Construction and Building Materials 28 (2012) 730–742

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Construction and Building Materials

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Seismic performance of concrete columns reinforced with hybrid shapememory alloy (SMA) and fiber reinforced polymer (FRP) bars

A.H.M. Muntasir Billah, M. Shahria Alam ⇑School of Engineering, The University of British Columbia, Kelowna, BC, Canada V1V1V7

a r t i c l e i n f o

Article history:Received 19 June 2011Received in revised form 20 September2011Accepted 2 October 2011Available online 29 November 2011

Keywords:Shape memory alloyFRP barColumnBase shearDuctilityResidual displacementEnergy dissipation capacity

0950-0618/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2011.10.020

⇑ Corresponding author. Tel.: +1 250 807 9397; faxE-mail address: shahria.alam@ubc.ca (M. Shahria A

a b s t r a c t

Lack of corrosion resistance of structural steel has resulted in many structural failures. Fiber reinforcedpolymer (FRP) bar is emerging as a viable solution to replace steel to mitigate corrosion problems. SinceFRP is a brittle material, it can bring about catastrophic sudden structural failures. Conversely, post-earthquake recovery of structures is another prime objective for performance based seismic design.Shape memory alloy (SMA) has the unique ability to undergo large inelastic deformation and regain itsundeformed shape through stress removal. Here a hybrid reinforced concrete (RC) column configurationis presented in order to reduce permanent damages, and enhance its corrosion resistance capacity wherethe plastic hinge region will be reinforced with SMA or stainless steel and the remaining regions with FRPor stainless steel rebar. An analytical investigation has been carried out to develop such hybrid RCcolumns and analyze them under seismic loadings. The results are compared in terms of baseshear-tip displacement, base shear demand/capacity ratio, ductility, residual displacement, and energydissipation capacity to those of a similar RC column reinforced with stainless steel. The results indicatethat such corrosion resistant hybridized column can substantially reduce the residual displacement withadequate energy dissipation capacity during earthquakes.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

In recent years, the seismic design guidelines have been focus-ing on performance-based design in order to predict and bettermanage the post-earthquake functionality and condition of struc-tures. The primary objective in current design philosophies is toensure that collapse is prevented, and not necessarily to limit dam-age for post-earthquake recovery. Current seismic design philoso-phies recommend that under the maximum expected lateralseismic force, structures are expected to suffer damages but willnot collapse, and not necessarily it will limit damages for post-earthquake recovery. Consequently, recent developments in per-formance-based seismic design and assessment approaches haveemphasized the importance of properly assessing and limitingthe residual (permanent) deformations that are typically sustainedby a structure after a seismic event [1]. During a seismic event thecolumns act as the primary member of the frame structure inresisting the seismic forces. Most of the structural collapses thatoccurred in recent earthquakes (e.g., Kobe, 1995; Northridge,1995; Loma Prieta, 1989) resulted from poor column perfor-mances. For example, following the Kobe earthquake, over 100reinforced concrete columns with a residual drift ratio (displacement

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: +1 250 807 9850.lam).

normalized by column height) of over 1.75% were demolished eventhough they did not collapse [2].

Steel is being used as reinforcement for hundreds of years. Butlack of corrosion resistance has been a major disadvantage of steeland this undesirable property of steel has led to early deteriorationand degradation of reinforced concrete (RC) structures. Stainlesssteel (SS) is often used as reinforcement in concrete buildings incoastal regions to avoid early corrosion. A number of bridges havebeen constructed and repaired in North America and Europe usingstainless steel (SS) for its higher resistance to corrosion comparedto regular steel. Practical applications have showed that the use ofSS increases the total project cost only by 3% [3]. FHWA [4] re-ported that SS rebar is capable of providing full service life of astructure when the concrete gets deteriorated. Moreover, applica-tion of SS in construction industry has been gaining popularity dueto its heat and corrosion resistance, ability to withstand impact andshock and little or no maintenance cost [3]. Ease of fabrication andinstallation with longer durability and adequate ductility, SS hasbeen emerging as an ideal reinforcing material.

As an alternative to reinforcing steel, fiber reinforced polymer(FRP) has been introduced in construction industry. FRP possessesseveral unparallel properties such as high corrosion resistance,invisibility to electrical and magnetic field, high strength and lightweight. These advantages along with mass production, and ease ofavailability and installation, have made them an alternative optionas reinforcing bars in concrete structures [5]. The use of FRP rebar

A.H.M. Muntasir Billah, M. Shahria Alam / Construction and Building Materials 28 (2012) 730–742 731

can be an enamouring solution for concrete structures operating inadverse environments, such as bridge piers in coastal regions andbuildings containing magnetic and sensitive equipment, eventhough the behavior of FRP bars as longitudinal reinforcement incompression members is still an important issue to be addressed.Although FRP has several advantages over regular steel, FRP rein-forcement has not yet been used in structures with the aim of mit-igating their dynamic response under service or extreme dynamicloads such as vibration control or seismic design applications,respectively [6]. As compared to steel reinforced structures, FRPreinforced concrete structures are more flexible, less ductile andhave low energy dissipation capacity. The absence of yieldingand an inelastic branch in the stress–strain behavior of FRP can re-sult in a sudden and brittle failure without adequate warning andconsequently with little dissipation of energy. This lack of ductilityand energy dissipation has impeded the wide use of FRP in dy-namic applications such as reinforcing structures in areas of mod-erate to severe seismic demand.

If FRP RC structures are designed in such a way that they arecapable of undergoing large displacement with adequate energydissipation capacity during a seismic event, they will not onlyeliminate the problem of corrosion, but also make the structuressafer against earthquakes. Thus, it will substantially scale downthe repair and maintenance cost of structures and at the same timeboost up the confidence level of designers while designing suchFRP reinforced structures [7]. Confidence can be gained throughdemonstrations or experimental investigations. Alam et al. [8]experimentally demonstrated that FRP could be coupled with aductile material like steel, or shape memory alloy (SMA) to incor-porate inelasticity and thus, introduce ductility. Experimentalinvestigations were carried out by Nehdi et al. [7] where a RCbeam-column joint reinforced with SMA bar at the plastic hinge re-gion of the beam and FRP in other regions were constructed andtested where the results show that the joint could dissipate signif-icant amount of energy.

