Recent earthquakes in Nepal and elsewhere in the world havecaused extensive damage to a large number of existing unreinforcedmasonry (URM) buildings. The majority of those URM buildingshave been constructed with little or no attention to seismic consid-erations. This has resulted in a large inventory of buildings thatlack ability to withstand strong seismic jolts. Therefore, there isan urgent need to improve the performance of URM structuresby retrofitting and strengthening them to resist potential earthquakedamage.
Seismic performance of these structures largely dependents onthe strength and behavior under in-plane loading. However, thebehavior of masonry walls under in-plane loading can generallybe divided into two categories: shear and flexure. Whether a wallis dominated by shear or flexure is largely dependent on the aspectratio (L=H) and vertical compression on the masonry (σn). Forslender walls (L=H < 1.0) with relatively light axial stress, behav-ior is usually dominated by flexure and the strength is limited byeither rocking or toe crushing preceded by a flexural cracking.For stocky walls (L=H > 1.5) with moderate to heavy axialstress, shear usually dominates through bed-joint sliding or diago-nal tension modes of failure (Magenes and Calvi 1997; Zhuge2010; Tomaževič and Gams 2009). Among these four inelasticfailure modes (Fig. 1), rocking and sliding shear are classified asdeformation-controlled phenomena because large lateral deformation
of walls and piers is possible without a significant loss in strength(Foraboschi 2009). By contrast, diagonal tension and toe-crushingbehavior modes are known as force-controlled phenomena becausethe ultimate failure can be abrupt with little or no subsequentdeformation. Stair-stepped diagonal cracking can also be consid-ered as a deformation-controlled action because frictional forcesalong bed joints are conserved with vertical compressive forces.However, diagonal tension cracking must be classified as a force-controlled action unless stair-stepped cracking can be distinguishedfrom diagonal cracking through units [FEMA 274 (FEMA 1997)].It should be noted that not all these failure modes will involvecollapsing of the masonry shear wall, and the final failure may bea combination of several failure modes.
A more general way of representing the various failure modesof masonry shear wall is given in Fig. 2, where the interactionsbetween normal and shear stresses on the masonry bed jointsare shown. It is evident in Fig. 2 that the failure modes in masonryshear walls are generally dictated by the magnitude of verticalcompression force applied over the wall. However, a rationallydeveloped failure criteria should be able to predict the tensile, com-pressive, and shear types of failure.
FEMA 356 (FEMA 2000) provides most up-to-date guidelinefor analysis of masonry structures considering performance-baseddesign. Three performance levels are defined and used as discretepoints to guide a rehabilitation design based on the expected per-formance of a building. Performance levels are based on theamount of damage to both the structural and nonstructural ele-ments. The three defined levels for primary structural elementsare immediate occupancy (IO), life safety (LS), and collapse pre-vention (CP). The guideline states two procedures for determiningthe acceptability of URM walls as a function of these performancelevels: linear static procedure and nonlinear static procedure. Forany of the procedures, a load–deformation backbone curve is re-quired to determine the strength and expected level of performanceof the component. Detailed description of these procedures can befound in Chapters 3 and 7 of FEMA 356 (FEMA 2000). However,
1JSPS RONPAKU Fellow, Graduate School of Engineering, HokkaidoUniv., Sapporo 060-8628, Japan (corresponding author). E-mail: [email protected]
2Professor, Faculty of Engineering, Hokkaido Univ., Sapporo 060-8628,Japan.
design guidelines based on component performance for fiber-reinforced polymer (FRP)-retrofitted masonry wall are still laggingand this study aims to highlight on this backdrop to some extent.
The aim of seismic retrofitting is to upgrade the ultimatestrength/deformation of the structure by improving the structure’sability to undergo inelastic deformation without fully collapsingduring an earthquake. FRP, which is a composite material consist-ing of a polymer matrix imbedded with high-strength fibers, suchas glass, aramid, carbon (Fig. 3) to achieve certain properties betterthan either of the base materials, is getting popular for retrofittingof the existing masonry structures. The benefit of using FRPs aspotential strengthening material comes from the reduction in han-dling costs; despite additional material costs, they are easy to installdue to lightweight (Burgoyne and Balafas 2007). Externallybonded polyethylene terephthalate-FRP (PET-FRP) with a largefracturing strain (>10%) is a retrofitting technique that has drawnsignificant attention as a unique alternative to carbon FRP (CFRP)or glass FRP (GFRP). High fracturing strain in PET-FRP yieldssubstantial inelastic deformation in masonry, which compensatesthe inherent brittleness of these structures without compromisingthe other advantages of FRP. In Japan, the use of PET-FRP hasemerged as an alternative to traditional FRPs, such as carbonand GFRP, where ductility is a major concern than strength(Anggawidjaja et al. 2006).
Various approaches have previously been undertaken to inves-tigate masonry strengthened with FRPs. There is no harmonizedtest method available to determine the shear performance ofmasonry elements under lateral loading (Bosiljkov et al. 2008;Tomaževič and Gams 2009). A TNO report (TNO 2004) gives
Fig. 1. Different failure modes of URM: (a) flexural cracking; (b) rocking followed by toe crushing; (c) shear sliding; (d) diagonal tension
L/H < 1.0
1.0 < L/H < 1.5
Flexural cracking followedby rocking and toe-crushing
an exhaustive discussion and literature review of various methodsfor testing of masonry shear wall that have been undertaken by dif-ferent researchers. None of them simulates real-time behavior, buthad been chosen because they reproduce static or kinematic boun-dary conditions, which can be easily interpreted with a perceivedanalytical model. Stratford et al. (2004) tested masonry wallsstrengthening with GFRP sheet, which shows a considerable in-crease (65%) in shear capacity without significant improvementin ductility. In these masonry walls, the failure was primarily attrib-uted to debonding of the GFRP sheet. Alcaino and Santa-Maria(2008) applied CFRP strips in two different configurations over16 masonry walls. An alternative to this system is to attach theFRP strips over the surface of the wall in a diagonal fashion (Dizhuret al. 2013; Altin et al. 2008; Akın et al. 2013) or in a grid system(Benedetti and Steli 2008). The FRP strips here act as a truss madefrom unidirectional fibers.
When FRPs are bonded to the surface of the wall, diagonal ten-sion failures or compressive crushing failures at wall toe are quitecommon (Hamid et al. 2005; Wang et al. 2006). In addition, pre-mature debonding of FRP, a brittle but unavoidable failure mode,was commonly observed during the test and, in general, FRP couldnot reach its ultimate strength (Ehsani et al. 1997; Stratford et al.2004; ElGawady et al. 2005; Foraboschi and Vanin 2013). Exper-imental tests indicate that the failure patterns are affected by but notlimited to the strength, orientation, amount, and anchorage lengthof FRP (Alcaino and Santa-Maria 2008; Marcari et al. 2007). Long-term durability of FRP strengthening work is a great concern ofmodern-day researches. One new finding that affects both masonryand concrete members that are externally strengthened using FRP isdelayed debonding, which is a phenomenological developmentof critical crack throughout the lifetime of the structure alongthe substrate–FRP interface that causes the external FRP to losebond stress and eventually reduce the lifetime of strengtheningworks (Foraboschi 2015). Shrestha et al. (2014) pointed out thatthere is considerable influence of moisture in deteriorating the bondproperty and reduce the durability of FRP-retrofitted works. In gen-eral, the possible failure mode for masonry strengthened with FRPcan be a combination of several mechanisms such as excessivecracking due to tensile stresses in the wall, crushing of masonryin the compression zone, shear slip of masonry, FRP debonding(instantaneous and delayed), and FRP rupture [CNR DT 200 R1(Italian National Research Council 2013)].
