Engineering Structures 30 (2008) 1632–1646 www.elsevier.com/locate/engstruct Improving seismic performance of deficient reinforced concrete columns using carbon fiber-reinforced polymers Okan Ozcan a,1,2 , Baris Binici b,∗ , Guney Ozcebe b,3 a Department of Civil Engineering, Akdeniz University, Antalya, 07058, Turkey b Department of Civil Engineering, Middle East Technical University, Ankara, 06531, Turkey Received 30 March 2007; received in revised form 19 October 2007; accepted 26 October 2007 Available online 20 February 2008 Abstract Reinforced concrete columns lacking transverse reinforcing steel do not possess necessary ductility to dissipate seismic energy during a major earthquake. The study reported herein investigates the use of carbon fiber-reinforced polymer (CFRP) wrapping as a method of retrofitting nonductile square reinforced concrete columns with low strength concrete and plain bars. Five specimens representative of transverse steel deficient flexure dominated columns in existing buildings were tested under lateral cyclic displacement excursions and constant axial load. The main parameters under investigation were the number of layers of CFRP wrap and presence and absence of the axial load on the column during strengthening. It was found that upon CFRP retrofit, deficient columns were able to withstand larger deformation demands without strength degradation. The test results were also compared with the results obtained from the analytical study that was conducted using a simple plastic hinge model considering the effects of FRP confinement and slip of plain bars. A good agreement between analytical and experimental results was observed. c  2007 Elsevier Ltd. All rights reserved. Keywords: Reinforced concrete column; Strengthening; Fiber-reinforced polymer; Plain bars 1. Introduction Earthquakes occurred in Turkey (Kocaeli 1999, Duzce 1999, Bingol 2003) revealed that inadequate strength and ductility of reinforced concrete (RC) columns due to poor detailing of transverse reinforcement can cause extensive structural damage and even collapse in reinforced concrete frame buildings. As most of the columns of existing buildings in Turkey are of this type, there is a need to understand the behavior of low strength RC columns (10–15 MPa concrete compressive strength) having plain bars and insufficient confining steel for seismic assessment of deficient buildings. Furthermore, rapid ∗ Corresponding author. Tel.: +90 3122 102457; fax: +90 3122 107991. E-mail addresses: ookan@metu.edu.tr (O. Ozcan), binici@metu.edu.tr (B. Binici), ozcebe@metu.edu.tr (G. Ozcebe). 1 Currently on leave at the Middle East Technical University for graduate study. 2 Tel.: +90 3122 105464; fax: +90 3122 107991. 3 Tel.: +90 3122 102461; fax: +90 3122 105401. retrofit methods need to be established and presented to the service of engineers. Previous research on strengthening and repairing methods emphasized that the composite column jacket retrofit systems can be as effective as conventional steel jacketing in improving the seismic response characteristics of RC columns equipped with deformed bars and having normal strength concrete [1– 10]. In all tests presented in these studies, specimens with properly designed continuous FRP jackets met or exceeded deformation capacities compared to those obtained from specimens with code compliant transverse reinforcement design. Test results showed that seismic resistance of FRP retrofitted columns improved significantly as a result of confining the entire column section and eliminating premature rebar buckling. It was concluded that FRP composites were very effective for both the rehabilitation and repair of existing columns. However, none of the details of the columns tested in the literature can successfully simulate the typical deficiencies and construction practice encountered in Turkey (i.e. plain bars, 90 degree hooks in stirrups, and low strength 0141-0296/$ - see front matter c  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.engstruct.2007.10.013