Shape memory alloy (SMA) has high resistance to corrosion,which proves to be a potential candidate to be used as reinforce-ment in concrete structures. Superelastic SMA possesses the un-ique ability to experience large deformation and retrieve itsoriginal shape upon load removal [9]. This is a distinct propertythat makes SMA a smart material and a strong contender for useas reinforcement in RC structures particularly at critical locations(plastic hinge region), which is prone to experience more damagesduring an earthquake. But the high cost of SMA limits its use [8].Therefore, the concept of hybrid construction comes into the sce-nario where SMA can be used particularly in the plastic hinge re-gion of a RC structural element, which can undergo largedeformation during an earthquake and potentially recover theplastic deformation after the earthquake, thus, requiring minimumamount of repairing.

In the present study, the authors propose several alternative hy-brid reinforcement configurations to achieve improved ductility(ability to undergo large deformation without collapse) with highcorrosion resistance in a RC column. Due to the corrosion problem,regular steel may not be an optimal solution; while stainless steel,SMA and FRP are probable candidates for consideration. Since FRPis a brittle material, it cannot be used in the plastic hinge region.Therefore, the possible configurations can be: (i) using SMA inthe plastic hinge region and FRP in other regions (SMA–FRP), (ii)using SMA in the plastic hinge region and stainless-steel in otherregions (SMA–SS), (iii) using stainless-steel in the plastic hinge re-gion and FRP in other regions (SS–FRP), and (iv) Stainless-steelalone in all regions (SS) of a RC column. In the case of using SS-–FRP configuration in a column, this can reduce maintenance costssince there will be no significant corrosion. However, if the struc-ture is subjected to an earthquake, the stainless-steel will undergo

large inelastic deformation and will not be able to recover its ori-ginal shape, thus, will experience permanent deformations. Onthe other hand, the use of SMA as reinforcement will not only elim-inate the corrosion problem, but also allow recovering inelasticdeformation at the end of earthquakes. Another advantage is thatsuch structural element can sustain repeated earthquake loadingeven though the SMA bar is being strained beyond its yield limit.In contrast, stainless steel will accumulate deformation and loseits serviceability.

Little research has been directed towards splicing of FRP barwith another ductile material [7] and using such spliced connec-tions in a column so as to make the structure corrosion resistanceand ductile with enhanced deformation capacity. In this study, asuitable coupler will be utilized, which was developed by Alamet al. [8] for connecting FRP with SMA or stainless steel, then thespliced SMA–FRP, SMA–SS, SS–FRP and only SS rebar have beenused as main longitudinal reinforcements in RC column. The primeobjective of this study is to investigate the seismic behavior of suchspliced RC columns of various reinforcement configurations whereSE SMA or SS were placed particularly in its plastic hinge zone andFRP or SS in other regions, and compare their performances to thatof a SS RC columns in terms base shear- displacement, base sheardemand/capacity ratio, ductility, residual displacement, and en-ergy dissipation capacity.

2. Research significance

This study investigates the efficacy of using SMA and FRP asreinforcement in reinforced concrete building columns in reducingits residual displacement and enhancing seismic performance anddurability. Although FRP RC structure can mitigate the problemassociated with steel corrosion, the reduced compressive strengthand stiffness of FRP bars make FRP RC columns more susceptible toinstability. Except JSCE [10] no other design codes such as NorthAmerican codes permit the use of FRP in compression members.Moreover ACI 440.1R-06 [11] recommends not using FRP bars aslongitudinal reinforcement in columns or as compression rein-forcements in flexural members as there were not enough experi-mental evidences that support its use. However, recent studiesdemonstrate that columns reinforced with FRP bars behave simi-larly to that of regular steel reinforced concrete column under axialload [12].

If ductility can be introduced in FRP RC structure, it will not onlymake such structures highly resistant to corrosion, but also be ableto dissipate significant amount of energy during earthquakes. Largeductility and energy dissipation capacity warrant SMA to be usedin conjunction with FRP reinforcement. The use of superelastic(SE) SMA in plastic hinge length in lieu of regular steel can providesufficient ductility and the use of FRP will lead to the constructionof corrosion free RC columns. As columns of a structure are mostsusceptible to severe damage and permanent deformations, the re-sults of this study will assist the application of SMA–FRP hybridreinforcement in reinforced concrete structures to mitigate theproblems associated with reinforcement corrosion and seismicloading.

3. Scope of previous work

Various experimental and numerical studies have been per-formed on concrete frame components with transverse and longi-tudinal FRP reinforcement [5,10–17]. Benmokrane et al. [13]carried out experimental and theoretical investigation to comparethe flexural behavior of concrete beams reinforced with FRP andconventional reinforcement. Their results showed that good bondexists between FRP rebar and surrounding concrete. Brown and

732 A.H.M. Muntasir Billah, M. Shahria Alam / Construction and Building Materials 28 (2012) 730–742

Bartholomew [14] investigated the flexural behavior of FRP rein-forced simply supported beams under third point loading. TheFRP reinforced sections behaved similar to those of steel reinforcedsections except they encountered much larger deflections due tolower elastic modulus of FRP. Al Sayed et al. [15] replaced the lon-gitudinal steel rebar as well as the tie of a column with GFRP (GlassFRP). They tested the column under axial load and found that thereplacement reduced the column capacity. Aiello and Ombres[16] conducted experiments with six beams having different con-figurations of longitudinal bar of steel only, FRP only, and hybridFRP-steel. Steel stirrups were used in all specimens. Experimentalresults showed that the hybrid reinforced beams experienced high-er ductility but lower service deflection than that of the steel-freeFRP beam. A parametric study carried by Mirmiran et al. [17]showed that despite FRP RC columns are more pliable to ductilityfailure than the steel RC columns, the design practice of using mo-ment magnification factors is also applicable to FRP RC columns.Grira and Saatcioglu [18] investigated concrete columns havinglongitudinal steel reinforcement by using both steel grids and CFRPgrids as stirrups for confinement. They used several grid configura-tions and tested the column specimens under cyclic loading. Theyconcluded that the performance of columns reinforced with CFRPstirrups was comparable to that of columns reinforced with steelstirrups. Saikia et al. [19] tested concrete beams reinforced withhybrid bars consisting of glass fiber reinforced polymer (GFRP)strands of 2 mm diameter wound helically on a mild steel core of6 mm diameter. It was observed that the hybrid RC beam exhibitedlower stiffness compared to that of the steel RC beam. Failure ofhybrid RC beam was primarily due to the delamination of concreteat the reinforcement level, leading to anchorage failure, and result-ing in loss of bond between the hybrid bar and concrete. De Lucaet al. [12] carried out experimental investigations on full scale RCcolumn reinforced with GFRP longitudinal and transverse rein-forcement under axial load to investigate its compressive behaviorand understand the contribution of GFRP ties to the confinement ofthe concrete core. Experimental results showed that the behaviorof GFRP RC columns were quite comparable with that of steel rein-forced columns.

