Quasi-static and pseudo-dynamic

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    Quasi-static and pseudo-dynamic testing of infilled RC frames retrofitted

    with CFRP material

    H. Ozkaynak a, E. Yuksel a,, O. Buyukozturk b, C. Yalcin c, A.A. Dindar d

    a Faculty of Civil Engineering, Istanbul Technical University, Istanbul, Turkeyb Civil and Environmental Eng., Massachusetts Institute of Technology, MA, USAc Department of Civil Engineering, Bogazici University, Istanbul, Turkeyd Department of Civil Engineering, Istanbul Kultur University, Istanbul, Turkey

    a r t i c l e i n f o

    Article history:

    Received 21 April 2010

    Received in revised form 5 July 2010

    Accepted 16 November 2010

    Available online 23 November 2010

    Keywords:

    A. Carbon fiber

    B. Plastic deformation

    B. Strength

    B. Retrofitting

    a b s t r a c t

    The intact infill walls in reinforced concrete (RC) frames have beneficial effects to overall behavior in

    terms of stiffness, strength and energy dissipation in the event of seismic actions. The rationale of this

    paper is to increase effectiveness of the carbon fiber reinforced polymer (CFRP)-based retrofitting tech-

    nique so that intact infill walls of vulnerable mid-rise RC buildings are transformed into a lateral load

    resisting system. The seismic behaviors of cross-braced and cross diamond-braced retrofitting schemes

    applied on infilled RC frames have been investigated experimentally. The research consisted of quasi-sta-

    tic (QS) tests wheredrift-basedcyclic loading reversals were used and pseudo-dynamic (PsD) tests where

    acceleration intensity-based loading was used. Twelve 1/3-scaled RC frames were built and tested as bare

    and infilled control frames, and as cross-braced and cross diamond-braced retrofitted specimens. Signif-

    icant findings were noted while comparing the QS and PsD tests. The maximum restoring force and drift

    couples that were obtained from PsD tests showed a close behavior pattern, regardless of the level of

    inertial masses, when compared with QS tests. The energy dissipation capacity of the specimens that

    was obtained from PsD test resulted somewhat less than the one tested with QS for the same level of

    damage. The performance of the retrofitted frames that was obtained from the experimental studywas evaluated with code-specified performance limits. Accordingly, it was concluded that the cross dia-

    mond-bracing scheme is an effective retrofitting technique that brings the bare frame from collapse pre-

    vention (CP) to life safety (LS) performance levels. Finally, analytical predictions as per FEMA 356

    guideline were performed and good agreement was obtained with experimental results.

    2010 Elsevier Ltd. All rights reserved.

    1. Introduction

    Past earthquakes showed that infill walls used in RC frames had

    many advantages in terms of improvements in global stiffness, lat-

    eral strength and energy dissipation capacities of the structures

    when they are placed regularly throughout the structure and/or

    they do not cause shear failures of columns, [1]. Several experi-

    mental researches conducted on infilled RC frames also showed a

    significant improvement in the overall behavior. Shake table tests

    on infilled RC frames performed by Hashemi and Mosallam [2] re-

    sulted that the infill walls increased the structural stiffness by

    nearly four times, shortened natural period by nearly 50% and in-

    creased the damping coefficient from 46% to 12%.

    In many existing RC buildings, especially those designed and

    built before the contemporary earthquake codes, there is a lack

    of seismic detailing in structural load carrying system and struc-

    tural members coupled with low material quality and workman-

    ship [3]. Infill walls, during any credible earthquake, may

    experience excessive damage and/or out of plane movements. Ret-

    rofitting these walls using CFRP materials could further improve

    the contribution of infills to the overall seismic behavior of the vul-

    nerable RC buildings. Mosallamet al. [4] applied PsD test technique

    to experimentally investigate a two-bay, two-storey gravity load-

    designed steel frame infilled with unreinforced concrete block ma-

    sonry walls. It was concluded that the imparted and hysteretic

    energies correlated well with the observed damage state. It was

    also concluded that the variation of these quantities with the in-

    crease of PGA levels might be considered as a global measure to

    quantify the damage state of the structure. Taghdi et al. [5] tested

    four concrete block masonry and two RC walls simulating low-rise

    non-ductile walls. Two masonry walls were unreinforced and two

    were partially reinforced. One wall from each pair was retrofitted

    using a steel strip system consisting of diagonals and vertical

    strips. Stiff steel angles and anchor bolts were used to connect

    the steel strips to the foundation and top of loading beam. The tests

    1359-8368/$ - see front matter 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.compositesb.2010.11.008

    Corresponding author. Tel./fax: +90 212 285 6761.

