical modeling. If the failuremechanism of the structural element is accurately simulated, and the loads which acted on theelement during the experiment are known, the information obtained by testing the physicalmodel can be reliably referred to the prototype.

Similitude in dynamic behavior requires similar distribution of masses and stiffnessesalong the height of the prototype and model. Similitude in failure mechanism, however,requires similar working stress level, i.e. working stress/compressive strength ratio in thestructural walls of the prototype and model masonry building.

The distribution of masses in a typical prototype brick-masonry structure has beenestimated by taking into account a single-room section of a building, with structural wallsat a typical distance between them. As indicated by this estimation, the typical values of

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Fig. 5 Model walls after the cyclic lateral resistance test. (a) Non-strengthened and CFRP laminatestrengthened wall (b)

Fig. 6 Lateralresistance-displacementhysteresis loops, obtained duringcyclic lateral resistance tests ofmodel walls.(a) Non-strengthened wall andCFRP laminate strengthenedwall (b)

floor/wall mass ratio range within mFP /mwP = 1/2.5–3. In the particular case studied, thevalue of floor/wall mass ratio of the tested models was mFP /mwP = 1/4.1 at the first andmFP /mwP = 1/2.1 at the second floor level.

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Table 1 Typical scale factorsSq : general equations and valuesfor 1:4 modeling scale

Quantity General equation Simple model

Length (L) SL = LP /LM SL = 4

Strain (ε) Sε = εP /εM 1

Strength (f) Sf = fP /fM 1

Stress (σ ) Sσ = fP /fM 1

Modulus of elasticity (E) SE = Sσ /Sε 1

Specific weight (γ ) Sγ = γP /γM 1

Force (F) SF = S2LSf S2

L = 16

Time (t) St = SL√

Sγ Sε/Sf SL = 4

Frequency (ω) Sω = 1/SL 1/SL = 0.25

Displacement (d) Sd = SLSε SL = 4

Velocity (v) Sv = Sε√

Sf/Sγ 1

Acceleration (a) Sa = Sf /SLSγ 1/SL = 0.25

Typical values of compressive stresses in the load-bearing walls of a two-storybrick-masonry building range within σoP = 0.13–0.15 MPa, depending on the weight ofthe floors and story height. Taking into consideration the values, obtained by testing existingbrick masonry walls, it can be seen that these values represent 1.3–10% of the expected valuesof the compressive strength of old brick-masonry. In the particular case studied, however,the average compressive stresses in the walls of the models were smaller: σoM = 0.085 MPa,i.e. 1.4% of the compressive strength of model masonry. It can be seen, however, that thelevel of the working stress/compressive strength ratio was at the lower limit of the expectedrange of values for prototype buildings. Therefore, the requirement for similitude in failuremechanism has been also accomplished.

Since all models have been tested in equal loading conditions, their seismic behavior can bedirectly compared. However, when referring the values of physical quantities measured on themodels to prototype, model scale factors, given in Table 1, should be taken into consideration.If a general quantity qM has been measured on the model, the following relationship appliesfor the quantity qP which refers to the prototype (Langhaar 1951):

qP = qMSq (1)

where Sq is a scale factor from Table 1.

2.4 Damp-proof sheet, seismic isolators and CFRP laminate strips

As a damp-proofing element, a commercially available PVC sheet, 2 mm thick and cut tofit the dimensions of the cross section of the walls, has been used. The PVC sheet has beenplaced in the mortar bed joint between the second and the third course of units of the wallsin the ground floor of model M2. The mechanical characteristics of PVC material havenot been tested, however, the sliding mechanism and friction characteristics of the damp-proof course in relation to vertical stresses in the walls have been determined by testing(Fig. 7). As expected, the average shear stress at sliding depended on the level of verticalstresses in the wall (Fig. 8). At 1% of compressive strength of masonry (compressive stressσo = 0.06 MPa), the value amounted to τ = 0.06 MPa, at 8% (σo = 0.49 MPa) to 0.28 MPa,and at 25% (σo = 1.53 MPa) to τ = 0.76 MPa. For comparison: the actual compressive stress

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Fig. 7 Testing of frictional characteristics of the damp-proof course

Fig. 8 Correlation between thecompressive and shear stresses atsliding along the damp proofcourse

level in the walls of model M2 has been estimated to σo = 0.085 MPa, and the actual averagevalue of the shear stress at the maximum base shear measured during the shaking table testhas been estimated to τ = 0.014 MPa. The correlation between the friction characteristicsof the damp-proof course and the actual stress state in the model’s walls explains why thedamp-proof course did not affect the observed behavior of the model.

