Analytical model for predicting the shear strength of FRP ...scientiairanica.sharif.edu/article_1960_d478e5706761cda628e73b6bb5... · a FRP strengthened concrete beam-column joint

Scientia Iranica A (2015) 22(4), 1363{1372

Sharif University of TechnologyScientia Iranica

Transactions A: Civil Engineeringwww.scientiairanica.com

Analytical model for predicting the shear strength ofFRP-retro�tted exterior reinforced concretebeam-column joints

H. Hejabi and M.Z. Kabir�

Department of Civil and Environmental Engineering, Amirkabir University of Technology, Hafez Ave., Tehran, Iran.

Received 23 September 2012; received in revised form 28 June 2014; accepted 5 January 2015

KEYWORDSAnalytical model;Reinforced concrete;FRP;Retro�t;Beam-column joint;Shear stress.

Abstract. In this study, a nonlinear procedure for analysis of a membrane element ofa FRP strengthened concrete beam-column joint is proposed, based on a softened trussmodel. The procedure employs three equations for equilibrium, three for compatibility,and six for the constitutive laws of materials. The model is capable of analysing thenonlinear behaviour of RC beam-column joints under cyclic loading, and has three majorattributes: nonlinear association of stress and strain in the presence of FRP, contributionof concrete damage by means of a softening coe�cient, and consideration of the bond e�ectbetween steel, FRP and concrete. The proposed model is applied to some previously testedand strengthened beam-column joints, and shows good predictions of their shear strength.The e�ect of various parameters on the response of a reinforced concrete beam-columnjoint, such as a column axial load, amount of FRP reinforcement, and FRP properties, hasbeen studied in a parametric manner. It is observed that even a low quantity of FRP canenhance the shear capacity of the joint signi�cantly. Also, it is observed that the axial loadincreases the con�nement of the joint care, which, in turn, increases the shear capacity ofthe joint.c 2015 Sharif University of Technology. All rights reserved.

1. Introduction

The importance of beam-column joints in the earth-quake resistance capacity of reinforced concrete frameswas �rst recognized from earthquakes in the 1960s.Since that time, researchers have made e�orts todevelop and improve seismic design provisions for RCjoints. The prime reason behind joint failure wasidenti�ed as the inadequate shear strength of the joint.Inadequate joint shear strength is generally due toinsu�cient and inadequately detailed reinforcementin the joint region. Further, due to insu�cientreinforcement, particularly transverse reinforcement

*. Corresponding author. Tel.: +98 21 64543016E-mail addresses: [email protected] (H. Hejabi);[email protected] (M.Z. Kabir)

in the joint, joint brittleness increases, which, inturn, signi�cantly reduces the overall ductility of thestructure. The rehabilitation of beam-column jointsrepresents a feasible approach to mitigate hazardsin existing structures and to provide safety for theiroccupants. In the last few decades, the strengtheningof existing seismically de�cient joints has receivedconsiderable attention. Several rehabilitation schemesfor strengthening joints were proposed one of whichis FRP strengthening. The use of FRP compositesfor strengthening is a relatively modern concept and,generally, most e�ective, due to advantages like fastand easy application, high strength/weight ratio, andcorrosion resistance.

Analytical modelling of FRP-strengthened jointshas been limited. Gergely et al. [1,2] computed FRPcontributions to the shear capacity of RC joints by

1364 H. Hejabi and M.Z. Kabir/Scientia Iranica, Transactions A: Civil Engineering 22 (2015) 1363{1372

an analogy to steel stirrups. Ghobarrah and Said [3]proposed a design methodology for the �bre jacketingof existing beam-column joints in RC moment resistingframes. Ghobarrahand El-Amoury [4] proposed asimple design methodology for upgrading reinforcedbeam-column joints using GFRP sheets. Antonopoulosand Trianta�llou [5] proposed a method that uses stressequilibrium and strain compatibility to yield the shearstrength of the RC joint with externally bonded FRP.Almusallam and Al-Salloum [6] extended this model topredict some governing parameters, such as diagonaltensile stress, variation of shear stress in the joint atdi�erent stages of loading, and strains in transverseand longitudinal steel bars.