Experimental and numerical investigations over the last twodecades have also demonstrated numerous possibilities of utilizingSMAs in civil engineering structures for protecting buildings andbridges against earthquakes [20,21]. Saiidi and Wang [22] incorpo-rated SMA bars in the plastic hinge region of RC columns and con-ducted shake table tests. They found that SMA can significantlyreduce the earthquake induced damage and dissipate adequateamount of energy. Nehdi et al. [7] proposed corrosion free SMA–FRP hybrid RC beam-column joint reinforced with a SE SMA barat the plastic hinge regions of the beam and FRP in the other re-gions of the beam and column. Alam et al. [9] investigated the dy-namic performance of an eight-storey RC frame building reinforcedwith steel and SMA in its beam-column joints. Wierschem andAndrawes [23] proposed SMA fiber-reinforced polymer (SMA–FRP) that comprised of polymer matrix and small diameter super-elastic SMA fibers. They proposed that 100% of the reinforcementwill be provided by SMAs for applications with high ductility andenergy dissipation demands (e.g. seismic applications), whereasSMA fibers could be used in conjunction with other types ofreinforcement such as glass or carbon fibers for the applicationswhere only moderate damping is required (e.g. vibration controlunder service loads). Roh and Reinhorne [24] investigated theuse of SE SMA bar at the base segment of precast segmental bridgepier to improve the energy dissipation capacity of unbounded post-tensioned segmental columns. They developed new modelingtechniques of SMA consisting of postyielding, slip-lock and gap-closing springs that operate simultaneously. Their results indicatethat the inclusion of SMA bar provides good recentering, high

ductility and stable energy dissipation. Saiidi et al. [25] investi-gated the effectiveness of reinforced concrete bridge columns withsuperelastic shape memory alloy (SMA) reinforcement and engi-neered cementitious composites (ECC) in plastic hinges. Their re-sults showed that column with combined SMA bars and ECCexhibited a larger drift capacity while experiencing substantiallyless damage compared with conventional columns.

4. Geometry of hybrid RC column

This section briefly describes various configurations of hybridRC column used in this study. Three different hybrid longitudinalreinforcement combinations are considered here such as (a)SMA–SS: SMA (only in plastic hinge region) with stainless steel(SS) bar, (b) SMA–FRP: SMA (only in plastic hinge region) withFRP bar, and (c) SS–FRP: stainless steel (SS, only in plastic hinge re-gion) with FRP bar. The performance of the reinforcement withinthe plastic hinge region will naturally govern the behavior of thecolumn under lateral load. This is why SMA or SS are used partic-ularly in the plastic hinge region and FRP or stainless steel in otherregions. The column is an interior column of the ground floor of asix storey building which has five bays in both directions with thesame bay length of 5 m each. This RC building was analyzed as perNBCC (2005) [26] and the columns were designed according to CSAA23.3-04 (2004) [27] as ductile moment resisting frames. Thebuilding is located in the city of Vancouver, and its seismicity is ob-tained from NBCC (2005) [26]. The columns have been designedwith the maximum moment and shear forces developed duringthe analysis considering all possible load combinations specifiedin NBCC (2005) [26]. The size of longitudinal rebars and spacingof transverse reinforcement was selected following current coderequirements CSA A23.3-04 (2004) [27].

The detailed drawing of the column is shown in Fig. 1, which isa 450 mm square reinforced with 8-M20 stainless steel (diameter19.5 mm), or SMA20 (diameter 20.6 mm) or #6 FRP (diameter19.1 mm) longitudinal bars (gross reinforcement ratio of 1.2%).The plastic hinge length (Lp) was calculated according to Paulayand Priestley equation [28]:

Lp ¼ 0:08Lþ 0:022dbfy ð1Þ

where L is the length of the member, db represents the rebar diam-eter in mm and fy is the yield strength of the rebar in MPa.

The total height of the column section was 3200 mm and theplastic hinge length was calculated as 468 mm according to Eq.(1). To ensure flexural dominated behavior and avoid shear failure,an aspect ratio (cantilever height to equivalent column diameter)of 6.3, a longitudinal reinforcement ratio of 1.22%, and lateral steelmeeting the current seismic provisions were used. The columnrotation is fixed at the top but movement is free. Fig. 1 showsthe longitudinal distribution of reinforcement for hybrid columns.In the case of SMA-SS combination SE SMA was used as longitudi-nal reinforcement at the plastic hinge region. In this paper, unlessotherwise stated, SMAs are mainly referred to Ni–Ti SMA (com-monly known as Nitinol). Mechanical couplers were used to con-nect SMA with SS. For the SMA-FRP combination SMA was usedin the plastic hinge region and GFRP in the other parts. Mechanicaladhesive type couplers were used to connect SMA with FRP. In thecase of FRP-SS combination, stainless steel was used at the plastichinge zone and a mechanical-adhesive type coupler was used forconnecting FRP with SS.

5. Hybrid rebar connection

Different types of couplers are available for connecting rebars.Various types of mechanical couplers are used in RC construction

468mm

2732 mm

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BLp

SS tie

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SMA /SS

Fig. 1. Geometry of the hybrid RC column.

A.H.M. Muntasir Billah, M. Shahria Alam / Construction and Building Materials 28 (2012) 730–742 733

for connecting smooth or deformed bars. But for splicing differenttypes of rebars conventional splicing technique is not suitable. Asimple, quick and economic technique is required for connectingSMA with stainless steel and FRP bars in order to facilitate theuse of SMA in critical locations of RC structures. Screw lock cou-plers are suitable for splicing rebar as they can be readily appliedin the construction and require no special installation equipment.