The size and deformability properties of rubber seismic isolators, used to isolate modelsM3 and M5, have been determined on the basis of dynamic analysis of the model’s responseto chosen prototype earthquake. The target characteristics were such as to reduce the funda-mental frequency of vibration of the control non-isolated model by approximately 10-times.As can be seen in Table 3, where the measured initial values of the first natural frequency ofvibration of the models are reported, this has been achieved to a great degree.

The isolators were 100 mm in diameter and 92 mm high. They have been manufacturedof vulcanized rubber, 8 mm thick. In order to keep adequate stiffness in vertical direction, tenpieces of 2 mm thick steel sheets have been uniformly distributed along the height of eachisolator. The structure of typical isolator, cut after the preliminary rupture test, is shown inFig. 9.

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Fig. 9 The structure of the seismic isolator

Fig. 10 Seismic isolator duringcalibration test

Before placing and fixing the isolators in the position, the deformability and energy dis-sipation characteristics of each isolator have been determined by calibration tests (Fig. 10).Typical hysteresis relationships between lateral load and displacements are shown in Fig. 11a,whereas the correlation between the average values of lateral stiffness, measured at differentvalues of lateral displacements, are shown in Fig. 11b. Since no attempt has been made todesign the isolators for improved energy dissipation capacity, the hysteresis is relatively thin(Fig. 11a). As shown in Fig. 11b, the lateral stiffness was not constant but decreased withincreased lateral deformation. The measured average lateral stiffness at lateral displacementd = 10 mm has been KH = 0.0325 kN/mm, at displacement d = 30 mm, KH = 0.0284 kN/mm,whereas the average stiffness in vertical direction amounted to Kv = 1.62 kN/mm.

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Fig. 11 Results of seismic isolators’ calibration tests. (a) Lateral force-displacement hysteresis loops andaverage relationship between the lateral stiffness and displacements (b)

To confine the structure and tie the walls with horizontally and vertically placed CFRPlaminate strips, as well as to strengthen the walls with diagonally placed strips, readilyavailable 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 the instructions provided by the manufacturer.The tensile strength of material, Sika® CarboDur S, in the direction of fibers amounts to3000 MPa, and the modulus of elasticity to 165,000 MPa. Before gluing the strips, the surfaceof masonry has been thoroughly cleaned and penetrated with primer. Original epoxy adhesivematerial, SikaDur, has been used to glue the strips on the masonry.

2.5 Simulation of seismic loads and measurements

The shape of the ground acceleration time history, used to control the shaking table motion,corresponded to the 24 s long strong phase of the N–S component of the ground accelerationrecord, recorded at Petrovac during the Montenegro earthquake of April 15, 1979, withmaximum measured ground acceleration amax = 0.43 g (Fig. 12). The response spectrum ofthe record is in general agreement with the Eurocode 8 proposed design spectrum with aflat part in the range of natural periods of vibration from 0.15 to 0.6 s, depending on the

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Fig. 12 Strong phase of the N–S component of the Petrovac ground acceleration record of the April 15, 1979,earthquake of Montenegro and calculated ground displacement time history

type of the ground (Eurocode 8 2004). As the ambient and forced vibration measurementsof dynamic characteristics of old masonry buildings indicate, the period range of the flatpart of the design spectrum is in good agreement with the range of the first natural periodof vibration of buildings of various configuration and height, which varies from 0.2 to 0.5 s(Sheppard 1989, Taškov et al. 1984). Typical value of the first natural period of vibration forthe tested type of buildings is T = 0.25 s.