FRP jacketing of the joint area restricts thedilation tendency of the con�ned core by controlling theextent of internal damage. Damage or loss of concretesti�ness is in uenced by its micro-structural properties,which is best represented by the amount of damageor expansion of the concrete core area resisting theaxial loads. Internal damage is best represented usingthe radial strain to account for the lateral dilation ofcon�ned concrete and the corresponding loss of sti�nessof FRP-con�ned concrete sections.

The initial response of FRP-con�ned concretefollows a similar behaviour to uncon�ned concrete,since the radial expansion of the concrete core is in-signi�cant. By increasing the strain, micro-cracking ofthe concrete core starts to accumulate and the responseof con�ned concrete deviates from elastic theory bycon�nement e�ects in a direction perpendicular to the�bre direction.

In this study, the softened truss model proposedby Hsu [7] is expanded to take into account theFRP con�nement and steel and FRP bond e�ects,in analysing the FRP retro�tted RC joints. Theproposed model has three major attributes. The�rst is the nonlinear association of stress and strainin the presence of FRP. The second is the contri-bution of concrete damage by means of softeningthe coe�cient, which represents the steel and FRPexistence of the damage parameter, and the third isconsideration of the bond e�ect between steel andFRP and concrete. The proposed model is appliedto some previously tested, strengthened beam-columnjoints and shows good predictions of the shear strengthof these joints.

2. Mechanics of RC joints strengthened withFRP strips

2.1. Basic assumptionsA typical beam-column joint is illustrated in Figure 1.The joint is idealized as a 3D element with dimensiond (width of column), b (width of beam), and h (heightof beam), as shown in Figure 2. Shear stresses

Figure 1. Moment and shear acting at joint.

Figure 2. Dimensions of joint.

are assumed to be a bidirectional action, which isdeveloped by the bond between reinforcement andconcrete. For simplicity, it is assumed that the shearstress, � , is uniformly distributed over the boundariesof the joint.

In this model, it is assumed that the consideredelement has an orthogonal grid of reinforcement in land t directions, parallel to the element boundariesand uniformly distributed in each reinforcing direction.Thus, the cracked concrete behaves as an orthotropicmaterial, whose material axes are aligned in the direc-tion of principal stress (Figure 3). Also, it is assumedthat the direction of principal strain is assumed tocoincide with the direction of principal stress.

Figure 3. The coordinate system d�r in thepost-cracking stage in reinforced concrete membraneelements.

H. Hejabi and M.Z. Kabir/Scientia Iranica, Transactions A: Civil Engineering 22 (2015) 1363{1372 1365

2.2. Problem formulationThe states of average stress and average strain inconcrete are expressed by the second order tensors:

" =�

"l 0:5 lt0:5 lt "t

�; (1)

� =��l �lt�lt �t

�; (2)

where �l and �t are the average concrete normalstresses in the l and t directions, respectively; �lt isthe shear stress; "l and "t are the average normalstrains in the l and t directions, respectively; and ltis the average shear strain in the l� t coordinate. Theconvention used here is tension positive.

�l + �t = �d + �r; (3)

�l�t � �2lt = �d�r; (4)

"l + "t = "d + "r; (5)

"l"t � 0:25 2lt = "d"r: (6)

The proposed model employs 12 equations to determinethe features of the behaviour of membrane elementssubjected to 2-dimensional loading. These equationsare 3 for equilibrium, 3 for compatibility and 6 forthe constitutive laws of materials, which are based onbiaxial, nonlinear and accurate constitutive equations.

2.3. Equilibrium equationsThe equilibrium equations are as follows:

�l=�d cos2 �+�r sin2 �+(�c+�l�cs)fl+�flffl; (7)

�t=�d sin2 �+�r cos2 �+(�b+�t�bs)ft+�ftfft; (8)

�lt = (��d + �r) sin� cos�; (9)

where �cs is the column reinforcement ratio inside thejoint core; �c is the total main column reinforcementratio; �bs is the stirrup reinforcement ratio; �b isthe total main beam reinforcement ratio; �fl is theFRP reinforcement ratio in the longitudinal direction;�ft is the FRP reinforcement ratio in the transversedirection; �l is the factor that relates the magnitudeof stresses in the column reinforcement outside thecore to the average stresses of the reinforcement insidethe core at the beam centerline; �t is the factor thatrelates the magnitude of stresses in the main beamreinforcement to the average stirrup stresses at thecolumn centerline; fl and ft are the average stresses inthe longitudinal and transverse steel bars, respectively;ffl and fft are the average normal stress in the FRPin the longitudinal and transverse directions; and � isthe angle of inclination between the longitudinal axisand d axis.