5.1. Splicing of SMA with stainless steel rebar

Connecting SMA with stainless steel rebar is a challenge sinceSMA is extremely hard for machining and also it loses strengthdue to notches. Therefore, conventional mechanical couplers arenot suitable for connecting SMA with stainless steel rebar. Alamet al. [8] carried out several experimental investigations in searchof suitable couplers for splicing SMA with steel rebar. They foundthat single barrel screw lock couplers with few modifications couldproduce the best result. They performed several experiments tomake a proper connection that can sustain cyclic tensile load upto SMA’s SE strain range. Among different arrangements, theyfound that two extra rows of holes drilled through the coupler inparallel to the existing row at an angle of 60� away from the centerof the coupler (Fig. 2) produces the best result. Nine flat end screwbolts were used to connect one SMA rebar to the coupler. Similartechnique was adopted in this study for splicing SMA with SSwhere five sharp end screws were used for connecting SS. Simplepull out test was carried out to determine the slippage of SMAand SS bar inside the coupler. In order to measure the slip insidethe coupler a linear variable displacement transducer (LVDT) wasattached to the bar. The slippage value was calculated by deducting

Fig. 2. SBC for splicing SMA and Stainless steel rebar.

the bar elongation from the measured slippage. The stress versusslippage of SMA and SS bars are shown in Figs. 3a and b, respec-tively. The connection was developed in such a way that it canwithstand cyclic load up to SMA’s SE strain range (503 MPa). Thatis why Fig. 3a terminates at 10 mm of slippage which correspondsto the SMA’s SE strain range. In each case the result shows that theslippage inside the coupler was not significant at the initial stage ofloading. The slippage increased with the increase of the load.

5.2. Splicing of SMA with FRP rebar

Another alternative hybrid configuration is SMA–FRP in order tofacilitate the development of corrosion free and ductile RC column.However, connecting brittle FRP bar with ductile SMA is anotherchallenging task. Alam et al. [8] carried out several experimentalinvestigations and came up with a mechanical–adhesive typecoupler having two parts. In one part SMA is connected with thecoupler using multi-row flattened screws whereas in the otherpart, FRP is connected with the SS coupler with epoxy (Fig. 4a).Similar arrangement was used for SMA–FRP hybrid reinforcementconfigurations in this study. The stress versus slippage of FRP barinside the adhesive coupler is shown in Fig. 4b. The slippage ofFRP bar inside the coupler was obtained from the attached LVDT.

5.3. Splicing of stainless steel with FRP rebar

Similar mechanical–adhesive type coupler was used for con-necting SS with FRP bar where the slippage of SS and FRP bars in-side the coupler can be presented as Figs. 3b and 4b, respectively.

5.4. Calculation of total slippage of hybrid rebars

The stress slip relationships of different bars inside the couplersare illustrated in Figs. 3a, b and 4b. For hybrid rebar configurationse.g. SMA–FRP, SMA–SS, SS–FRP, the stress-slip relationships forvarious combinations were calculated. For example, for connectingSMA with FRP, the slip of SMA (s1) inside the coupler for a partic-ular stress (r) was calculated (Fig. 3a). Then for the same stress (r)the slip of FRP (s2) inside the coupler was calculated (Fig. 4b). Add-ing these two terms, the total slip (s) for a particular stress,

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ss (M

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ss (M

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(a) (b)Fig. 3. Stress versus slippage of (a) SMA bar, (b) SS bar and (c) FRP bar inside the coupler.

Adhesive coupler part

Screw lock coupler part

SMA bar

FRP bar

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Fig. 4. (a) Mechanical–adhesive type coupler for splicing SMA/SS with FRP bar, (b) test setup of coupler in a universal testing machine for determining slippage.

734 A.H.M. Muntasir Billah, M. Shahria Alam / Construction and Building Materials 28 (2012) 730–742

s = s1 + s2 was obtained. Fig. 5 shows the total slip in a SMA–FRPcombination. For instance, at a stress level (r) of 400 MPa, the slipof SMA bar (s1) inside the coupler was 3.13 mm (Fig. 3a) and theslip of FRP (s2) inside the coupler was 1.58 mm (Fig. 4b). Bysumming up the two quantities, the total slip (s) of SMA-FRP

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Fig. 5. Calculation of total slip for SMA–SS connection, (a) Stress–slip for SMA, (b)Stress–slip for FRP, (c) Combined stress slip curve for SMA–FRP connection.

combination inside the coupler was obtained as 4.71 mm (Fig. 5).Following the same procedure stress–slip relationship of all thecombinations were calculated as presented in Fig. 6. Whilecalculating the slip, it was assumed that the tension force inthe bar and at the coupler location is constant. These stress–slip

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Fig. 6. Stress–slip relationship of hybrid coupler connections for (a) SMA–FRP, and(b) SS–FRP. FE model of hybrid RC column.

Table 1Material properties for various Hybrid RC column configurations.

Material Property

A.H.M. Muntasir Billah, M. Shahria Alam / Construction and Building Materials 28 (2012) 730–742 735

relationships were used to evaluate the parameters of rotationalsprings, which were incorporated in the FE model to illustrate thisbond-slip relationship, which is described in Section 6.2.

Concrete Compressive strength (MPa) 38.3Corresponding strain 0.0029Tensile strength (MPa) 3.33Elastic modulus (GPa) 23.1

SE SMA Modulus of elasticity (GPa) 54.2Austenite-to-martensite starting stress (MPa) 414Austenite-to-martensite finishing stress (MPa) 530Martensite-to-austenite starting stress (MPa) 380Martensite-to-austenite finishing stress (MPa) 130Superelastic plateau strain length (MPa) 6.2

FRP Modulus of Elasticity (GPa) 52.2Ultimate tensile strength (MPa) 728Ultimate compressive strength (MPa) 364

Stainless steel Elastic modulus (GPa) 190Yield stress (MPa) 402Ultimate stress (MPa) 661Ultimate strain 0.55

6. Finite element analysis

A finite element (FE) program SeismoStruct [29] has been em-ployed to study the performance of hybrid RC column with splicedreinforcements. This particular FE program is able to consider bothgeometric nonlinearities and material inelasticity and can predictlarge displacement behavior of structures. Fiber modeling ap-proach has been employed to represent the distribution of materialnonlinearity along the length and cross-sectional area of the mem-ber. Concrete has been modeled using a uniaxial nonlinear con-stant confinement model, programmed by Madas [30], thatfollows the constitutive relationship proposed by Mander et al.[31] and the cyclic rules proposed by Martinez-Rueda and Elnashai[32]. SMA has been modeled according to the model of Auricchioand Sacco [33]. The 1-D superelastic SMA model that is consideredin this study is shown in Fig. 7. The parameters used to define theSMA model are fy (austenite to martensite starting stress); fP1(aus-tenite to martensite finishing stress); fT1(martensite to austenitestarting stress); fT2(martensite to austenite finishing stress), super-elastic plateau strain length or maximum residual strain, el; mod-ulus of elasticity, Ea. Stainless steel was modeled as a bilinearmodel following the experimental results of Talja and Salmi [34].FRP was modeled as a simplified uniaxial elastic material modelwith similar elastic modulus of elasticity in tension and compres-sion. The parameters used to define the material model have al-ready been presented in Table 1. A bond-slip relationship hasbeen incorporated in the analytical model of spliced hybrid RC col-umn, which is represented by a rotational spring at its joint. Theadapted FE model is presented in Fig. 8a. A constant axial load of400 kN was applied at the top of the column to simulate the gravityload.