As a result of a processing error which has occurred when testing the control modelM1, the earthquake record has not been scaled in accordance with the rules of the simplemodel similitude (see Table 1). Instead, it has been scaled as if complete models had beentested. In order to directly compare the behavior of the tested models, it has been decided that,despite the error, the same procedure be used also for testing the remaining models. However,additional analysis to study the possible influence of such non-compliance in modeling theseismic loads on the test result has been carried out.

Namely, by taking into consideration the fact that the models have been made of materi-als with strength characteristics similar to the prototype and taking into account the valuesof scale factors, given in Table 1, it can be seen that the earthquake acceleration time his-tory, used to control the shaking table motion, actually represented 48 s long earthquake(TP = TM St = 12 ∗ 4 = 48 s) with maximum ground acceleration equal to amax = 0.11 g(aP = aM Sa = 0.43/4 = 0.11 g). Consequently, the resonance amplitudes of the responsespectra of the earthquake used to control the shaking table did not coincide with the naturalperiods of vibration of the models, as has been planned when designing the tests. As canbe seen in Fig. 13, where the response spectra of the actual shaking table motion with in-dicated range of natural periods of vibration of the tested models are shown, the maximumamplification range of the spectra is shifted outside the range of periods of vibration of thenon-isolated models. If the response spectra would have been compressed by 4-times, as

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Fig. 13 Typical response spectra of the shaking table motion. (a) First phase of testing: group of modelsmodels M1, M2, and M3, and (b) second phase of testing: group of models M4 and M5. The range of the firstperiods of vibration of the tested models is indicated by dashed lines

required by the rules of the model similitude, this would not be the case. The natural periodsof vibration of isolated models M3 and M5 also fall outside the maximum amplificationrange, which, however, has been planned when designing the models. It can be seen that,in the particular case studied, the fundamental periods of vibration of the tested models areeither lower (models M1, M2 and M4) or greater than the period range of amplification ofthe modeled earthquake (models M3 and M4). Therefore, approximately the same level ofamplification of motion when subjected to shaking-table motion of the same intensity can beexpected in the case of both, non-isolated and isolated models.

Shaking table motion has been displacement controlled. To obtain the displacement timehistory, used to control the shaking table, the acceleration time history of the model earthquakehas been integrated and the maximum calculated displacement value used to adjust theintensity of motion. As can be seen in Fig. 14, good correlation between the earthquakeacceleration record, used as the input for the calculation of control displacements and theactual accelerations, measured on the shaking table during the test, has been obtained.

Test run designated R100 usually represents the modeled prototype earthquake. In theparticular case studied, R100 represented 48 s actual prototype earthquake record with peakground acceleration ag = 0.11 g. Each model has been tested by a sequence of earthquakeswith gradually increased intensity of motion obtained by scaling the calculated displacementtime history. The notation in each test run represents the percent of the maximum displacementvalue of the modeled earthquake. For example, test run R50 means 50% of intensity of themodeled, i.e. R100 earthquake.

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Fig. 14 Correlation between the input earthquake accelerogram and accelerations, measured on the shakingtable

The tests have been carried out in two phases with a 2-year long time interval betweenthem. Models M1, M2, and M3 have been tested within the first phase, whereas models M4and M5, strengthened with CFRP strips, have been tested within the second one. During thefirst testing phase, mechanical problems affected the shaking table motion. Disturbances inmotion resulted into the unexpected spectral amplification in the high frequency range ofvibration, which, in the particular case studied, coincided with the first natural frequenciesof vibration of the models (Fig. 13a). The disturbances are important at zero level of criticaldamping. However, the spectra in the high frequency (low period) range are smoothed ifthe damping is taken into consideration. If 10% or more of critical damping is taken intoaccount, which is a typical value measured the free vibration test of models, the differencesin the response spectra of the first and second phase of testing did not significantly affectthe response of the tested models (see Table 3 for the critical damping of the tested models).As can be seen in Fig. 13b, the mechanical problems of the shaking table motion have beenfixed in the second phase of testing.