2.4. CompatibilityThe two-dimensional compatibility condition expressesthe relationship among the average strains in di�erentcoordinate systems. In order to �nd out the longitudi-nal and transverse steel and FRP strains at failure, thestrain compatibility condition should be investigated asfollows:

"l = "d cos2 �+ "r sin2 �; (10)

"t = "d sin2 �+ "r cos2 �; (11)

lt2

= (�"d + "r) sin� cos�; (12)

where "r and "d are the average and normal strainsin the r and d directions, respectively. The angle ofinclination between the longitudinal axis and the d axisis computed as follows:

tan�1 � ="l � "d"t � "d : (13)

Because of the necessity to express the strain in thel�t coordinate in terms of strain and stress in the r�dcoordinate, the following equations are used (based onEqs. (3), (5), (7), (8), (10) and (11)):

"l = "r +"r � "d�r � �d (�l � �r � �lfl � �flffl) ; (14)

"t = "r +"r � "d�r � �d (�t � �r � �tft � �ftfft) : (15)

2.5. Constitutive lawsThe relationship between stress and strain along theprincipal compressive direction can be described bythe uni�ed constitutive law for FRP con�ned concreteproposed by Wei and Wu [8] in the following form:8<:�d = Ec"c + f0�Ec"0

"20"2c 0 � "c � "0

�d = f0 + E2("c � "0) "0 � "c � "cu(16)

"0 =

(f0 + fcu+Ec"cu)�p(f0+fcu+Ec"cu)2�8f0Ec"cu2Ec

;(17)

E2 =fcu � f0

"cu � "0; (18)

where Ec is the secant modulus of concrete; f0 and"0 are the transitional stress and strain, respectively;fcu and "cu are concrete ultimate stress and strain,respectively; and E2 is the slope of the second portionin the con�ned concrete stress-strain curve. Thismodel is based on the regression analysis of available


experimental data for three parameters, including ulti-mate stress, flu, ultimate strain, "Cu, and transitionalstress, f0. Due to the paper length limits, detailson calculation of current parameters are not presentedhere. This information can be found in Wei and Wu [8].

Observations on cracked reinforced concrete incompression indicate lower strength and sti�ness thanunusually compressed concrete, which is called thecompression softening phenomenon [9]. It is believedthat the shear strength of the beam-column jointshould also be governed by the softening e�ect ofconcrete. The softening coe�cient is computed asfollows [9]:

� =1 + �s � fy=f0

0:85� 0:34� (�"d="0): (19)

The shear resistance of reinforced concrete membraneelements also in uences the tensile stress-strain rela-tionship of concrete. A typical tensile stress-straincurve of concrete consists of two parts: 1) The ascend-ing linear portion up to the cracking tensile strain and,2) The descending nonlinear portion. The stress-strainrelationship can be expressed by:8><>:�c = Ec"c "r � "cr

�c = fcr�"cr"c

�0:4"r � "cr

(20)

where:

Ec =2f 0c"0

; "cr = 0:00008; fcr = 0:31pf 0c: (21)

Reinforcement steel is usually modelled as a linearelastic, linear strain hardening material with a yieldstress, fy.8<:fs = Es"s "s � "y

fs = fsy "s � "y(22)

However, when reinforcing bars are surrounded by con-crete, the average behavior of the stress-strain relationis quite di�erent. The most in uential di�erence be-tween bare reinforcement steel and concrete surroundedsteel is the lowering of the yield stress below fy in thelatter, both in tension and compression. Yielding of anRC member occurs when the steel stress at a crackedsection reaches the yield stress of the bare bar. How-ever, the average steel stress at a cracked element stillmaintains an elastic stress that is less than the yieldstrength, because the concrete matrix located betweenthe cracks is still partially capable of resisting tensileforces, owing to the bond between the concrete andthe reinforcement. Determination of element sti�nesson the basis of the yielding of steel at a cracked section,

where a local stress concentration appears in the steel,may result in overestimating the structural responsein the post-yielding range. Since this phenomenon isaccelerated with increased deformation, an analysis ofRC members subjected to cyclic loading accompanyingrelatively large deformations requires the use of averagestress-strain relations. Accordingly, the average stress-strain relation of steel needs to be de�ned for tracingthe cracking behavior of RC beams and/or columnsup to the ultimate limit state. This can be accom-plished using a smeared crack model in which the localdisplacement discontinuities at cracks are distributedover some tributary area within the �nite element, andwhere the behavior of cracked concrete is representedby the average stress-strain relations. Consideringthese factors, the following linear average stress-strainrelation, introduced by Belarbi and Hsu [10] fromexperimental data, is used:8<:fs = Es"s "s � "n

fs = fn + (0:02 + 0:25B)Es("s � "n) "s � "n(23)