Although the analysis has been performed using a freely avail-able software, the authors have verified the software with severalexperimental results that consist of static and dynamic loading ofstructures. For instance, static pushover test of RC bridge bent byBillah [35], shake table test of SMA reinforced concrete columnby Alam et al. [36], quasi-static reversed cyclic loading test ofSMA reinforced concrete beam-column joint by Alam et al. [36],and shake table test of a three storey RC frame by Alam et al. [9].Besides, the software SeismoStruct was employed by the winnerin the Practitioners category at the recent blind prediction contest

Fig. 7. 1D-Superelastic model of SMA [29].

deployed by PEER and NEES regarding the shaking-table testing ofa full-scale circular bridge pier [37].

6.1. Moment versus rotation

Finite element model of various hybrid columns was developedaccording to the data provided in Table 1. All the columns were di-vided into 12 elements longitudinally. In all the cases three ele-ments represent the plastic hinge region and the other ninerepresented the remaining portion. Each element was dividedtransversely into 400 by 400 fiber elements. Fig. 8b shows a con-crete column with hybrid reinforcement under lateral load. At itstension face the rebars are subjected to tension, which will try topull the rebar out from the coupler and this will cause slippage in-side the coupler. For a given lateral load the tension stress in therebar can be calculated numerically. Since the stress-slip relation-ship for a hybrid system is known, this will determine the slippageof rebar inside the coupler. Once the slippage, s is known, the rota-tion of the column due to slippage can be determined by dividing swith effective depth d. i.e. rotation, h = s/d and the correspondingmoment can be calculated by multiplying the lateral load withthe column height. Fig. 9 shows the moment–rotation relationshipfor various configurations. This relationship was used to find outthe parameters of rotational spring used at the bottom of columnas shown in Fig. 8a. This rotational spring was introduced at thebottom of the column section to represent the stress-slip relation-ship of bars inside the coupler used for splicing.

6.2. Calibration of bar-slip mechanism

Youssef and Ghobarah [38], Calvi et al. [39] and Lowes et al. [40]proposed several beam-column joint models using equivalent mo-ment rotational spring to represent the joint behavior in both lin-ear and nonlinear range. Based on these models, a simplified slipmodel is adopted in this study which is capable of predicting thebond-slip behavior of rebars within the couplers. The rotationalspring was modeled by defining a joint element in SeismoStruct[29]. The joint element connects two initially coincident structuralnodes, and requires the definition of an independent force–displacement (or moment-rotation) response curve for each of itslocal six degrees-of-freedom (F1, F2, F3, M1, M2, M3). In order todefine the joint element, modified Takeda hysteresis curve wasadopted in this study to define the moment-rotation relationshipof joint element. This is a modified bilinear curve described by

Fig. 8. (a) FE model of hybrid RC column, (b) bar slip due to column lateral load.

736 A.H.M. Muntasir Billah, M. Shahria Alam / Construction and Building Materials 28 (2012) 730–742

Otani [41] which follows the unloading rules proposed by Emoriand Schnobrich [42].

Five parameters are necessary to characterize the behavior ofresponse curve which are, the yielding strength Fy; the initial stiff-ness Ky; the postyielding to initial stiffness ratio a; the out loopstiffness degradation factor b0; and the inner loop stiffness degra-dation factor b1. From the moment-rotation relationship of differ-ent hybrid configuration (Fig. 9), the required parameters todefine the response curve (Fy, Ky and a) were obtained. The stiff-ness degradation factors b0 and b1 are specified as 0.1 and 0.9,respectively, as to make the unloading stiffness close to the initialstiffness during the cyclic loading. Similar modeling technique wasadopted by Yu [43] to simulate the behavior of beam-column jointusing rotational spring under cyclic loading using SeismoStruct[29]. They showed that this type of numerical modeling showsgood agreement with experimental results.

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500

600

0 0.01 0.02 0.03 0.04 0.05

Mom

ent (

kN-m

)

Rotation (rad)

SMA-SS

SS-FRP

SMA-FRP

Fig. 9. Moment versus rotation envelopes for various hybrid configurations.

6.3. Validation with experimental result

One 3=4 scale beam column joint was designed, constructed andtested under reversed quasi-static cyclic load by Nehdi et al. [7].SMA rebars were placed at the plastic hinge region and GFRP barsin the remaining portion connected using adhesive type coupler.Fig. 11 shows the performance of the beam column joint under ap-plied reversed quasi-static cyclic load in terms of beam tip loadversus displacement. The applied load was displacement con-trolled and the load history is shown in Fig. 10. In order to validatethe numerical model adopted in this study, the SMA–FRP hybridcolumn was modeled using finite element software and analyzedunder same reversed quasi-static cyclic load where the SMA isplaced at the plastic hinge region of the column and GFRP bars inthe remaining portion. Fig. 11 also portrays the predicted beam-tip load versus displacement of the numerical model, which showssimilar results compared to that of the experimental results [7].

-50

-40

-30

-20

-10

0

10

20

30

40

50

0 2 4 6 8 10 12

Stor

ey d

rift r

atio

Time (sec)

Fig. 10. Reversed cyclic load history used for analysis.

-80

-60

-40

-20

0

20

40

60

80

-60 -40 -20 0 20 40 60

Load

(kN

)

Displacement (mm)

Experimental result

Predicted result

Fig. 11. Comparison of load–displacement relationship of experimental [7] andpredicted results.