To study the influence of different shapes of spectra, resulting from different shakingtable motions during the first (group of models M1, M2 and M3) and the second phase oftesting (group of models M4 and M5), Arias intensity of shaking, IA, and input energy, Einp,induced by the actuator’s work in each subsequent test run of each phase of testing, havebeen compared. Arias intensity has been calculated by (Arias 1970):

IA = π

2g

t0∫

0

a2m(t) dt (1)

Equation 2, which determines the input energy per unit of mass (Bertero and Uang 1992):

Einp,um =t0∫

0

|am(t)||vm(t)| dt, (2)

where:

vm(t) =t∫

0

am(τ ) dτ , (3)

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and:

– am(t), - the shaking-table acceleration time history;– vm(t), - the shaking-table velocity time history, and– t0, the duration of the motion in each individual test run,

has been used as a basis for the calculation of input energy, which has been defined as awork of hydraulic actuator needed to drive the shaking table together with the model, fromthe beginning of tests to the end of test run under consideration. Hence, the actuator’s work,i.e. cumulative input energy from the beginning to the end of each individual test run, isgiven by:

Einp = me

t0∫

0

|am(τ )vr(τ )| dτ, (4)

where:

– Einp, the cumulative input energy from the beginning to the end of the test under consid-eration,

– am(τ ) - the shaking-table acceleration time history,– vr(τ ) - the relative velocity of the response of the model, idealized as a single degree of

freedom system,– t0 - the duration of excitation during each individual test run,– me - the equivalent mass of the model, idealized as a single degree of freedom system.

The response of the model has been taken into account by idealizing the tested structure withan equivalent single degree of freedom system (Tomaževic 1987). The equivalent mass ofthe model, idealized as a single degree of freedom system, has been calculated by:

me =n∑

i=1

mi�i, (5)

where �i is the measured modal shape vector, and mi is a mass, concentrated at i-th floorlevel (see below). The values of vr(τ ) have been calculated on the basis of the measuredrelative displacement response of the model at the location of the equivalent mass of theequivalent single degree of freedom system.

The calculated values of Arias intensity, IA, and input energy, Einp , in each test run aregiven in Table 2. In the same table, the average values of accelerations and displacementsof the shaking table motion, measured during the testing of models in both series of tests,are also given. It can be seen that, regarding the maximum measured values of accelerationsand displacements, as well as intensity of motion, test run R75 in the first series of tests(group of models M1, M2, and M3) corresponded to test run R100 in the second series(group of models M4 and M5). Taking this into consideration, it can be concluded thatboth groups of models have been tested in a similar way, by subjecting them to a similarsequence of earthquake motion with similar intensity of motion in each subsequent testrun.

All models have been instrumented with a set of displacement transducers and accelerom-eters (Fig. 15), fixed to the models at the level of floors. The missing live load at the levels offloors has been modelled by means of concrete blocks of adequate mass, which have beenfixed to wooden joists with steel bolts so that the in-plane rigidity of floors has not been sig-nificantly affected. In all cases, the mass of floors, including concrete blocks, and masonry

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Table 2 Maximum accelerations, amax , and displacements of the shaking table, dmax , Arias intensity, IA,and input energy, Einp , of the shaking table motion, measured during individual test runs

Testrun

First phase of tests: group of models M1, M2and M3

Second phase of tests: group of models M4and M5

amax (g) dmax (mm) IA (m/s) Einp (Nm) amax (g) dmax (mm) IA (m/s) Einp (Nm)

R005 0.028 0.795 0.0072 1.2 0.027 0.600 0.0051 0.8

R025 0.131 3.480 0.2132 34.2 0.109 2.854 0.1380 20.4

R050 0.386 7.116 0.9018 130.4 0.239 5.660 0.5437 81.6

R075 0.505 10.827 2.0613 286.7 0.380 8.509 1.2221 180.1

R100 0.688 14.620 2.7559 360.8 0.483 11.322 2.1638 318.0

R150 – – 0.727 17.034 4.8413 711.6

R200 – – 1.015 22.721 8.5955 1259.2

R300 – – 2.682 34.002 21.6463 2658.8

R350 – – 3.555 39.492 30.7944 3411.1

Fig. 15 Instrumentation of models

walls, concentrated at floor levels, amounted to m2 = 287 kg at the second and m1 = 448.4 kgat the first floor level. The total mass of the model, on the basis of which the weight of themodel, W , above the base has been calculated, amounted to mtot = 856.8 kg. In order toprevent the damage to instruments and shaking table at the moment of collapse, concreteblocks have been loosely hanged on the crane. All models have been oriented so that thedirection of shaking table motion coincided with longer dimension of the model. In other

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words, seismic loads acted in the direction of load-bearing walls, pierced with window anddoor openings.