B =1�

�fcrfy

�1:5

(24)("n = "y(0:93� 2B)fn = Es"n

(25)

where fs and "s represent the average strain and stress,respectively, and fy and "y are the yield stress andthe corresponding yield strain of a bare steel bar,respectively. fs becomes fl and ft when applied tolongitudinal and transverse steel.

The FRP is assumed to behave elastically untilfailure or debonding. It will fail by tensile fracturewhen the tensile stress reaches the tensile strength.

ff = Ef"f ; (26)

ff and "f represent the average FRP strain and stress,respectively, and becomes ffl, fft, "fl and "ft whenapplied to longitudinal and transverse FRP sheets.Also, depending on how it is treated, according tothe model by Islam and Wu, FRP debonding load iscalculated as follows:8<:ff;deb = 0:565bff

00:1c (Ef tf ) lb � Lbe

ff;deb = 0:565bff00:1c (Ef tf )

�lbLbe

�lb � Lbe

(27)

Lfe = 0:395(Ef tf )0:54

f 00:09c

; (28)

where bf is FRP layer width. FRP systems that arenot anchored have been observed to delaminate from


the concrete before the loss of aggregate interlock ofthe section. For this reason, bond stresses should beanalyzed to determine the e�ective strain level that canbe achieved. The e�ective strain is calculated using abond-reduction coe�cient, kv, applicable to shear:

"fe = kv"fu � 0:004: (29)

The bond-reduction coe�cient is a function of theconcrete strength, the type of wrapping scheme used,and the sti�ness of the laminate. In this research,the bond-reduction coe�cient is computed from theequations of ACI440-02 [11] proposed by Khalifa etal. [12]:

kv =k1k2Le

11900"fu� 0:75: (30)

This equation relies on the active bond length, Le,which is the length over which the majority of the bondstress is maintained, and two modi�cation factors, k1and k2, that account for the concrete strength and thetype of FRP scheme used, respectively. Expressions forthese parameters are given as follows:

Le =23300

(tfEf )0:58 ; (31)

k1 =�f 0c27

�2=3

; (32)

k2 =h� 2Le

h: (33)

Furthermore, it is noted that if, at the moment ofstrengthening, the joint is already loaded, a set of initialnormal strains, e0t and e0l, in the transverse (beam)and longitudinal (column) direction, respectively, ini-tial shear strain, 0, can be de�ned and enters theequilibrium equations. These parts act as constantparts in solving equations and have no e�ect on solutionprocedure.

3. Solution procedure

The analytical formulation given above was imple-mented in a computer program speci�cally developedfor the analysis of RC joints strengthened with FRPstrips. The user inputs a series of material andgeometric characteristics, and the program traces thestate of stress and strain in the joint until failure. Inputto the program consists of:

1. The geometric variables, �c, �b, �cs, �bs, �fl, �ft; b,h, w;

2. The bond condition variables, �t and �l (Eqs. (7)and (8));

3. The material properties, fc, fcr, and e0 for concrete;Es, fyl, fys, and fyt for steel, and Ef , ffu, andff;deb for FRP;

4. The axial forces, Nl and Nt; and5. The initial strains, e0l and e0t, in the joint (at the

moment of strengthening).

The solution procedure is described in the follow-ing steps (Figure 4):

1. Assume "d;

Figure 4. Maximum shear stress calculation owchart.


Table 1. Specimen description.