A.H.M. Muntasir Billah, M. Shahria Alam / Construction and Building Materials 28 (2012) 730–742 737

The maximum beam-tip load and residual deformation were calcu-lated as 52.2 kN and 18.4 mm compared to the experimental val-ues of 57.8 kN and 17.5 mm, respectively. The results obtainedfrom finite element analysis show good agreement with the exper-imental results with a small variation of 10.7% and 5.1%, respec-tively in predicting the beam-tip load and residual deformation.Moreover the cumulative energy dissipation was calculated as6.93 kN m from the load displacement curve obtained from numer-ical analysis whereas the experimental result was 7.54 kN m,which is only 8.8% variation between the numerical and experi-mental result.

6.4. Pushover analysis

Prior to dynamic analyses of hybrid RC columns, nonlinear sta-tic pushover analyses have been performed on each of the hybridconfigurations. The comparative performances of various hybridcolumns are shown in Fig. 12. From this figure it is evident that dif-ferent hybrid configurations have different peak base shear capac-ity. Before concrete cracks, all the hybrid columns exhibited similarstiffness. Once the concrete cracks, the SMA rebars become effec-tive in resisting tensile forces, which resulted in reduced stiffnessfor SMA–SS and SMA–FRP columns, compared to that of SS column.As the stiffness and yield capacity of SMA, FRP and SS are different,the base shear capacity of different hybrid columns are also differ-ent. Table 2 shows the values for base shear and displacements atwhich the cracking, yielding and crushing of concrete took place indifferent hybrid RC columns. The performance criteria of differentRC columns were obtained by tracking the base shear and displace-

0

20

40

60

80

100

0 100 200 300 400

Bas

e Sh

ear (

kN)

Displacement (mm)

SMA-FRPSS-FRPSMA-SSSS

Cracking

Yielding

Crushing

Fig. 12. Pushover analysis results of various hybrid configurations.

ments at cracking of cover concrete, yielding of reinforcement andcrushing of core concrete. Before concrete cracked, all the hybridconfigurations exhibited similar stiffness. Once concrete cracked,the SMA rebars became effective in resisting forces. Since SMAhas lower stiffness, there is some reduction in the overall stiffnessof the column. SMA–FRP combination shows higher displacementof 93 mm before yielding at a base shear of 74 kN. But for yieldingSS–FRP combination could sustain a yielding displacement of45 mm while the base shear was 67.5 kN. Before the crushing ofconcrete the SMA–FRP combination suffered a significant amountof displacement of 207 mm which was 21% and 25.5% greater thanthat of SMA–SS and SS–FRP combination, respectively. It is evidentfrom the result that the combination with SMA at the plastic hingeregion suffered significant amount of displacement before yieldingcompared to the other ones. This is due to the lower stiffness ofSMA compared to that of SS.

The performances of various hybrid configurations were alsocompared in terms of ductility (ability to undergo large displace-ment while maintaining the postyield load level). Structural ductil-ity plays a significant role in the evaluation of seismic design forcesand evaluation of performance during an earthquake event. Ductil-ity is also a contributing factor in the energy dissipation capacity ofthe structure as well. In this study ductility is used as a demandparameter to compare the performances of four hybrid columnconfigurations. Ductility, a dimensionless factor, is defined as theratio of ultimate displacement to the yield displacement. The yielddisplacement corresponds to the first yielding of tension reinforce-ment. Different researchers have defined the ultimate displace-ment in different way. Most of the design guidelines [44,45] andmany researchers [46–48] have defined the ultimate displacementas the maximum displacement corresponding to maximum baseshear. While others [49,50] defined it as the 85% of the peak load.Some researchers [51,52] defined it as the displacement corre-sponding to a load that drops 20% from the peak load. In this studythe ultimate displacement is considered as the displacement corre-sponding to a load that drops 20% from the peak load. Fig. 13 showsthe variation in ductility for all different hybrid RC columns. It isevident from Fig. 13 that SS RC column showed higher ductilitycompared to others as expected. SS-FRP combination showed goodductility which was higher than that of SMA–FRP and SMA–SScombination.

6.5. Dynamic time history analysis

Dynamic time history analyses were performed to evaluate theperformances of different hybrid columns under earthquake exci-tation. In order to conduct the dynamic analysis, a point mass(25 ton) was assigned to each column. All permanent weight onthe floor level that moves with the column top was lumped atthe column top. Real accelerograms were used for the dynamicanalysis of various column configurations. These accelerogramswere chosen such that they were representative of the seismic mo-tions of the location of the structure. The dynamic analyses werecarried out for the 20 selected earthquake records as shown inTable 3. The records selected belong to a bin of relatively large mag-nitudes 6.5–6.9, and moderate epicentral distances in the range of15–32 km. These accelerograms were chosen such that they repre-sent the seismic characteristics of the site of the structure. The ra-tio between the peak ground acceleration (PGA) and peak groundvelocity (PGV) is an indicator of the frequency content of seismicmotion. The characteristic seismic motions for the western partof Canada have a PGA/PGV ratio around 1.0 [53]. The selectedensemble of earthquake records is presented in Table 3 wherethe PGA/PGV ratio varies between 0.8 and 1.3. These 20 ground mo-tion records were obtained from the PEER strong motion database.Fig. 14 shows the acceleration response spectrum (5% damped) for

0

1

2

3

4

5

6

7

8

9

10

SMA-FRP SS-FRP SMA-SS SS

Duc

tility

Column Type

Fig. 13. Ductility comparison of various hybrid configurations.

Table 2Comparative performance of different hybrid columns.

Concrete cracking Steel yielding Concrete crushing

Column type Displacement (mm) Base shear (kN) Displacement (mm) Base shear (kN) Displacement (mm) Base shear (kN)

SMA–FRP 6 29.15 93 74 207 74.5SS–FRP 6 32.8 45 67.6 165 71.5SMA–SS 3 30.17 60 63.68 171 75.14SS 3 36 24 78.5 108 82.66

738 A.H.M. Muntasir Billah, M. Shahria Alam / Construction and Building Materials 28 (2012) 730–742

the selected ground motion sets. All the ground motions werescaled using SeismoMatch [54] to match the response spectrumof Vancouver, B.C. where the structure is located. This software isable to adjust any ground motion accelerograms to match a spe-cific design response spectrum using wavelets algorithm proposedby Abrahamson [55] and Hancock et al. [56]. The dynamic time his-tory analyses results are used to compute the demand capacity ra-tio in terms of base shear, energy dissipation capacity and residualdisplacement.