In this study, PET-FRP and CFRP have been used as two distinctstrengthening materials. The strength of CFRP is higher than PET-FRP, whereas PET-FRP possess a relatively higher fracturing strainthan CFRP. The purpose of this study is to show the differencein behavior of masonry shear walls strengthened by these twodifferent FRPs and the superiority of the one FRP over the other.Ultimate load-bearing capacity, deformation at peak load, andmode and mechanism of failure are observed in this study for differ-ent arrangements of the FRPs.
Experimental Program
Specimen Details
In this experimental study, a total of 12 masonry walls havingnominal dimensions of 1,270 × 1,020 × 120 mm3 were fabricated.All of the walls were made with a single-layer running bond ofbricks having dimensions of 240 × 120 × 74 mm3 with an averagecompressive strength of 30 MPa. A 10-mm thick mortar joint witha compressive strength of 22 MPa was used throughout. Becauseof the anchorage at the wall top and bottom, the topmost and
bottommost layers of the brick were ineffective in transferringthe shear. Therefore, the effective height (872 mm) of the wall con-sisted of the remaining 10 courses of brick, and offered an aspectratio (L=H) of 1.45. Although the size of the masonry wall chosenfor this experimental purpose does not necessarily reflect thenominal size of actual masonry structures, an aspect ratio of1.45 does resemble that of the real-time masonry shear wall as fail-ure in this size of wall predominantly governed by either bed-jointshear sliding or diagonal tension failure [Figs. 1(c and d)], whichcan be seen in many masonry failure during the past earthquakes(Fig. 4). Table 1 provides the details of the walls. In this table, RWstands for reference wall, P for wall strengthening with PET-FRP,C for wall strengthening with CFRP, D for diagonal strip configu-ration, G for grid system, F for fully wrapped wall, and S for solidbricks.
Specimen Preparation
Both PET-FRP and CFRP sheets having unidirectional fibers wereapplied using a wet layup procedure on wall specimens in threedifferent configurations as shown in Fig. 5. At first, the wall spec-imens were cleaned by removing loose mortar and dirt with thehelp of a wire brush. Epoxy putty (filler) was used to fill the
Fig. 4. Typical diagonal tension failure in a squatting wall duringEmiliana earthquake in Italy, 2012
Table 1. FRP Configurations and Summary of Test
Results for masonry specimens
Wallidentifier
Peakload (kN)
Deformation atpeak load (mm) FRP configuration
RWS1 48 8.7 Reference wallRWS2 30 5.4 Reference wallPSD1 114 8.9 PET crossdiagonalPSD2 101 24.7 PET crossdiagonalPSG1 129 10.0 PET in grid systemPSG2 88 9.7 PET in grid systemPSF 168 6.5 PET fully wrappedCSD1 95 2.9 Carbon crossdiagonalCSD2 94 4.4 Carbon crossdiagonalCSG1 134 2.5 Carbon in grid systemCSG2 99 7.0 Carbon in grid systemCSF 107 2.7 Carbon fully wrapped
depression on the wall surface to provide a smooth plane surfaceready for FRP laying. A thin layer of primer was then applied allover the wall to seal the pores in the masonry. Epoxy resin havingthe properties indicated in Table 2 was then applied over the wallwherever necessary. Fiber sheets were cut to the width of 80 mm fordiagonal strip and 70 mm for the grid system. Resin was then ap-plied with a roller brush all over the sheet strip so that it was fully
saturated by the resin. As soon as the application of the resin wascompleted, the FRP strips were laid over the wall and wrappedtightly to keep them in place. For the diagonal strip, the wall cor-ners were cut in the same dimension width (80 mm) as the strip[Fig. 5(a)] to avoid sharp edges and problem in wrapping.
FRP Anchorage
After the required curing period, the wall was transferred to theloading frame as shown in Fig. 6, specially designed for this ex-perimental study. The wall top and bottom were anchored to thetop and bottom channel beams, respectively, with the help of 2422-mm bolts as shown in Figs. 6 and 7. Between the wall andthe channel beam, a 16-mm thick steel plate was inserted so thatpressure could be applied on the wall by tightening the bolts. Gyp-sum plaster was used to fill the gap between the wall and the steelplate. For FRP-strengthened walls, the FRP itself was wrapped in aU-shaped anchorage, so there was almost no chance of slippage ofthe FRP during the experiment. Although this kind of anchoragesystem is not very much practical for the real structure, it has beenchosen so for this experimentation to avoid premature anchoragefailure of FRP prior to attaining full shear strength. For fully
Fig. 5. FRP reinforcement layout: (a) FRP crossdiagonal bracings (CSD and PSD); (b) FRP strips in grid system (CSG and PSG); (c) fully wrappedby FRP (CSF and PSF)
Table 2. Properties of FRPs and Adhesives (Data from Nippon Steel &Sumikin Materials Co. Ltd. Japan 1997; Maeda Kosen Co. Ltd. Japan2005)
wrapped masonry walls, horizontal FRP sheets were installedon the wall in 360° wrapping (Fig. 8) and vertical sheets wereanchored to the top and bottom channel beams with 22 mm boltsas shown in Fig. 7.
Application of Load
A precompression of 40 kN, which is equivalent to a uniform pres-sure of 0.25 MPa, was applied on the top of the wall through twohydraulic jacks, prior to incrementally increasing lateral loading(Fig. 6). Lateral load was applied gradually, increasing at a constantrate of 5 kN=min until it reached at peak. In postpeak regime, aconstant displacement of 2 mm=min was applied until completefailure. The two vertical loads restricted the rotation of the top beamonly to some extent, as the beam was not restrained against rotationduring the evolution of lateral load. A fixed-free experimental con-dition was maintained throughout the loading process.
Instrumentation and Data Acquisition
Sufficient number of uniaxial, biaxial and rosette strain gauges,LVDTs, and load cells (50-t capacity) were used to record allthe necessary information during the testing (Fig. 9). A lateral loadwas applied in a pull-out manner by a 50-t capacity center-holejack, with a remote pump, until the complete failure of the masonrywall. All of the data (displacement, strain, and load) were recordedthrough a digital data logger.
Experimental Observation
Failure of URM Walls (RWS1 and RWS2)
These two reference specimens were tested under similar boundaryconditions to make a comparison with rest of the walls mentionedin Table 1. No potential crack was observed in either of the wallsuntil the load reached about 80% of the peak load. Only after that, aflexural crack appeared at the heel of the wall, and propagated to-ward the toe as shown in Figs. 10(a and b), and the load droppedslightly. Lateral load was further increased to ultimate load, and thecrack opening widened as the crack propagated all the way to thetoe of each wall. After the applied load reached its peak, it did notdecrease much with the increased displacement, which furtheredthe shear slip along the crack plane for wall RWS1 [Fig. 10(a)].In specimen RWS2, no shear slip was observed rather the crackpropagated in a stepped fashion and traveled all the way to thetoe and a faster crushing at the wall toe was observed, which causeda rapid decrease in load. This kind of failure in URM wall undershear is reported in a number of papers (Stratford et al. 2004;Salmanpour et al. 2013; Bosiljkov et al. 2003; ElGawady et al.2007, etc.).