Specimen Beam Column Anchorage fcMPa

Reinforcement FRP

H(mm)

b(mm)

w(mm)

b(mm)

�sl �stfyl

(MPa)fys

(MPa)�fl �ft

Ey(MPa)

"fu

Al(IC1) 160 350 300 160 No 30 1.6% 1.6% 420 420 - - - -Al(IC2) 160 350 300 160 No 25 1.6% 1.6% 420 420 - - - -Al(IS1) 160 350 300 160 No 30 1.6% 1.6% 420 420 1.25% 0 61.5Al(IS2) 160 350 300 160 Yes 25 1.6% 1.6% 420 420 1.25% 0 61.5ANT(C) 300 200 200 200 No 21.6� 1.54 1.54 585 585 - - - -

ANT(F11) 300 200 200 200 No 22.8� 1.54 1.54 585 585 0.13 0.13 230 0.016ANT(F21) 300 200 200 200 No 27� 1.54 1.54 585 585 0.26 0.13 230 0.016ANT(F22) 300 200 200 200 No 27.2� 1.54 1.54 585 585 0.26 0.26 230 0.016

ANT(F22W) 300 200 200 200 Yes 29.2� 1.54 1.54 585 585 0.26 0.26 230 0.016�Cube specimen strength.

2. Assume "r;3. Calculate softening coe�cient from Eq. (19);4. Calculate �d from Eq. (16);5. Calculate �r from Eq. (20);6. Calculate "l from Eq. (14) by consideration of bond

e�ects from Eqs. (25) and (32);7. Calculate fl from Eq. (25);8. Calculate ffl from Eq. (28);9. Calculate � from Eq. (13);

10. Calculate "t from Eq. (15) by consideration of bonde�ects from Eqs. (25) and (32);

11. Calculate ft from Eq. (25);12. Calculate ffl from Eq. (28);13. Calculate "r from Eq. (5);14. If the di�erence between the calculated "r and

the assumed value of "r is small, then proceed.Otherwise, go to the beginning of the loop andincrease "r;

15. Control steel rupture; if ultimate strain is reached,go to step 18, otherwise, proceed;

16. Control FRP rupture; if ultimate strain is reached,go to step 18, otherwise, proceed;

17. Control concrete crashing; if ultimate strain isreached, go to step 18, otherwise, go to step 1;

18. Calculate shear stress for assumed "d from Eq. (9).

4. Veri�cation of the analytical model

In order to have su�cient con�dence in the above-presented algorithm, it was necessary to compare theshear capacity from the proposed procedure with someexperimental test results available in the literature.However, experimental data on FRP-strengthenedbeam-column joints have been relatively limited. Forthis purpose, the experimental work of Almusallam

Table 2. Veri�cation of the analytical model: Analysisresults.

�

Specimen Experimental(MPa)

Analytical(MPa)

Anal./Exp.�

Al(IC1) 6.36 6.26 0.98Al(IC2) 4.95 5.18 1.05Al(IS1) 7.39 7.88 1.06Al(IS2) 7.17 6.85 0.96ANT(C) 3.2 3.18 0.99

ANT(F11) 4.64 4.60 0.99ANT(F21) 5.47 5.24 0.96ANT(F22) 5.37 5.40 1.01

ANT(F22W) 6.15 5.87 0.95�Analytically calculated shear strength divided byexperimentally one.

and Al-Salloum [13] and Antonopoulos and Trianta�l-lou [14] on exterior joints was selected for validation.The key data for analysis of these specimens are givenin Table 1, and comparison between experimentalresults and the results of analysis with the proposedprocedure are given in Table 2. A detailed descriptionof the specimens; geometries and material propertiesof concrete, steel and FRP; test set up; and instru-mentation details can be seen in Almusallam and Al-Salloum [13] and Antonopoulos and Trianta�llou [14].Based on this comparison, it is concluded that theagreement between the proposed algorithm and testresults is good, and the proposed procedure may beused to design an externally bonded retro�t for beam-column joints.

5. Parametric study

In this section, the e�ect of the quantity of FRP re-inforcement on a parametric basis, using the proposedprocedure, is studied. For this reason, a joint withthe following characteristics were studied: The joint is


Figure 5. E�ect of FRP reinforcement on shear strengthof the joint.

assumed to be reinforced equally in the column and thebeam (�c = �b = 0:015). Each FRP layer consists ofunidirectional carbon �bers in an epoxy matrix, andhas an elastic modulus equal to 180 GPa and ultimatestrain equal to 1.2%. Joint dimensions are 500 (columnheight) � 500 (beam height) � 250 (column and beamwidth). The elastic modulus and yield stress of columnand beam reinforcements are 200 GPa and 420 MPa,respectively. The compressive strength of concrete isassumed equal to 25 MPa, and its ultimate strain isassumed to be 0.0021.