3 Test results

The control model M1 exhibited typical behavior of old masonry buildings with woodenfloors without wall ties: in the beginning of tests when subjected to low intensity earthquakeground motion, the behavior has been monolithic. However, with increased intensity ofshaking during test run R50, first vertical cracks developed in the upper part of the model. Asa result of separation of walls during test run R75, the upper story of the model disintegratedin the subsequent test run R100 and collapsed (Fig. 16a).

The tests of model M2 have shown that the damp-proof course in the form of a simplePVC sheet placed in the mortar in the bed joint cannot be regarded as a seismic isolatingdevice. Although the compressive stresses in the walls with installed damp-proof course

Fig. 16 Collapse of models without wall ties. (a) Control model M1, model M2 with damp-proof course (b),and isolated model M3 (c)

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Fig. 17 CFRP laminate strips confined models at the end of shaking-table test. (a) Model M4 and isolatedmodel M5 (b)

were low, the measurements have indicated only unsignificant sliding along the damp-proofcourse (within the order of accuracy of measurements), which did not affect the responseof the model. The walls in the upper story disintegrated and the story collapsed at the sameintensity of excitation as has been the case of control model M1 (Fig. 16b).

Although improved behavior of model M3, placed on rubber seismic isolators has beenexpected, model M3 exhibited practically the same poor behavior as non-isolated modelsM1 and M2. However, a slight difference in the sequence of damage propagation has beenobserved. Whereas damage propagated gradually in relation to intensity of motion in thecase of models M1 and M2, the collapse of model M3 during test run R100 has been sudden,without cracks occurring during the previous test runs (Fig. 16c).

The seismic behavior of models M4 and M5 confined with CFRP laminate strips, how-ever, has been significantly improved. The models did not suffer severe damage or collapseeven when subjected to ground motion with accelerations, which by more than three timesexceeded the accelerations withstood by models M1, M2 and M3 without wall ties (Fig. 17a,b). Since the capacity of the shaking table has been attained, tests had to be terminated at thatpoint. In the case of the non-isolated model M4 the steel anchor angle profiles, by means ofwhich the vertical strips have been fixed to the foundation slab, pulled out (Fig. 18) and themodel started rocking on the foundation slab. Consequently, masonry crushed at the cornersand severe cracks occurred in the lintel parts of the walls.

In the case of model M5 on seismic isolators, one of the isolators detached (Fig. 19).However, almost no damage has been observed in the model’s walls. It has to be noted, thatalso in the case of model M4 no structural damage has been observed before the pulling outof anchor angle profiles. By comparing the results of tests of CFRP laminate strengthenedand non-strengthened model walls (see Fig. 6), it seems that in the particular case studied, theimproved behavior is the result of confining the model structure with horizontal and verticalCFRP strips, and not the result of diagonally placed strips on the wall piers.

The changes in dynamic characteristic of the tested models, measured before the tests andafter each subsequent test run, are presented in Table 3. The values of the first natural fre-

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Fig. 18 Detail of pulling out of the anchoring system of CFRP laminate strips in one of the corners of modelM4

Fig. 19 Detached isolator of model M5 after the shaking table test

quency of vibration f and coefficient of equivalent viscous damping ζ (in % of critical damp-ing), have been determined by hitting the model with impact hammer and analyzing the mea-sured response. As can be seen, the measured first natural frequencies of vibration of the mod-els are well in agreement with the expected prototype values: fP = fMSω = 16 × 0.25−21 × 0.25 = 4−5.25 Hz, which corresponded to natural periods of vibration of typical oldmasonry buildings (TP = 0.19–0.25 s; Taškov et al. 1986). Fourier analysis of accelerationand displacement records has been used to obtain these data. Unfortunately, the results ofmeasurements of dynamic characteristics of isolated, non-strengthened model M3 have beenlost due to technical problems.