Fig. 7. Experimental setup of FRP-reinforced masonry wall
Diagonally Braced Walls (CSD and PSD)These four walls were externally strengthened by two diagonalCFRP and PET-FRP strips each, of 80 mm width in a fashionshown in Fig. 5(a). No noticeable crack appeared on either ofthe walls until the lateral load reached about 75% of the ultimatestrength. In the case of the wall CSD1, two flexural cracks appearedat the wall top on the loading end and at the wall bottom on the farend [Fig. 10(c)] followed by some debonding of the CFRP tensiondiagonals. No rupture or damage of the CFRP was observed. Forwall CSD2, some fine-line cracks appeared at the top of the wall onfar end and started to propagate downward in a stepped fashion.Almost simultaneously, another crack, parallel and above to thisone, appeared and began to propagate downward in a same mannerbut with a lesser degree of crack opening displacement. Once theload increased to a maximum of 94 kN, a sudden rupture of thediagonal tension strip on the both sides of the wall took placeand the load suddenly dropped to the half of the peak load.Accumulated compressive stress caused some crushing at walltoe just before the rupture of the CFRP strip, which can be seenFig. 10(d).
In the case of the walls reinforced with PET-FRP strips (PSD1and PSD2), at a load of nearly 50% of the peak load, similar diago-nal cracks were observed to appear on these two walls along thebottom of the compression diagonal. These traversed along thecompression diagonal in a stepped fashion [Figs. 10(e and f)].In PSD2, another crack appeared on the top of the compressiondiagonal and propagated all the way to the toe of the wall followedby some toe crushing. The peak loads of 114 and 101 kN, respec-tively were not attenuated much with the applied displacement forquite a long time, and at that stage the experiment was ceased.Some debonding in tension diagonal PET strips was observed inboth of these two walls.
Wall Strengthened by FRP Grid System (CSG and PSG)These four walls were strengthened with CFRP and PET-FRP stripsof 70 mm width each in a grid system as shown in Fig. 5(b). In thecase of the CFRP-reinforced wall (CSG1), at a load, amounting to70% of peak load, some fine-line diagonal cracks appeared at thecentral area of the wall. More cracks appeared across the tensiondiagonal with simultaneous debonding of the CFRP strips, whilethe load reached at a level of 130 kN [Fig. 11(a)]. Almost simulta-neously, one of the horizontal CFRP strips ruptured with a loudexplosive sound, and the load dropped to 109 kN. With furtherdisplacement, the crack width increased and, in some places, theCFRP totally separated from the wall. In CSG2, at a load equalto about 75% of the peak load, flexural crack along the bottomof the wall appeared, which was followed by another flexuralcrack at the midheight of the wall and a web crack at the center[Fig. 11(b)]. With further displacement, the peak load did notchange from 99 kN, which only widened the crack opening. Norupture of CFRP was noticed, except some debonding in horizontalstrips at few places.
For PSG1, at a load of about 120 kN, which is tantamountto 93% of the peak load, some debonding of the PET-FRP stripoccurred along the tension diagonal. Almost simultaneously, aflexural cracking followed by some discontinuous diagonal crack-ing appeared at the lower half of the wall on the heel side, and trav-eled all the way to the wall toe in a stepped fashion as shownin Fig. 11(c). Similar but a single flexural crack was observedin the wall PSG2 at a load of 63 kN, as shown in Fig. 11(d). Minorcrushing at wall toe was noticed as soon as the peak loadreached 88 kN.
Walls Fully Wrapped with FRPs (CSF and PSF)These two walls were strengthened after fully wrapped with CFRPand PET-FRP sheets according to Fig. 8. For CSF, the stiffness aswell as the lateral load capacity was so high that at a later stage thegrip at the wall bottom prematurely failed before the wall attained
Rupture of CFRP
(a) (b) (c)
(d) (e) (f)
Fig. 10. Schematic illustration of crack patterns in unreinforced and diagonally reinforced masonry walls: (a) RWS1; (b) RWS2; (c) CSD1; (d) CSD2;(e) PSD1; (f) PSD2
its full strength [Fig. 11(e)]. Because the wall was fully wrapped bythe CFRP sheets, damage, if any, could hardly be detected by visualobservation. By contrast, the wall PSF showed constant stiffnessuntil it reached about 60% of its peak shear capacity. Only afterthat, did the stiffness show a reduction, but no damage or crackwhatsoever was seen on the wall [Fig. 11(f)]. Once it reachedits full strength, further displacement only resulted in load reduc-tion and the wall failed at its grip at the bottom similar to the wallCSF. It was quite clear from fully wrapped wall that higher the per-cent of FRP, higher is the stiffness as well as some enhancement inshear strength of masonry wall. However, higher amount of FRPmaterials do compromise with the ductility of the wall and thesematerials do not fit well with the purpose of seismic strengthening.
Load–Deformation Response
URM WallsThe load–deformation characteristics of reference walls (RWS1and RWS2) are depicted in Fig. 12(a). It is quite evident fromthe figure that, before appearance of any potential flexural crack,the load–deformation relationship is almost linear for both the twowalls. It is interesting to note that RWS1 shows quite ductile behav-ior while maintaining a constant lateral strength due to shear slip atthe wall bottom. By contrast, RWS2 shows a rapid reduction inlateral shear after a crushing took place at the wall toe.
Fig. 13 shows the variation of vertical precompression with theprogress of lateral load. It is evident from Fig. 13 that the verticalload on the far end increased with the progressive shear, while thevertical load went down on loaded end. This is because with theincrease of lateral load, the top beam on the wall underwent a smallrotation, which exerted a vertical upward pressure on the far-endjack on the wall. However, the average vertical load (broken linein Fig. 13) did not change much from the initial stipulated valueof 20 kN.
Strengthened Masonry WallsThe load–deformation characteristics of diagonally braced walls(CSD and PSD) are plotted in Fig. 12(b). It is quite evident fromthe figure that the lateral load capacity increased more than twotimes for all the four retrofitted walls. The lateral load increasedquite linearly until a potential crack appeared in those walls. A re-duction in stiffness can be seen thereafter, which depicts the inelas-tic response of the strengthened masonry walls. One importantcharacteristic here is that the wall diagonally braced with CFRPstrips shows a brittle type of failure where load reduction was quiteabrupt with no softening at all. This was due to the crushing at walltoe immediately followed by a rupture of the tensile CFRP strips onboth sides of the wall [Fig. 10(d)]. By contrast, walls strengthenedby PET-FRP show quite ductile nature of inelastic behavior, fol-lowed by a gradual softening regime, retaining some residualstrength. Moreover, no rupture of the PET strip was observed inPSD walls, expect for some debonding phenomenon elsewherein the tension diagonals.
Fig. 12(c) portrays the load–deformation response of the wallsthat were strengthened with CFRP and PET-FRP in grid system.Initial stiffness was fairly linear at the beginning and maintaineda constant stiffness up to two third of the peak load. Only after thata reduction in stiffness can be seen and a ductile nature of inelasticresponse continued for quite a long time. It is also interesting tonote that the ductility is not compromised substantially whenthe walls are strengthened with higher amount of CFRP such asin grid system than the amount of CFRP in diagonal configuration.This phenomenon also holds true for PET-FRP walls (PSG1and PSG2).