    E-mail address: yukselerc@itu.edu.tr (E. Yuksel).

    Composites: Part B 42 (2011) 238263

    Contents lists available at ScienceDirect

    Composites: Part B

    j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o m p o s i t e s b

    http://dx.doi.org/10.1016/j.compositesb.2010.11.008mailto:yukselerc@itu.edu.trhttp://dx.doi.org/10.1016/j.compositesb.2010.11.008http://www.sciencedirect.com/science/journal/13598368http://www.elsevier.com/locate/compositesbhttp://www.elsevier.com/locate/compositesbhttp://www.sciencedirect.com/science/journal/13598368http://dx.doi.org/10.1016/j.compositesb.2010.11.008mailto:yukselerc@itu.edu.trhttp://dx.doi.org/10.1016/j.compositesb.2010.11.008
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    showed that the complete steel strip system was effective in signif-

    icantly increasing the in-plane strength and ductility of low-rise

    unreinforced and partially reinforced masonry walls, and lightly

    reinforced concrete walls. Saatcioglu and Serrato [6] carried out

    an experimental investigation on gravity-load-designed RC frames,

    infilled with concrete block masonry. The aim of that study was to

    develop a seismic retrofit strategy involving the use CFRP sheets.

    The retrofit technique consisted of CFRP sheets, surface bonded

    to the masonry wall, while also anchored to the surrounding con-

    crete frame by means of specially developed CFRP anchors. The re-

    sults indicated that the infilled frames without a seismic retrofit

    developed extensive damage in the walls and surrounding frame

    elements. Furthermore, the elastic rigidity was reduced consider-

    ably resulting in softer structure and failure occurred in non-duc-

    tile frame elements, especially in columns. Retrofitting using

    CFRP sheets controlled cracking and increased lateral bracing while

    improving the elastic capacity of the overall structural system. The

    retrofitted specimens exhibited approximately three times in lat-

    eral force resistance than that of control specimens. Erdem et al.

    [7] conducted an experimental study on 1/3-scaled, two-story,

    three-bay frames to compare two types of strengthening tech-

    niques. One of the frames was strengthened with RC infill while

    the other one was strengthened with CFRP-strengthened hollow

    clay blocks. It was observed that both strengthened frames be-

    haved similarly under reversed cyclic lateral loading. The stiffness

    of the strengthened frames was at least 10 times than that of the

    bare frame. Although the strengths of both specimens were almost

    the same, the strength degradation of the CFRP retrofitted frame

    beyond the peak lateral force level was more pronounced. Almu-

    sallam and Al-Salloum [8] investigated the effectiveness of glass fi-

    ber-reinforced polymers (GFRP) in strengthening of unreinforced

    masonry infill walls in RC frames which are subjected to in-plane

    seismic loading. Test results showed great potential for externally

    bonded GFRP sheets in upgrading and strengthening the infill

    walls. Wei et al. [9] studied the response of different FRP orienta-

    tions on the masonry wall elements. It was concluded that the

    diagonally-meshed specimen had a greater ductility than others.Binici et al. [10] developed an efficient CFRP retrofitting on hollow

    clay brick infill walls which could be utilized as lateral load resist-

    ing elements. The practical retrofitting scheme was developed to

    limit the inter-storey deformations with CFRP-strengthened infill

    walls that were integrated to the boundary frame members by

    means of CFRP anchors. It was observed that the CFRP retrofitting

    reduced the damage-induced deficient columns by means of con-

    trolling storey drifts. Yuksel et al. [11] tested infilled RC frames

    with and without retrofitting. The effect of various CFRP retrofit-

    ting schemes was discussed. They concluded that the cross bracing

    and cross diamond-bracing type of retrofitting had more advanta-

    ges compared with the others.

    The rationale of this paper is to increase the efficiency of the

    CFRP-based retrofitting technique in which the infill walls of vul-nerable mid-rise RC buildings could be transformed into a lateral

    load resisting system. In order to achieve this goal, CFRP sheets

    were used in two diffe