6.6. Base shear capacity demand ratio

The base shear demand is defined as the average maximumbase shear obtained for earthquake loading and the capacity isdefined as the amount of base shear obtained from pushover

Table 3Selected earthquake ground motion records. Source: PEER strong motion database, http://

No. Event Year Record station

1 Imperial valley 1979 Plaster City2 Imperial valley 1979 Plaster City3 Imperial valley 1979 Westmoreland Fire Sta.4 Imperial valley 1979 Westmoreland Fire Sta.5 Imperial valley 1979 El Centro Array#136 Imperial valley 1979 El Centro Array#137 Loma Prieta 1989 Agnews state hospital8 Loma Prieta 1989 Coyote Lake Dam9 Loma Prieta 1989 Sunnyvale Colton Ave

10 Loma Prieta 1989 Sunnyvale Colton Ave11 Loma Prieta 1989 Anderson Dam12 Imperial valley 1979 Chihuahua13 Loma Prieta 1989 SF Inter. Airport14 Loma Prieta 1989 Hollister Diff. Array15 Imperial valley 1979 Cucapah16 Loma Prieta 1989 WAHO17 Loma Prieta 1989 Holister South & Pine18 Loma Prieta 1989 16 LGPC19 Superstition Hill 1987 Wildlife liquefaction arr20 Superstition Hill 1987 Wildlife liquefaction arr

a Moment magnitudes,b Closest distances to fault rupture.

analyses. Fig. 15 shows the base shear capacity demand ratio ofthe different hybrid configurations for 20 different ground mo-tions. For most of the ground motion considered the SS reinforcedcolumn showed higher base shear capacity demand ratio. SMA–SSreinforced column showed good performance under dynamic load-ing. For instance, for ground motion-6 the base shear capacity de-mand ratio of SMA–FRP, SS–FRP, SMA–SS and SS reinforced columnwas 1.91, 2.02, 1.97 and 2.05, respectively.

6.7. Drift demand

Fig. 16 shows the drift demand of various hybrid columns under20 ground motions considered in this study. Here, ‘drift’ refers tothe ratio of lateral displacement of the column top to the columnheight, and the ‘drift demand’ is defined as the maximum columntop drift during an earthquake. It can be observed that in mostcases all the four different configurations provide comparable re-sults. The average top drift (%) of SS RC column is 1.02%, whereasthe superelastic SMA–SS and SMA-FRP column has an averagetop drift of 1.19% and 1.14% respectively. The SS-FRP has an aver-age top drift of 1.21%. At a PGA of 0.638 g (Eq no-18) all the col-umns experienced larger drift. In this case the SS RC columnexperiences a maximum drift of 3.8% whereas the SMA-FRPcolumn has a maximum drift of 3.2%. In most cases the SMA RCcolumn experienced higher top drift than SS RC column. SMA hasa low modulus of elasticity as compared to SS, which makes SMARC column less stiff than those of SS RC column and consequentlyresulting in higher drift demand.

6.8. Residual drift

Residual drift is a very important parameter for performancebased earthquake engineering as it dictates the functionality of a

peer.berkeley.edu/svbin.

M�a R�b (km) PGA (g) PGA/PGV

6.5 31.7 0.042 1.36.5 31.7 0.057 1.056.5 15.1 0.082 1.26.5 15.1 0.11 0.86.5 21.9 0.117 0.86.5 21.9 0.139 1.066.9 28.2 0.159 0.96.5 22.3 0.179 0.936.9 28.8 0.207 0.846.9 28.8 0.209 0.876.9 21.4 0.244 1.26.5 28.7 0.254 0.846.9 25.8 0.236 0.936.9 25.8 0.279 0.796.9 23.6 0.309 0.856.9 16.9 0.37 1.036.9 28.8 0.371 0.976.9 16.9 0.605 1.19

ay 6.7 24.4 0.134 1.0ay 6.7 24.4 0.132 1.03

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 1 2 3 4

Spec

tral

Acc

eler

atio

n (g

)

Period (sec)

EQ1 EQ2EQ3 EQ4EQ5 EQ6EQ7 EQ8EQ9 EQ10EQ11 EQ12EQ13 EQ14EQ15 EQ16EQ17 EQ18EQ19 EQ20Vancouver spectra

Fig. 14. Spectral acceleration for the chosen earthquake records.

0

0.5

1

1.5

2

2.5

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Bas

e Sh

ear C

/D

Earthquake No.

SMA-FRPSS-FRPSMA-SSSS

Fig. 15. Base shear C/D of various hybrid configurations.

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Drif

t (%

)

Earthquake no.

SMA-FRPSS-FRPSMA-SSSS

Fig. 16. Maximum top drift (%) of various hybrid configurations.

A.H.M. Muntasir Billah, M. Shahria Alam / Construction and Building Materials 28 (2012) 730–742 739

member after an earthquake. A limited number of studies havebeen performed by a few researchers specifically with regard toevaluating residual drift. Fig. 17 presents the results of the dynamicresponses of different hybrid configurations under the Loma Prietaearthquake (record No. 18 in Table 3). From Fig. 17 it is evident thatthe maximum displacements experienced by all types of columnswere more than 100 mm. However, there is a marked differencein the residual displacement experienced by four different hybridcolumns where all the SMA RC columns experienced negligibleresidual deformation whereas both SS RC columns experiencedapproximately 30 mm of residual drift. Fig. 18 shows the residual

displacement of four different columns under 20 ground motions.In all cases SMA reinforced column suffered negligible residualdrift as compared to SS reinforced column. In each case the residualdrift in SMA reinforced column was less than 0.5% where in somecases the SS reinforced column exceeded 1%. For instance, underthe Loma Prieta earthquake (record No. 18 in Table 3) the SS col-umn experienced a maximum residual drift of 0.9% which was76%, 87% and &7% larger than SMA–SS, SMA–FRP and SS–FRP col-umn, respectively. From this analysis it can be concluded thatSMA in the plastic hinge length is very effective in reducing theresidual drift which in turn will keep the structure functional evenafter a seismic event.