By contrast, Fig. 12(d) demonstrates the load–deformation re-sponse of the walls fully wrapped by FRPs. The shear capacity ofthose walls increased by more than three times of that of unrein-forced walls. The deformation does not portray the real picture as itwas stated earlier that these two walls underwent some rotation attheir bottom and a premature failure at the grip followed. Even
Fig. 11. Schematic representation of crack patterns in grid system and fully wrapped walls: (a) CSG1; (b) CSG2; (c) PSG1; (d) PSG2; (e) CSF;(f) PSF
then, a lesser amount of deformability of these walls can be fore-seen from their load–deformation characteristics. However, it canbe predicted that wall strengthened with PET sheet behaves in aductile manner than its counterpart CFRP wall and that wallstrengthened with PET-FRP can resist more lateral load than theCFRP wall.
Analysis and Discussion of the ExperimentalResults
From the foregoing observation of the experimental results, it wasevident that the load-carrying capacity of the masonry walls sub-jected to in-plane shear can be greatly increased by applying eitherCFRP or PET-FRP sheet on their external surfaces, but the ductilityof those walls are substantially compromised, once they are bondedwith CFRP sheet. It was also observed that the failure modes of thestrengthened masonry walls are largely affected by the strength,orientation, and anchorage system of FRP. In the following section,more in-depth analysis of failure mechanism and in-plane shear re-sistance are discussed in light of the aforementioned experimentalobservation and existing knowledge on masonry.
Analysis of URM
The difference in failure modes for the reference specimens RWS1and RWS2 can be attributed to the location where the first flexuralcracks appeared in these two specimens and their line of propaga-tion [Figs. 10(a and b)]. In RWS1, first flexural crack appeared atthe very bottom course of the wall and propagated horizontally in astraight fashion toward the wall toe [Fig. 10(a)], which facilitatedthe wall to slip along the crack path. Once a crack forms at the wallheel, no tension force perpendicular to the crack can be transmittedacross it, and the load drops by a little, but as long as the crack isnarrow, the wall can still transmit some shear forces in its own planethrough friction of the surface roughness that can be characterizedas elastoplastic behavior. With further deformation and slip, the
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Fig. 12. Load–deformation responses of unreinforced and retrofitted masonry shear walls: (a) reference panels RWS1 and RWS2; (b) panels CSDand PSD with diagonal FRP; (c) panels CSG and PSG with FRP grid system; (d) panels CSF and PSF with fully wrapped FRP
Fig. 13. Variation of vertical load with progressive shear
interlocking planes suffer substantial damage and are flattened tosome extent after which the wall reaches a state of kinematic equi-librium where only some residual frictional shear resistance pre-vails (Rahman and Ueda 2014), which does not diminish withthe applied displacement as can be seen in Fig. 12(a). In the refer-ence wall RWS2, the first flexural crack appeared three coursesabove the bottom course on the wall heel side and propagated to-ward the wall toe in a stepped fashion [Fig. 10(b)]. This failurepattern prompted a rocking phenomenon in the wall instead ofshear sliding and consequently a softening behavior was observedin the postpeak regime [Fig. 12(a)]. This behavior was caused by atoe crushing in the wall. It is worth noting here that the locationwhere the first flexural crack will appear is somehow difficult topredict, as was the case for these two reference walls. This is largelybecause of the nonhomogeneous nature of masonry fabrication thatcreates several plane of weakness along the horizontal bed joints.
Thus, it can be said that an earlier flexural crack is normallyfollowed by either a toe crushing or a sliding shear type of failure.Stratford et al. (2004) observed similar failure mode in masonrywalls. Moreover, it is quite evident in Fig. 12(a) that the shearstrength for masonry sliding is higher than that of rocking andtoe crushing because shear sliding endures a resistance alongthe entire crack plane, whereas toe crushing occurs due to accumu-lation of compression stress on a very small area at the crack tip onthe wall toe. It is also observed that the existing equations [Eqs. (1)and (2)] proposed by FEMA 356 (FEMA 2000) to calculate masonryin-plane shear strengths for different modes of failure can fairlyapproximate the experimental results (48 and 30 kN) and the pre-dicted results were 47 and 27 kN for RWS1 and RWS2, respectively.
Shear strength for bed-joint sliding
VM ¼ Agτ sld ¼ Agðcþ σn tanϕÞ ð1Þ
Shear strength for toe crushing
VM ¼ α1σnAg
�LH
��1 − σn
0.7f 0mc
�ð2Þ
Some of the parameters used in these equations depend on testresults. In absence of candid test results for masonry compressivestrength and cohesion, Eq. (3) [Eurocode 6 (CEN 2005)] andEq. (4) (Ali et al. 2012) can safely be used for a conservative valueof these two parameters.
f 0mc ¼ 0.55ðfbÞ0.7ðfmÞ0.3 ð3Þ
c ¼ 0.0337ðfmÞ0.6 ð4Þ
where VM = shear strength of the URM wall; L, H, and b = length,height, and thickness of the masonry wall, respectively; f 0
mc =compressive strength of masonry prism; Ag = sectional area ofwall = (L × b); α1 = factor for boundary condition (0.5 for canti-lever wall and 1.0 for fixed wall); τ sld = average bed-joint slidingshear stress of the URM wall; c = cohesion between mortar andbrick; σn = vertical compression on masonry wall; ϕ = frictionalangle at the sliding surface (approximately 20°, Hansen 1999);and fb and fm = uniaxial compressive strength of brick and mortar,respectively.
Considering the ongoing discussion, the following conclusionscan be made:• Masonry with an aspect ratio (L=H) nearly 1.5 and with a low
vertical pressure (0.25 MPa) can have either shear sliding or toe-crushing type of failure.
• Shear sliding offers more resistance than toe crushing and theexisting FEMA 356 (FEMA 2000) provisions can fairly predictthe test results.
• More tests are necessary to observe diagonal tension failure inmasonry with the same aspect ratio but with higher verticalstresses.