As expected, a trend of degradation of the first natural frequency of vibration and increasein values of coefficient of equivalent viscous damping can be observed with increased intensityof excitation in all cases, except in the case of isolated model M5, which has suffered almostno damage during testing. The differences in initial values of natural frequencies of vibrationof models M1 and M2 can be mainly attributed to differences in mechanical characteristic ofmodel masonry materials (coefficient of variation of the compressive strength of the modelbricks was 0.38). The increase in stiffness in the case of model M4, confined and strengthened

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Table 3 First natural frequency of vibration f (in s−1) and coefficient of equivalent viscous damping ζ (in %of critical damping) measured on the models before the beginning of shaking table tests and after characteristictest runs

Model Before 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 19.5a 18.9 12.6

ζ (%) 3.8 5.0 9.5a 10.2 8.9

M5 f (s−1) 2.2 2.1 2.1a 2.1 2.1

ζ (%) 11.6 10.2 10.1a 10.0 10.3

a Values are measured after test run R100, which corresponded to test run R75 in the case of testing of modelsM1 and M2. No measurements have been carried out in the case of model M3

with CFRP laminate strips, however, can be attributed to the effect confining elements. Thefact that no changes in natural frequency and damping have been observed in the case ofthe isolated model M5, is the result of seismic isolators, which reduced the response andprevented structural damage of the model.

Typical measured acceleration responses of the tested models are shown in Figs. 20–23.As a measure of intensity of excitation, shaking table acceleration time history is also plottedin each figure. In Figs. 20 and 21 the responses of control model M1 and CFRP laminateconfined model M4 to seismic excitation of the same intensity are compared (test runs R75and R100, respectively; see Table 2 for the comparison of parameters defining the intensityof shaking). In Figs. 22 and 23, however, the responses of CFRP laminate confined modelsM4 and M5 to seismic excitation at maximum intensity (test run R300) are presented.

The maximum measured story acceleration values (absolute maximum values) for allmodels are summarized in Table 4. To correlate the actual amplification of motion, maximummeasured accelerations of the shaking table motion are also given in the table. The sameobservation as in the case of analyzing the amplication effects of the calculated shaking tableresponse spectra can be made. As can be seen in Table 5, where the amplification factors aregiven as a ratio between the measured acceleration at the top of the model and accelerationof the shaking table, the values are similar to values indicated in Fig. 13.

Although the amplification of shaking table motion has not been substantial, it can benoticed, however, that even in the case of models M1, M2, and M3, which exhibited poorseismic behavior, the accelerations measured at the top were relatively high. Since the max-imum acceleration response values, measured during the test run R100 where the top floorsof models without wall ties disintegrated, do not represent the monolithic response of thestructure any more, the values are not included in the analysis.

Because of dynamic spectral characteristics of the model earthquake and tested modelsit is difficult to directly assess the beneficial effect of seismic isolators in the case of thestrengthened models M4 and M5. By correlating the induced accelerations, it is evidentthat—neglecting the fact that the natural periods of vibration of the models are either shorteror longer than the period range of maximum amplification of the model earthquake—muchsmaller accelerations have been induced in the case of the base-isolated model M5 than in thecase of model M4 without base isolation. By comparing the acceleration responses of modelsM4 and M5, the effect of isolators can be clearly seen. Whereas during the test runs R75

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Fig. 20 Acceleration response of model M2 to seismic excitation during test run R75

Fig. 21 Acceleration response of model M4 to seismic excitation during test run R100

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Fig. 22 Acceleration response of model M4 to seismic excitation during test run R300

Fig. 23 Acceleration response of model M5 to seismic excitation during test run R300

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Table 4 Maximum absolute accelerations of the shaking table ag,max, and response values aM,max, mea-sured at floor levels and at the top of isolators during the characteristic test runs (in g)

