Seismic Behavior of Concrete Columns Reinforced by Steel-FRP Composite Bars Ze-Yang Sun 1 ; Gang Wu 2 ; Zhi-Shen Wu, M.ASCE 3 ; and Min Zhang 4 Abstract: Steel-fiber-reinforced polymer (FRP) composite bars (SFCBs) are a novel reinforcement for concrete structures. Because of the FRP’ s linear elastic characteristic and high ultimate strength, they can achieve a stable postyield stiffness even after the inner steel bar has yielded, which subsequently enables a performance-based seismic design to easily be implemented. In this study, lateral cyclic loading tests of concrete columns reinforced either by SFCBs or by ordinary steel bars were conducted with axial compression ratios of 0.12. The main variable parameters were the FRP type (basalt or carbon FRP) and the steel/FRP ratio of the SFCBs. The test results showed the following: (1) compared with ordinary RC columns, SFCB-reinforced concrete columns had a stable postyield stiffness after the SFCB’ s inner steel bar yielded; (2) because of the postyield stiffness of the SFCB, the SFCB-reinforced concrete columns exhibited less column-base curvature demand than ordinary RC columns for a given column cap lateral deformation. Thus, reduced unloading residual deformation (i.e., higher postearthquake reparability) of SFCB columns could be achieved; (3) the outer FRP type of SFCB had a direct influence on the performance of SFCB-reinforced concrete columns, and concrete columns reinforced with steel-basalt FRP (BFRP) composite bars exhibited better duc- tility (i.e., a longer effective length of postyield stiffness) and a smaller unloading residual deformation under the same unloading displace- ment when compared with steel-carbon FRP (CFRP) composite bar columns; (4) the degradation of the unloading stiffness by an ordinary RC column based on the Takeda (TK) model was only suitable at a certain lateral displacement. In evaluating the reparability of important structures at the small plastic deformation stage, the TK model estimated a much smaller residual displacement, which is unsafe for important structures. DOI: 10.1061/(ASCE)CC.1943-5614.0000199. © 2011 American Society of Civil Engineers. CE Database subject headings: Concrete columns; Composite materials; Bars; Fiber reinforced polymer; Deformation; Stiffness; Seismic effects. Author keywords: Steel-FRP composite bar; Seismic action; Soncrete column; Postyield stiffness; Residual deformation. Introduction Damage to a concrete column could result in the loss of vertical bearing capacity and even the collapse of a bridge. A reasonable seismic design should guarantee a structure’ s postearthquake ser- viceability. Residual displacements in piers have been considered secondary to the maximum ductility demand in the seismic design of structures (Kawashima 2000; Kawashima et al. 1998). Because of the elastoplastic characteristics of ordinary steel bars, the dam- age to an ordinary RC column is uncontrollable after the steel bar yields, and the damage results in a large residual deformation (Priestley et al. 1996; Christopoulos et al. 2003; Wu et al. 2009). In performance-based seismic design, a structure is designed to achieve different levels of performance when subjected to different levels of seismic demand. The realization of performance-based seismic design requires methods to quantify both the degree of damage and the repair effort. For example, the earthquake-resistant design codes in Japan [Japan Society of Civil Engineers (JSCE) 2000] suggest that the reparability of an important bridge should be examined (i.e., checked for a residual displacement < 1% of the column height) during the design stage. Previous studies conducted by Kawashima et al. (1998), Christopoulos et al. (2003), and Pettinga et al. (2007) showed that the postyield stiffness ratio (i.e., the ratio of the postyield stiff- ness to the initial elastic stiffness) of a bridge column is the main parameter of postearthquake residual deformation. When the postyield stiffness ratio of a bridge column is larger than 5%, the stability of the residual deformation response under different types of ground motion is enhanced without any significant change to the overall maximum response of the column (Pettinga et al. 2007). For concrete structures under the same lateral dis- placement, higher postyield stiffness indicates smaller residual displacements. Several approaches for reducing the residual deformations of bridge columns have been explored. They include the application of prestressed, unbonded tendons at column bases (Kwan and Billington 2003; Iemura et al. 2006), segmental precast, unbonded posttensioned concrete bridge columns (Ou et al. 2007), highly ductile materials at column bases (Billington and Yoon 2004), and hybrid reinforcement frames with glass fiber-reinforced poly- mers (GFRPs) and ordinary steel bars (Nehdi and Said 2005). The steel-FRP composite bar (SFCB), a novel rebar, was proposed to 1 Ph.D. Candidate, Southeast Univ., Key Laboratory of Concrete and Prestressed Concrete Structures of the Ministry of Education, Nanjing, China. 2 Professor, Southeast Univ., Key Laboratory of Concrete and Pre- stressed Concrete Structures of the Ministry of Education, Nanjing, China (corresponding author). E-mail: g.wu@seu.edu.cn 3 Professor, Southeast Univ., Key Laboratory of Concrete and Prestressed Concrete Structures of the Ministry of Education, Nanjing, China; and Dept. of Urban and Civil Engineering, Ibaraki Univ., Hitachi, Japan. 4 Engineer, Beijing Texida Technology Research and Development Co., Ltd., Beijing, China. Note. This manuscript was submitted on September 8, 2010; approved on December 15, 2010; published online on January 12, 2011. Discussion period open until March 1, 2012; separate discussions must be submitted for individual papers. This paper is part of the Journal of Composites for Construction, Vol. 15, No. 5, October 1, 2011. ©ASCE, ISSN 1090-0268/ 2011/5-696–706/$25.00. 696 / JOURNAL OF COMPOSITES FOR CONSTRUCTION © ASCE / SEPTEMBER/OCTOBER 2011