Analysis of Strengthened Masonry
Experimental results summarized in Table 3 illustrate that the in-plane shear capacity of the tested walls was significantly enhancedby the proposed strengthening technique but the ductility behaviorwas not essentially the same. Figs. 10 and 11 depict the crack pat-terns and failure modes for all the retrofitted walls. It is interestingto see that the mode of failure in masonry when strengthened withdiagonal FRP is diagonal tension and they are quite similar to eachother (except for the case of CSD1, where there might be high pos-sibility that the anchorage at the bottom of the diagonal tensionstrip was somehow loosen that prompted a flexural tension crack-ing). Bonding FRP material onto the surface of the walls in diago-nal fashion made it stronger against flexural cracking due to the tiedaction of the FRP tension diagonals and triggered a diagonal crack-ing along the compression diagonal. Accumulated compressionstresses at the toe region of the wall eventually caused a toe crush-ing and to some extent ruptured the CFRP tension diagonal as forthe case of wall CSD2. Although there is not much difference inshear strength among these four walls, significant difference can benoticed in deformation capacity. Table 3 shows that the FRP stiff-ness in CFRP diagonal is three times more than that of PET-FRPdiagonal, but the mean deformation at peak load is only one fifth ofPET-FRP-strengthened wall. This is due to the fact that PET pos-sesses a higher fracturing strain (more than six times) than CFRP(Table 2). It is worth noting here that high stiffness not necessarilyoffers high shear strength, rather the reverse is very likely. Anotherinteresting thing here is that the stain in FRP remains quite lowbefore the inception of any potential cracking in masonry. Oncethere is a crack, the internal stress is redistributed and shifts its po-sition from masonry to FRP, thus increase the strain in FRP, espe-cially in tension diagonal strips as illustrated in Fig. 14. During thattime, the debonding phenomenon was also observed in FRP thatcaused a fluctuation in distribution of internal strain in FRP. Itis also important to mention here that even at the time of masonryfailure when the lateral load reaches its peak, the effective strainremains quite low and amounts to only some fraction of the ulti-mate fracturing strain of FRP. Based on this principle, most of theanalytical models assume a low value of effective strain in FRP
Table 3. Summary of Test Results for Masonry Walls
while calculating the shear contribution by FRP [CNR DT 200 R1(Italian National Research Council 2013); ACI 440 (ACI 2008)].These phenomena hold true for both CFRP and PETas well. There-fore, many authors (Stratford et al. 2004; Marcari et al. 2007)argued that it is impossible to accurately predict the FRP contribu-tion to shear strength as they prove by their experimental test thatthe contribution of the FRP to the shear strength of masonry wall isfar less than its ultimate tensile strength. Therefore, it can be con-cluded that• The mode of failure in masonry may change from shear sliding
to masonry diagonal cracking when they are externally strength-ened by FRP diagonal strips.
• FRP with high elastic modulus and low fracturing strain such asCFRP offers less ductility and possibly less in-plane shearstrength, which is not a good option for masonry strengthening.
• PET-FRP shows good ductile behavior at both prepeak andpostpeak regimes and significantly enhances masonry shearcapacity.
• FRP debonding is a common phenomenon for both CFRP andPET, but the rupture of PET sheet is very unlikely, whereasoccasional rupture of the CFRP cannot be ruled out.In the case of the FRP grid system, mixed mode of failure can be
seen in Fig. 11 for both CFRP and PET-FRP-reinforced walls, andtheir load–deformation responses are quite similar to each other[Fig. 12(c)]. This is due to the fact that beyond certain limit ofFRP amount, the failure mode of masonry wall commutes fromdiagonal tension to predominantly flexural cracking irrespectiveof the type of FRP. Numerical modeling and analysis of masonryis the best tool to evaluate this threshold value. The horizontal stripsof FRP played a crucial role here that restricted the wall from fail-ing in a diagonal tension mode. These horizontal strips made thewalls stronger in shear than in flexure. Although the vertical stripscontributed in flexural strength to some extent, eventually it couldnot out strength the shear capacity, and caused the wall to fail inflexure. It is also interesting to make a comparison betweenFigs. 12(b and c) that once masonry walls are strengthened withCFRP in grid fashion, its brittleness in diagonal configurationchanged into a ductile one. High FRP stiffness (11,420 MN) did
not change the overall stiffness of the masonry wall substantially,but rather it helped the wall to deform as a composite system insuch a way that can be compared as elastic–perfectly plastic defor-mation. However, the deformability of high-stiff CFRP will be lessthan that of PET as PET has higher deformation rate than CFRP.Another important point here to note is that increasing the amountand stiffness of FRP material on the surface of the wall does notnecessarily increase the shear strength of the strengthened wall.So, it is very much crucial to find out a suitable FRP material thathave a moderate strength with a good deformability and have to beused in optimum amount that will be coherent with the fragilenature of masonry. One interesting observation that could be madehere is that, in absence of vertical strips, the wall would have ex-perienced in-shear sliding along the planes that are not offset by thehorizontal strips (Alcaino and Santa-Maria 2008). Thus, it can beconcluded that:• Higher amount of FRP material bonded on the external surface
of the masonry wall does not serve the purpose of strengthening;rather it will only increase the cost of the strengthening workand in some instance increase the stiffness of the masonry.
• Although the PET-FRP in grid system does not compromisewith the overall ductility of the masonry, it is not an effectivesystem of strengthening as it does not help much to enhance themasonry in-plane shear strength compared with the diagonalsystem. The same is true for the CFRP grid system as well.
• FRP with high stiffness may change the masonry mode of fail-ure from diagonal tension cracking to a flexural cracking.As for the case of fully wrapped walls, ductility is hugely com-
promised for either of the FRP material, that is, CFRP or PET.Fig. 12(d) does not depict the real deformation picture as it wasstated earlier that these two walls underwent some rotation at theirbottom and a premature failure at the grip was noticed. Even then,the deformability of these walls can be foreseen from these load–deformation plots. Therefore, it can be said that though PET-FRPwall can resist more lateral load than its counterpart CFRP, either ofthem is not a good option for masonry strengthening as long as theyare used in a fully wrapped manner.
One interesting point that came out from the analysis of theseexperimental results is that the contribution of FRP in masonryshear strength, irrespective of their types and arrangements, willonly manifest after a certain time, which is somewhat like 60%of the peak strength and that can be attributed to the service loadingcondition. Only after that some potential cracks show up in the ma-sonry and the FRP starts to take part in contributing to shear. That iswhy there is no significant difference in the initial stiffness of theURMaswell asmasonrywith FRP [Figs. 12(a–d)] andwhy the initialstrain in FRP is also very small (Fig. 14). Thus, it can be concludedthat FRP strengthening of the masonry can have only marginal effecton the shear strength at the service load condition but can contribute alot at some accidental overloading such as earthquake.
Analysis of Masonry Performance
Inelastic analysis of a structure becomes increasingly importantwith the emergence of performance-based engineering (PBE) asa technique for seismic evaluation and design [ATC-40 (ATC1996); SEAOC 1995; Terán-Gilmore et al. 2008; Magenes andCalvi 1997]. PBE uses the prediction of performance to forecastdecisions regarding safety and risk. In this approach, the buildingsand their components are designed with the objective of desiredperformance level for each seismic earthquake level, and with pos-sible seismic hazards based on some acceptance criteria [FEMA440 (FEMA 2005); Abrams 2001].
Fig. 14. Load–strain responses in diagonal FRP strips of retrofittedpanels at Point 2 on Fig. 9(a)
FEMA 356 (FEMA 2000) provides a set of guidelines for URMwalls that restricts the damage to three basic levels of performance.They are the IO limit state, the LS limit state, and the CP limit stateas shown on the normalized force–deformation curve on Fig. 15.The CP limit state for URM buildings is related to the distress statein which the building is on the verge of partial or total collapse.Substantial damage to the structure has occurred, potentiallyincluding significant degradation in the stiffness and strength ofthe lateral-force resisting system, when the lateral resistance of abuilding drops by 20%, that is, VCP ¼ 0.8Vy, where VCP is thebase shear in the building at the CP limit state, and the Vy isthe effective yield strength of the building determined from thecapacity curve.