Model Position R50 R75 R100 R150 R200 R300

M12nd floor 0.458 0.921 – – – –

1st floor 0.408 0.616 – – – –

Shaking table 0.386 0.505 0.688a – – –

M22nd floor 0.534 0.602 – – – –

1st floor 0.395 0.530 – – – –

Shaking table 0.386 0.505 0.688a – – –

M3

2nd floor 0.369 0.502 – – – –

1st floor 0.349 0.512 – – – –

Above isolator 0.284 0.459 – – – –

Shaking table 0.386 0.505 0.688a – – –

M42nd floor 0.277 0.473 0.601 0.917 1.308 2.148

1st floor 0.257 0.425 0.549 0.825 1.170 2.539

Shaking table 0.239 0.380 0.483 0.727 1.015 2.682

M5

2nd floor 0.250 0.381 0.496 0.695 0.901 1.266

1st floor 0.245 0.373 0.479 0.632 0.777 0.982

Above isolator 0.225 0.352 0.455 0.582 0.726 0.931

Shaking table 0.239 0.380 0.483 0.727 1.015 2.682

Prototype values: aP,max = 0.25 aM,maxa Collapse: disintegration of the walls on the top floor

Table 5 Acceleration response amplification factors values A = aM,max/ag,max measured during the char-acteristic test runs

Model Amplification

R50 R75 R100 R150 R200 R300

M1 1.19 1.82 – – – –

M2 1.38 1.19 – – – –

M3 0.96 0.99 – – – –

M4 1.16 1.24 1.24 1.26 1.29 0.80

M5 1.05 1.00 1.03 0.96 0.89 0.47

and R100 the isolators reduced the acceleration response by 20%, the reduction amounted to45% and 70% during the test runs R200 and R300 respectively.

Maximum measured absolute story displacement values are compared in Table 6. As wasthe case of the measured first natural frequency of vibration, smaller displacement responsevalues also indicate the increased stiffness of the structure as a result of confinement of modelM4.

To better analyse the differences in the behavior of the models, the relationships betweenthe base shear developed in the first story of the models and relative story drift have beenalso evaluated. Maximum values of the base shear evaluated on the basis of the measuredacceleration responses of the models during each test run are compared in Table 7. Baseshear is given in terms of the base shear coefficient, BSC, i.e. the ratio between the base shear

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Table 6 Maximum absolute story displacement response values, dM,max, measured at floor levels and at thetop of isolators during the characteristic test runs (in mm)

Model Position R50 R75 R100 R150 R200 R300

M12nd floor 1.442 10.062 – – – –

1st floor 0.613 4.607 – – – –

M22nd floor 0.353 2.119 – – – –

1st floor 0.186 1.322 – – – –

M32nd floor 18.010 28.240 – – – –

1st floor 16.965 26.890 – – – –

Above isolator 15.530 24.130 – – – –

M42nd floor 0.181 0.305 0.527 0.711 1.186 8.534

1st floor 0.155 0.247 0.356 0.472 0.754 5.417

M52nd floor 15.414 24.863 33.724 49.734 62.227 85.075

1st floor 14.905 24.008 32.516 47.671 59.333 79.732

Above isolator 14.383 23.073 31.184 45.636 56.376 73.549

Prototype values: dP,max, = 4 dM,max

Table 7 Maximum base shear coefficient evaluated on the basis of the measured response of the modelsduring characteristic test runs BSC = miamax,i/W

Model R50 R75 R100 R150 R200 R300

M1 0.245 0.662 – – – –

M2 0.225 0.453 – – – –

M3 0.305 0.434 – – – –

M4 0.228 0.380 0.488 0.739 1.050 2.032

M5 0.108 0.322 0.415 0.564 0.702 0.895

BS developed in the model during shaking and the weight of the model above the base W :BSC = BS/W . Base shear has been calculated as the sum of products of masses, concentratedat the levels of floors mi and measured average maximum values of accelerations at the samelevel ai : BS = miai .

Typical base shear coefficient—story drift rotation angle hysteresis loops, calculated forthe cases of model responses, shown in Figs. 20–23, are presented in Figs. 24 and 25. Storydrift rotation angle, �, is defined as the ratio between the relative story drift, d , and storyheight h: � = d/h (in %). The resistance curves, which show the relationships between themaximum base shear developed in the model during individual test run and correspondingvalue of story drift, evaluated on the basis of the measured responses of all models, arepresented in Fig. 26.