First, the variation of joint shear strength withthe amount of FRP, keeping all other parameters thesame, is studied. The result of the analysis is shown inFigure 5. As shown, a small increase in the amountof FRP considerably increases the shear strength ofthe joint. This can be attributed to the con�nementof the joint core. As the quantity of FRP increases,con�nement increases, which, in turn, increases theshear strength of the joint. Also, the e�ectivenessof FRP improves as more �bers are placed in thebeam direction (perpendicular to the column direction,which includes axial load). As shown in Figure 5,the maximum shear stress diagram has two distinctiveslopes. These denote that the rate of increase inthe maximum shear stress of the joint slows downafter a speci�c amount of FRP reinforcement, and anincrease in the amount of FRP beyond this limit is notparticularly e�ective.

The variation of maximum shear stress of the jointwith column axial load is shown in Figure 6. It isindicated in the �gure that maximum joint shear stressincreases by increasing axial load. This is due to thefact that with an increase in axial load, con�nement ofthe joint core increases, which, in turn, increases theshear capacity of the joint.

Also, variations of the joint transverse, longitudi-nal and shear strain of a FRP retro�tted joint werestudied. As expected, the magnitude of maximumstrain decreases with the increase of FRP quantity.This can be attributed to redistribution of load dueto the additional reinforcement provided by the FRP.In Figure 7, variations of the beam ("l) and column ("t)

Figure 6. E�ect of axial load on shear strength of thejoint.

Figure 7. E�ect of FRP reinforcement on beam andcolumn reinforcement strains.

maximum strains in a FRP retro�tted RC joint with�bers in the beam direction (to ensure the maximumcon�nement e�ectiveness of the retro�t scheme) arepresented. As shown in this �gure, both strains arereduced, but, the rate of degradation in the beamdirection is much more than the equivalent rate in thecolumn direction.

Finally, the e�ect of �ber direction was studiedparametrically. For this reason, a joint retro�tted withconstant amounts of FRP, but with di�erent portionsin the beam and column directions, is described asfollows (Vt is FRP volume portion in the columndirection, and Vl is FRP volume portion in the beamdirection):

1. Vt : Vl = 1, which represent 100% �bers in thecolumn direction;

2. Vt : Vl = 0:75, which represent 75% �bers in thecolumn direction and 25% in the beam direction;

3. Vt : Vl = 0:5, which represent 50% �bers in bothcolumn and beam directions;

4. Vt : Vl = 0:25, which represent 25% �bers in thecolumn direction and 75% in the beam direction;

5. Vt=Vl = 0, which represent 100% �bers in the beamdirection.

In Figures 8-10, variations of joint maximum


Figure 8. E�ect of FRP reinforcement con�guration onmaximum joint shear stress.

Figure 9. E�ect of FRP reinforcement con�guration onmaximum beam direction strain.

Figure 10. E�ect of FRP reinforcement con�guration onmaximum column direction strain.

shear stress in the concrete core, beam strains andcolumn strains are represented. As shown in these�gures, con�nement e�ects of the beam direction �bres(because of the presence of axial load in the columndirection) made retro�t in the beam direction e�cientfor the shear strengthening of an edge RC joint.

6. Conclusions

An algorithm for determining the shear strength ofFRP retro�tted exterior beam-column joints underseismic action is proposed. The algorithm uses thesoftened truss concept, proposed by Hsu [7], to analyzea reinforced concrete beam-column joint retro�ttedwith FRP. For more accuracy in analysis, the nonlinearassociation of stress and strain in the presence of FRP,the contribution of concrete damage and considerationof the bond e�ect between steel and FRP and concretewere considered. The model can provide valuableinsight into the seismic behavior of retro�tted exteriorbeam-column joints, and is able to evaluate the seis-micity of existing joints.

Parametric analyses using the proposed modelindicated that even low quantities of FRP material mayprovide signi�cant enhancement of the shear capacity.The e�ectiveness of external reinforcement depends onthe con�guration of layers on joint regions, the beamand the column, amount of column axial load, andthe relative quantities of steel and FRP reinforcementin the beam and column directions that cross thejoint panel. It was observed that with an increasein the quantity of horizontally directed FRP layersin the joint region, with the existence of axial loadin columns, con�nement increases, which causes anincrease in the shear strength of the joint. Also, themaximum shear stress diagram shows two distinctiveslopes, which indicates that the rate of increase inthe maximum shear stress of the joint slows downafter a speci�c amount of FRP reinforcement, andthat an increase in the amount of FRP beyond thislimit is not particularly e�ective. The formulationcould be extended for layered composites with di�erent�ber orientations with few changes in formulation. Itis also extendible to 3D analysis of retro�tted jointsby writing equations in tensor form and introducingdamage factors, which will be discussed in anotherarticle by the authors.