For seismic rehabilitation, simple nonlinear analysis is per-formed, based on a linear static approach, where lateral displace-ments are approximated in terms of member forces based on theequal displacement rule (i.e., elastic and inelastic displacementsare the same for structures beyond yield strength shown in Fig. 16).With this assumption, as noted in Eq. (5), elastic design force due toearthquake and gravity loads, Ve, will be equal to the inelasticstrength, Vi, times the ductility factor, m, which is defined asthe inelastic deformation at a given limit state, δi divided by theyield deformation, δy [Eq. (6)]. FEMA 356 (FEMA 2000) providesa set of ductility factors, m factors, for different limit states and fortwo different failure modes (rocking and shear sliding) of masonry.Table 4 presents the m factors and drift ratios (δ=H) recommendedby FEMA 356, along with the estimated performance of the shearwalls in this study
mVi ≥ Ve ð5Þ
where
m ¼ δiδy
ð6Þ
The nonlinear force–displacement curve (also known as push-over capacity curve) of the component of the structure shall be re-placed with an idealized relationship to calculate the effectivelateral stiffness, Ke, and effective yield strength, Vy, of the build-ing. Trilinear idealization of the lateral force–displacementcurves of a URM wall as well as of a FRP-strengthened wallsare shown in Figs. 17(a and b), respectively. In this study, the targetdisplacement, δt, which is intended to represent the maximum dis-placement likely to be experienced during the design earthquake,has been simply assumed to be equal to, δu, the deformation cor-responding to ultimate shear strength. The detailed idealizationtechnique and more accurate calculation of δt can be found inFEMA 440 (FEMA 2005) and ATC 40 (ATC 1996). From theidealized curve, the effective lateral stiffness, Ke, shall be takenas the secant stiffness calculated at a base shear force equal to60% of the effective yield strength Vy of the structure. In the caseof the unavailability of a pushover curve, a reasonable estimation ofin-plane shear stiffness of URM is suggested here, by Eq. (7), pro-vided all the parameters related to masonry are readily available(Al-Chaar 2002)
Ke ¼1.2σn
H3
3EmIgþ H
AgGm
ð7Þ
where Ig = moment of inertia of the uncracked section ofmasonry; Em = masonry elastic modulus; and Gm = masonry shearmodulus.
Performance of Strengthened Masonry
FEMA 356 (FEMA 2000) and FEMA 440 (FEMA 2005) guide-lines do provide methods for estimating the strength and deforma-tion capacity of existing URM walls and piers; however, they donot provide such information for components rehabilitated byFRPs. This is largely because such research data are limited. Onlysome strengthening techniques, design guidelines, and anchoragesystems can be found elsewhere in the model code or prestandardssuch as ACI 530 (ACI 2002), JSCE (2001), Euro Code 6 (CEN2005); CNR DT 200 R1 (Italian National Research Council2013), and ACI 440 (ACI 2008). From this experimental study,some of the parameters that were mentioned in the foregoing sec-tion regarding URM are discussed here. Ductility factors and CPare also addressed.
Damage Evolution and Ductility Capacity
From Table 4, it is evident that many of the URM shear walls havepoor performance in LS and CP limit states. Once the masonrywalls are strengthened by FRP strips, their performance gets better.Masonry wall strengthened by diagonal PET-FRP behaves in amore ductile manner than the rest of the strengthening walls. It alsoshows a very good postpeak CP behavior as the drift ratio is over1.5 with a drift ratio more than the FEMA recommended valueof 0.8 for URM. It is interesting to note that, when the shearwalls are strengthened with the PET-FRP grid system, their stiff-ness increased and made the seismic ductility demand for CP muchhigher. By contrast, walls strengthened by CFRP show low percentdrift ratio with a low ductility factor. As mentioned earlier, to beharmonized with masonry, a strengthening material needs to beidentified that offers a moderate stiffness with a good ductile
0.25 0.50 0.75 1.00
Drift ratio, ( /H) %
1.0
0.5
1.5
y
V
V
LS CPIO
Fig. 15. Performance indices
Ve
Vi
V
iy
Vy
Fig. 16. Equal displacement rule for linear elastic analysis
behavior. CFRP offers a higher stiffness with a low ductility per-formance, although having good enhancement of shear strengthover PET-FRP. Therefore, masonry externally reinforced withCFRP will not be a good alternative over PET-FRP.
Concluding Remarks
The in-plane shear behavior of 12 masonry walls having beenstrengthened by CFRP and PET-FRP sheets in three different
Table 4. Performance of Experimental Shear Walls for Different Limit States
Wall type ReferencesWall
identifierPerformance
indices
Performancelevel
Elasticstiffness
Ke (N=mm)
Postyieldstiffnessfactor, α1
Softeningstiffnessfactor, α2IO LS CP
URM FEMA 356 (FEMA 2000) — m factor 1.0 6.0 8.0 — — —URM FEMA 356 (FEMA 2000) — % Drift, (δ=H) 0.1 0.6 0.8 — — —URM This study RWS1 m factor 1.0 13 25 61,000 0.02 0.05URM This study RWS1 % Drift, (δ=H) 0.1 1.0 2.0 61,000 0.02 0.05URM This study RWS2 m factor 1.0 14 23 62,000 0.02 0.07URM This study RWS2 % Drift, (δ=H) 0.1 0.6 1.1 62,000 0.02 0.07URM Bosiljkov et al. (2004) BNL4 m factor 1.0 6.0 11.0 51,000 0.02 0.08URM Bosiljkov et al. (2004) BNL4 % Drift, (δ=H) 0.1 0.8 1.5 51,000 0.02 0.08URM Bosiljkov et al. (2004) BNL6 m factor 1.0 18 32 33,000 0.01 0.03URM Bosiljkov et al. (2004) BNL6 % Drift, (δ=H) 0.1 2.3 4.1 33,000 0.01 0.03URM Magenes et al. (2008) CL05 m factor 1.0 2.0 4.0 114,000 0.04 0.22URM Magenes et al. (2008) CL05 % Drift, (δ=H) 0.1 0.3 0.5 114,000 0.04 0.22URM Magenes et al. (2008) CL07 m factor 1.0 2.0 3.0 20,000 0.01 0.32URM Magenes et al. (2008) CL07 % Drift, (δ=H) 0.1 0.2 0.4 20,000 0.01 0.32URM Modena et al. (2005) 15_5 m factor 1.0 5.0 9.0 36,000 0.01 0.09URM Modena et al. (2005) 15_5 % Drift, (δ=H) 0.3 1.4 2.6 36,000 0.01 0.09URM Da Porto et al. (2009) 15_7 m factor 1.0 6.0 11.0 54,000 0.01 0.07URM Da Porto et al. (2009) 15_7 % Drift, (δ=H) 0.2 1.6 2.8 54,000 0.01 0.07URM Fehling et al. (2007) N1 m factor 1.0 1.2 2.3 73,000 0.09 0.23URM Fehling et al. (2007) N1 % Drift, (δ=H) 0.1 0.1 0.2 73,000 0.09 0.23URM Frumento et al. (2009) 18_3 m factor 1.0 3.5 6.0 251,000 0.05 0.12URM Frumento et al. (2009) 18_3 % Drift, (δ=H) 0.1 0.2 0.4 251,000 0.05 0.12Reinforced with diagonal FRP This study PSD1 m factor 1.0 3.6 8.0 63,000 0.03 0.08Reinforced with diagonal FRP This study PSD1 % Drift, (δ=H) 0.3 1.1 2.3 63,000 0.03 0.08Reinforced with diagonal FRP This study PSD2 m factor 1.0 13 20 70,000 0.01 0.05Reinforced with diagonal FRP This study PSD2 % Drift, (δ=H) 0.1 1.2 1.8 70,000 0.01 0.05Reinforced with diagonal FRP This study CSD1 m factor 1.0 5.7 11 153,000 0.05 0.12Reinforced with diagonal FRP This study CSD1 % Drift, (δ=H) 0.1 0.3 0.7 153,000 0.05 0.12Reinforced with diagonal FRP This study CSD2 m factor 1.0 3.7 8.0 149,000 0.09 0.15Reinforced with diagonal FRP This study CSD2 % Drift, (δ=H) 0.1 0.2 0.5 149,000 0.09 0.15Reinforced with grid FRP This study PSG1 m factor 1.0 5.0 12 83,000 0.10 0.19Reinforced with grid FRP This study PSG1 % Drift, (δ=H) 0.1 0.5 1.1 83,000 0.10 0.19Reinforced with grid FRP This study PSG2 m factor 1.0 6.0 21 99,000 0.13 0.15Reinforced with grid FRP This study PSG2 % Drift, (δ=H) 0.1 0.4 1.4 99,000 0.13 0.15Reinforced with grid FRP This study CSG1 m factor 1.0 6.0 18 810,000 0.05 0.06Reinforced with grid FRP This study CSG1 % Drift, (δ=H) 0.1 0.2 0.3 810,000 0.05 0.06Reinforced with grid FRP This study CSG2 m factor 1.0 6.0 16.0 744,000 0.08 0.09Reinforced with grid FRP This study CSG2 % Drift, (δ=H) 0.1 0.2 0.3 744,000 0.08 0.09
configurations with similar boundary conditions have been studiedunder monotonic lateral loading in a quasistatic test facility.Vertical loads were applied at two points prior to lateral loading.Load–deformation characteristics were observed for each walland subsequent damages were evaluated. Some of the performancecriteria set by FEMA 356 are assessed. Based on the foregoing re-sults and observations, the following remarks can be outlined:• Two unreinforced reference walls failed in shear sliding and toe
crushing preceded by a flexural cracking, which is largely seenas a common failure mode of the URM wall with aspect ratiobetween 1 and 1.5 having a moderate precompression verticalload. For FRP-strengthened masonry walls, the failure modechanged to predominantly a diagonal tension and toe crushingpreceded by a flexural cracking where debonding was the mostcommon phenomenon for the failure of FRP. In two occasions,only the rupturing of CFRP was observed.