6.9. Energy dissipation capacity

In the case of a reinforced concrete element, energy dissipationcapacity during a seismic event proves its structural integrity andshows the interaction between the reinforcement and concrete.The cumulative energy dissipation by various hybrid RC columnsduring seismic excitation was calculated by summing up the dissi-pated energy in successive load–displacement loops throughoutthe analysis. Fig. 19 shows the cumulative energy dissipationcapacity of four hybrid columns under 20 earthquake excitations.The results show similar trend under different ground motions.Larger hysteretic loops of SS in the plastic hinge region of SS–FRPcombination resulted in a higher amount of energy dissipationwhereas in the case of SMA–SS and SMA–FRP combinations, slip-page of SMA and FRP bars inside the coupler assisted in consider-able energy dissipation during seismic loads even though SMAexhibits smaller hysteretic loops compared to that of SS. Energydissipation capacity is one of the most important seismic designcriteria for a structure. Slippage inside the coupler can be consid-ered as a source of energy dissipation as evident from the experi-mental investigations by Nehdi et al. [7]. From Fig. 19 it can beobserved that in most cases SS systems could dissipate higheramount of energy with respect to the SMA systems. Nevertheless,the major benefit of SMA–FRP and SMA-SS combination is theirability to dissipate considerable amount of energy while encoun-tering negligible residual drift and permanent damage. For in-stance, under ground motion-18, the SS reinforced columndissipated a maximum of 13.8 kN m of energy which was 43%,22% and 32% higher than that of SMA–FRP, SS–FRP and SMA–SScombinations, respectively.

7. Discussion

The use of hybrid SMA–FRP, SMA–SS and SS–FRP as reinforce-ment in concrete is yet to be introduced in real structural applica-tions. Before the full-scale use of such hybrid reinforcements in theindustry, significant analytical and experimental efforts are still re-quired for establishing proper guidelines. The present study ex-plored the performance of RC column reinforced with SE SMA orSS at their plastic hinge region and FRP/SS in the other regionsalong with a SS RC column under dynamic loading. After compar-ing the performances of various hybrid RC column configurations,it is shown that replacing SS with SMA and FRP improves the struc-tural performance by considerable recovery of deformations dur-ing an earthquake. Hence, this type of SMA–FRP hybridreinforcement may be considered for applications with high ductil-ity demands such as in the case of seismic applications. The super-elasticity of SMA resulted in very small residual displacement andsubstantially reduced the damage in the plastic hinge region. Onthe other hand, FRP can provide the required corrosion resistancewhich is another major cause of structural failure. There are differ-ent approaches in the literature for improving the corrosion

Fig. 17. Pier top drift time histories of different hybrid configurations (a) SS, (b) SMA–SS, (c) SMA–FRP and (d) SS–FRP for ground motion 18.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Res

idua

l drif

t (%

)

Earthquake No

SMA-FRP SS-FRP SMA-SS SS

Fig. 18. Residual drift (%) for various hybrid configurations.

0

2

4

6

8

10

12

14

16

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Cum

ulat

ive

Ener

gy

Dis

sipa

tion(

kN-m

)

Earthquake No.

SMA-FRP SS-FRP

SMA-SS SS

Fig. 19. Cumulative energy dissipation for various hybrid configurations.

740 A.H.M. Muntasir Billah, M. Shahria Alam / Construction and Building Materials 28 (2012) 730–742

resistance of RC structures such as cathodic protection, low perme-ability concrete with added corrosion inhibitors, epoxy coating etc.However, this requires installation of equipment and power supply

along with regular operation and maintenance. Conversely, themain advantage of using SMA–FRP RC is that it is corrosion freeand should require no additional care or support. Although SMAreinforced column has low energy dissipation capacity as com-pared to SS reinforced column, they might undergo larger amountof cycles compared to that of SS reinforced column and eventuallydissipate equal amount of energy where its residual deformationwill still be negligible compared to SS reinforced column.

The dynamic analysis of columns reinforced with SMA or SSreinforcement in the plastic hinge zones and FRP in other regionswere performed with 20 earthquake records. However, dedicatedresearch efforts are still required to address many issues anduncertainties before the widespread use of Hybrid RC column. Thisstudy highlights the potential long-term benefits of using the pro-posed structural system in terms of enhanced durability of struc-ture and the reduced expected life-cycle cost. The authors hopethat the limited study presented herein will pave the way for fur-ther research on hybrid RC column for large scale industrialapplication.

8. Conclusion

The applicability of corrosion-free hybrid RC column is investi-gated analytically and an effort was made to compare the perfor-mance of SMA hybrid RC columns to that of SS reinforcedcolumns under earthquake loading. The study was carried out bymeans of inelastic pushover analysis and dynamic time historyanalysis employing 20 natural ground motions. The efficiency ofusing hybrid reinforcements in concrete structures was demon-strated in terms of adequate ductility, energy dissipation capacityand reduced permanent deformation. Based on the analytical studythe following conclusions are drawn:

1. The residual displacement in columns with superelastic SMAlongitudinal reinforcement in the plastic hinge zone was con-siderably less than that of conventional SS reinforced column.The results obtained in this study showed that the reductionwas as high as 87% when SMA is used in the plastic hinge zone.

A.H.M. Muntasir Billah, M. Shahria Alam / Construction and Building Materials 28 (2012) 730–742 741

The practical implication of the major reduction in residual driftis that the structure is more likely to remain serviceable afterthe earthquake when superelastic SMA longitudinal bars areused in the plastic hinge length.

2. SMA–FRP and SMA–SS combinations could dissipate consider-able amount of energy compared to that of SS combinationwhere the energy dissipation was governed by significant slip-page of FRP and SMA bars inside the coupler. Larger hystereticloops of SS-FRP combination resulted in a higher amount ofenergy dissipation compared to that of SMA’s smaller hystereticloops in SMA–FRP and SMA–SS combination.

3. The SS reinforced column showed higher ductility as comparedto other systems. However, the SMA reinforced columns provedto have sufficient ductility before failure. The ductility of bothSS–FRP and SMA–SS were close.

The analytical results demonstrated a considerable advantage ofusing SMA in plastic hinge zone compared to the conventional SSreinforced column. Under seismic excitations, using SMA in plastichinge zone improved the seismic behavior of the columns signifi-cantly compared to the others. The superiority of the proposedSMA–FRP hybrid technique is primarily attributed to the signifi-cant reduction in residual deformation and low maintenance cost,which could allow the structure to remain serviceable even after astrong earthquake. However, a thorough life-cycle-cost-analysis isrequired for the proposed hybrid RC column systems to assist inthe decision making process.

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