Nomenclature

lt Average shear strain in l� t coordinate(mm/mm)

�lt Average stress strain in l� t coordinate(MPa)

kv Bond-dependent coe�cient for shear"0 Strain at the maximum compressive

stress of non-softened concrete(mm/mm)

"c Strain level in the concrete (mm/mm)"cr Concrete cracking tensile strain

(mm/mm)


"d Average principal (compressive) strainin concrete in d direction (mm/mm)

"fe E�ective strain level in FRPreinforcement; strain level attained atsection failure (mm/mm)

"fu Mean rupture stain of FRPreinforcement (mm/mm)

"cu Ultimate concrete strain (mm/mm)

"l Average strain in longitudinal direction(mm/mm)

"r Average principal (tensile) strain inconcrete in r direction (mm/mm)

"t Average strain in transverse direction(mm/mm)

�d Average principal (compressive) stressin concrete in d direction (MPa)

�l Average stress in longitudinal direction(MPa)

�r Average principal (tensile) stress inconcrete in r direction (MPa)

�t Average stress in transverse direction(MPa)

b Beam widthbf FRP width

d Column widthEc Secant modulus of concrete (MPa)

Ef FRP strips elastic modulus in principal�ber direction (MPa)

Es Steel elastic modulus (MPa)

E2 Slope of second portion in con�nedconcrete stress-strain curve (MPa)

f 0c Uncon�ned strength of concrete incompression

fcr Mean tensile strength of concretefy Reinforcement yield stress

h Beam heightlb FRP bond length�cs Column reinforcement ratio inside the

joint core�c Total main column reinforcement ratio�bs Stirrup reinforcement ratio�b Total main beam reinforcement ratio�fl FRP reinforcement ratio in the

longitudinal direction�ft FRP reinforcement ratio in the

transverse direction

�l Factor that relates the magnitude ofstresses in the column reinforcementoutside the core to the average stressesof the reinforcement inside the core atthe beam centerline

�t Factor that relates the magnitudeof stresses in the main beamreinforcement to the average stirrupstresses at the column centerline

fl Average stress in longitudinal steelbars

ft Average stress in transverse steel barsffl Average normal stress in the FRP in

the longitudinal directionfft Average normal stress in the FRP in

the transverse direction� Angle of inclination between the

longitudinal axis and d axislb FRP bond lengthff;deb Maximum tensile stress in FRP when

debonding occursffu Ultimate tensile strength in primary

�ber direction

References

1. Gergely, J., Pantelides, C.P., Nuismer, R.J. and Reave-ley, L.D. \Bridge pier retro�t using �ber-reinforcedplastic composites", Journal of Composite Construc-tion, 2(4), pp. 165-174 (1998).

2. Gergely, J., Pantelides, C.P. and Reaveley, L.D. \Shearstrengthening of RCT-joints using CFRP composites",Journal of Composite Construction, 4(2), pp. 56-64(2000).

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Biographies

Hadi Hejabi received a BS degree in Civil Engineer-ing from Isfahan University of Technology, Iran, in2006, and an MS degree in Structural Engineering,in 2008, from the Department of Civil Engineeringat Sharif University of Technology, Tehran, Iran. Heis currently �nishing his PhD degree at Amir KabirUniversity of Technology, Tehran, Iran. His researchinterests include retro�tting of structures and life-lines.

Mohammad Z. Kabir is Professor and Chair ofCivil and Environmental Engineering Department atAmirkabir University of Technology, Iran. He receivedhis BS and MS degrees from Amirkabir Universityof Technology and his PhD degree from WaterlooUniversity in Canada. His research interests includestructural stability, structural analysis using FEM,experimental methods in structural engineering, com-posite structures, structural optimization, damage de-tection and rehabilitation of structures.

Documents

Analytical model for predicting the shear strength of FRP ...scientiairanica.sharif.edu/article_1960_d478e5706761cda628e73b6bb5... · a FRP strengthened concrete beam-column joint