• The in-plane shear strength of the strengthened walls is consid-erably improved by using either of the FRPs but the ductility iscompromised when CFRP is used as strengthening material.The experimental study demonstrates that PET-FRP has a betterseismic performance than CFRP, as it shows a better ductile be-havior than CFRP, especially in the postpeak region where thereis a structural demand for integrity and margin of safety againstcollapse. Although the CFRP increases the shear capacity of amasonry wall, it substantially reduces the ductility of the wall,which may eventually cause an explosive type of masonryfailure.
• The in-plane shear strengths observed in this experimental studyare almost equal to each other for the cases where the amountof FRP was greater than a certain limit. This information willassist to some extent to develop an analytical model for FRP-strengthened wall based on effective strain in FRP.
• The elastic stiffness of URM walls was largely modified by theuse of FRP, externally bonded over the surface of the walls but itwas observed that stiffness value beyond some specific rangedoes not increase the in-plane shear strength of masonry andit will only increase the cost of strengthening works.
• Diagonal bracing with PET-FRP sheet can be the best option,where not only capacity enhanced but also, at the same time,the wall is made quite ductile, substituting a catastrophic modeof failure by a ductile one.
• Another interesting point that is manifest from this experimentalstudy is that the FRP strengthening of the masonry can haveonly marginal effect on the structural performance at the serviceload condition but it can contribute a lot at some accidental over-loading such as earthquake, where the seismic demand is high.
• In all strengthening work, cost of the material and installation isa prime concern for both engineers and builders. The crossdia-gonal configuration requires least PET-FRP materials, whosematerial cost is less than CFRP, with the minimum amountof installation work among all the cases considered, includingwalls strengthened by CFRP strips. Wall fully covered by FRPsis not a viable option for external strengthening.
• Finally, it has been shown that between the two FRPs used forstrengthening of URM walls, PET-FRP has better seismic per-formance than CFRP especially at the LS and CP states ofthe wall.However, further study is needed for the advancement of knowl-
edge on FRP in general and on PET in particular concerning someof the unresolved issues that could not be addressed in this papersuch as brick–FRP interface behavior, optimum percent of FRPfor masonry, delayed debonding, and issues related to anchoragesystem of FRP with masonry. A concerted effort and a harmonizedapproach are necessary to make externally bonded FRPs as a
lucrative solution for many historically valued masonry structuresfor retrofitting and rehabilitation purposes. Although the presentwork is not very exhaustive, it can be one step forward in achievingthis goal.
Acknowledgments
The authors sincerely acknowledge the financial assistance fromthe CASR grant of KUETand from RONPAKU fellowship of JSPSto carry out this research. The authors are also thankful to NipponSteel & Sumikin Materials Co. Ltd. Japan and Maeda KosenCo. Ltd. Japan, for their support in providing necessary materialsfor CFRP and PET-FRP, respectively, that have been used in thisexperiment.
References
Abrams, D. P. (2001). “Performance based engineering concepts for unrein-forced masonry building structures.” J. Prog. Struct. Eng. Mater., 3(1),48–56.
ACI (American Concrete Institute). (2008). “Guide for the design and con-struction of externally bonded FRP systems for strengthening concretestructures.” ACI 440, Farmington Hills, MI.
Akın, E., Özcebe, G., Canbay, E., and Binici, B. (2013). “Numerical study onCFRP strengthening of reinforced concrete frames with masonry infillwalls.” J. Compos. Constr., 10.1061/(ASCE)CC.1943-5614.0000426,04013034.
Alcaino, P., and Santa-Maria, H. (2008). “Experimental response of exter-nally retrofitted masonry walls subjected to shear loading.” J. Compos.Constr., 12(5), 489–498.
Al-Chaar, G. (2002). “Evaluating strength and stiffness of unreinforcedmasonry infill structures.” Rep. No. ERDC/CERL TR-02-1, EngineerResearch and Development Center, U.S. Army Corps of Engineers,Vicksburg, MS.
Ali, Q., Badrashi, Y. I., Ahmad, N., Alam, B., Rehman, S., and Banori,F. A. S. (2012). “Experimental investigation on the characterizationof solid clay brick masonry for lateral shear strength evaluation.”Int. J. Earth Sci. Eng., 5(4), 782–791.
Altin, S., Anil, Ö., Kara, M. E., and Kaya, M. (2008). “An experimentalstudy on strengthening of masonry infilled RC frames using diagonalCFRP strips.” Compos. Part B, 39(4), 680–693.
Anggawidjaja, D., Ueda, T., Dai, J., and Nakai, H. (2006). “Deformationcapacity of RC piers wrapped by new fiber-reinforced polymer withlarge fracture strain.” Cem. Concr. Compos., 28(10), 914–927.
ATC (Applied Technology Council). (1996). “Seismic evaluation and retro-fit of concrete buildings.” ATC 40, Redwood City, CA.
Benedetti, A., and Steli, E. (2008). “Analytical models for shear–displacement curves of unreinforced and FRP reinforced masonrypanels.” Constr. Build. Mater., 22(3)175–185.
Bosiljkov, V., Page, A. W., and Bokan-Bosiljkov, V. (2003). “Performancebased studies of in-plane loaded unreinforced masonry walls.”MasonryInt., 16(2), 39–50.