For this purpose, the rotation of model structures due to vertical deformations of isolatorshas been taken into consideration in the case of isolated models M3 and M5. Since verticaldisplacements of the foundation slab of the models above the isolators have not been mea-sured, the rotation has been estimated on the basis of the measured vertical stiffnesses ofisolators and shear forces developed during vibration along the height of the models. Conse-quently, although the results are logical, the calculated corrections may only be consideredas an approximation.

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Fig. 24 Base shear coefficient—relative story drift rotation angle hysteresis loops. (a) Response of modelM2 during test run R75 and response of model M4 during test run R100 (b)

The comparison of test results clearly indicates the beneficial effect of confining andstrengthening old brick-masonry buildings with vertically, horizontally and diagonally placedCFRP laminate strips. Although the strips have been placed on the outer side of the wallsonly, the resistance of confined model M4 by more than 3-times exceeded the resistance ofcontrol model M1 without wall ties. CFRP laminate strips ensured the integrity of modelstructure even when subjected to seismic excitation which by more than 3.5-times exceededthe intensity of excitation causing the collapse of control model. Obviously, the system ofvertical and horizontal confining elements, together with diagonally placed strips which tookthe shear loads, ensured the structural integrity of the building and prevented the walls fromdiagonal cracking. When the anchor plate at the bottom of vertical confinement of model M4pulled off, the model started rocking as a rigid body. However, although crushing and fallingout of masonry units occurred at that point, the confinement maintained the integrity of thestructure until the end of test.

By comparing the resistance curves of CFRP laminate strips strengthened models M4and M5, the efficiency of seismic isolators can be estimated. In the case of model M4, builtdirectly on the foundation slab of the shaking platform, the available lateral resistance has

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Fig. 25 Base shear coefficient—relative story drift rotation angle hysteresis loops. (a) Response of modelM4 during test run R300 and response of model M5 during test run R300 (b)

Fig. 26 Comparison of resistance curves evaluated on the basis of the model shaking table tests

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been almost attained, whereas at similar intensity of shaking the shear developed in the firststory of isolated model M5 has been more than 2-times smaller.

4 Conclusions

Experiments have shown that seismic isolation alone is not enough to improve the seismicbehavior of old masonry buildings without wall ties. Experiments have also shown that asimple damp-proof course in the form of PVC sheet installed in the mortar bed joint cannotbe considered as seismic isolation. However, the shaking table tests of models, confined withhorizontal and vertical CFRP laminate strips and strengthened with diagonally placed stripsat the same time, indicated significantly improved seismic behavior. The CFRP laminatestrengthened models did not collapse even when subjected to ground accelerations whichby more than three-times exceeded accelerations causing the collapse of the models withoutwall ties.

The experiments indicated the possibility of replacing the steel ties, which are installedat floor levels of old masonry buildings as one of the usual seismic strengthening measures,by CFRP laminates. Placed also vertically and diagonally, CFRP laminate strips additionallystrengthen the structure, if properly anchored into the foundation system at the ends. Althoughthe model earthquake, used to drive the shaking table in the particular study, cannot beconsidered as typical design earthquake, the experiments also confirmed the long knownfact that seismic isolation of rigid masonry structures represents an efficient way to reduceseismic loads. However, the experiments showed that the usual measures to ensure structuralintegrity, such as the tying the walls with wall ties, should not be omitted.

The experiments indicated high efficiency of contemporary technical solutionswhen applied to old masonry buildings. However, they also pointed out that technologi-cal details, crucial for the efficiency of such methods, need to be resolved before the methodsbe widely applied to old masonry buildings. Namely, adequate solutions related with bondingand efficient interaction between the materials which have so extremely different mechanicalcharacteristics as CFRP laminates and masonry, need yet to be found.

Acknowledgments The research presented in the paper has been carried out within the framework of researchproject L2-0691 and research program P2-0274, financed by the Ministry of High Education, Science andTechnology of the Republic of Slovenia and co-financed by rubber industry, Sava Company Ltd., programConstrumat from Kranj, Slovenia. CFRP laminates and bonding materials have been given at disposition freeof costs by Sika AG, Slovenian branch from Trzin, Slovenia.

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