Bosiljkov, V., Page, A. W., Bokan-Bosiljkov, V., and Žarnic, R. (2008).“Review paper, progress in structural engineering and material: Struc-tural masonry evaluation of the seismic performance of brick masonrywalls.” Struct. Control Health Monit., 17(1), 100–118.
Bosiljkov, V., Tomazevic, M., and Lutman, M. (2004). “Optimization ofshape of masonry units and technology of construction for earthquakeresistant masonry buildings: Research report. Part one and two.” ZAG,Ljubljana, Slovenia.
Burgoyne, C., and Balafas, I. (2007). “Why is FRP not a financial success?”FRPRCS-8, Univ. of Patras, Patras, Greece.
CEN (European Committee for Standardization). (2005). “Design of ma-sonry structures—Part 1-1: General rules for reinforced and unrein-forced masonry structures.” EN 1996-1-1: 2005, Brussels, Belgium.
Da Porto, F., Grendene, M., and Modena, C. (2009). “Estimation of loadreduction factors for clay masonry walls.” Earthquake Eng. Struct.Dyn., 38(10), 1155–1174.
Dizhur, D., Griffith, M., and Ingham, J. (2013). “In-plane shear improve-ment of unreinforced masonry wall panels using NSM CFRP strips.”J. Compos. Constr., 10.1061/(ASCE)CC.1943-5614.0000400, 04013010.
Drysdale, R. G., and Hamid, A. A. (2008). Masonry structures, behaviorand design, 3rd Ed., Masonry Society, Simon & Schuster, EnglewoodCliffs, NJ.
Ehsani, M. R., Saadatmanesh, H., and Al-Saidy, A. (1997). “Shear behaviorof URM retrofitted with FRP overlays.” J. Compos. Constr., 1(1),17–25.
ElGawady, M. A., Lestuzzi, P., and Badoux, M. (2005). “Performanceof masonry walls under in-plane seismic loading.” Masonary Soc. J.,23(1), 85–104.
ElGawady, M. A., Lestuzzi, P., and Badoux, M. (2007). “Static cyclicresponse of masonry walls retrofitted with fiber-reinforced polymers.”J. Compos. Constr., 11(1), 50–61.
Fehling, E., Stuerz, J., and Emami, A. (2007). “Tests results on the behav-iour of masonry under static (monotonic and cyclic) in plane lateralloads.” Technical Rep. of the Collective Research Project ESECMaSE,Deliverable D7.1a, Institute of Structural Engineering, Univ. of Kassel,Kassel, Germany.
FEMA (Federal Emergency Management Agency). (1997). “NEHRP com-mentary on the guidelines for the seismic rehabilitation of buildings.”FEMA 274, Washington, DC.
FEMA (Federal Emergency Management Agency). (2000). “Prestandardand commentary for the seismic rehabilitation of buildings.” FEMA356, Washington, DC.
Foraboschi, P. (2009). “Coupling effect between masonry spandrels andpiers.” Mater. Struct., 42(3), 279–300.
Foraboschi, P. (2015). “Analytical model to predict the lifetime of concretemembers externally reinforced with FRP.” Theor. Appl. Fract. Mech.,75, 137–145.
Foraboschi, P., and Vanin, A. (2013). “Newmethods for bonding FRP stripsonto masonry structures: Experimental results and analytical evaluations.”Compos: Mech. Comput. Appl., 4(1), 1–23.
Frumento, S., Magenes, G., Morandi, P., and Calvi, G. M. (2009). Inter-pretation of experimental shear tests on clay brick masonry walls andevaluation of q-factors for seismic design, IUSS Press, Pavia, Italy.
Hamid, A. A., EI-Dakhakhni, W. W., Hakam, Z. H., and Elgaaly, M.(2005). “Behavior of composite unreinforced masonry–fiber-reinforcedpolymer wall assemblages under in-plane loading.” J. Compos. Constr.,10.1061/(ASCE)1090-0268(2005)9:1(73), 73–83.
Hansen, K. F. (1999). “Bending and shear test with masonry.” SBI Bulletin123, Danish Building Research Institute, Hørsholm, Denmark.
Italian National Research Council. (2013). “Guide for the design and con-struction of externally bonded FRP systems for strengthening existingstructures.” CNR-DT 200 R1, Rome.
JSCE (Japan Society of Civil Engineers). (2001). Recommendation forupgrading of concrete structures with use of continuous fiber sheets,Tokyo.
Magenes, G., and Calvi, M. (1997). “In-plane seismic response of brickmasonry walls.” Earthquake Eng. Struct. Dyn., 26, 1091–1112.
Magenes, G., Morandi, P., and Penna, A. (2008). “Tests results on thebehaviour of masonry under static cyclic in plane lateral loads.” Tech-nical Rep. of the Collective Research Project ESECMaSE, DeliverableD7.1c, Dept. of Structural Mechanics, Univ. of Pavia, Pavia, Italy.
Marcari, G., Manfredi, G., Prota, A., and Pecce, M. (2007). “In-plane shearperformance of masonry panels strengthened with FRP.” Compos. PartB, 38(7), 887–901.
Modena, C., Da Porto, F.,Garbin, E., Grendene, M., and Mosele, F. (2005).“Ricerca sperimentale sul comportamento di sistemi per muratura por-tante in zona sesmica.” Draft 2005/01, Univ. of Padua, Padova, Italy (inItalian).
Rahman, A., and Ueda, T. (2014). “Experimental investigation and numeri-cal modeling of peak shear stress of brick masonry mortar joint undercompression.” J. Mater. Civ. Eng., 26(9), 1–13.
Salmanpour, A. H., Mojsilovic, N., and Schwartz, J. (2013). “Deformationcapacity of unreinforced masonry walls subjected to in-plane loading: Astate-of-the-art review.” Int. J. Adv. Struct. Eng., 5(1), 1–12.
SEAOC (Structural Engineers Association of California). (1995). “Perfor-mance based seismic engineering.” Sacramento, CA.
Shrestha, J., Ueda, T., and Zhang, D. (2014). “Durability of FRP concretebonds and its constituent properties under the influence of moistureconditions.” J. Mater. Civ. Eng., 10.1061/(ASCE)MT.1943-5533.0001093, A4014009.
Stratford, T., Pascale, G., Manfroni, O., and Bonfiglioli, B. (2004). “Shearstrengthening masonry panels with sheet glass-fiber reinforcedpolymer.” J. Compos. Constr., 8(5), 434–443.
Terán-Gilmore, A., Oscar Zuniga-Cuevas, Z., and Ruiz-García, J. (2008).“Model for the nonlinear analysis of confined masonry buildings.” 14thWorld Conf. on Earthquake Engineering.
TNO. (2004). Shear test on masonry panels; literature survey and proposalfor experiments, TNO Building and Construction Research, Delft,Netherlands.
Tomaževič, M., and Gams, M. (2009). “Shear resistance of unreinforcedmasonry walls.” Ing. Sismica, 26(3), 5–18.
Wang, Q., Chai, Z., Huang, Y., Yang, Y., and Zhang, Y. (2006). “Seismicshear capacity of brick masonry wall reinforced by GFRP.” Asian J. Civ.Eng., 7(6), 563–580.
Zhuge, Y. (2010). “FRP-retrofitted URM walls under in-plane shear:Review and assessment of available models.” J. Compos. Constr., 14(6),743–753.
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