Post-yield stiffnesses and residual deformations of RC bridge columns reinforced with ordinary rebars and steel fiber composite bars

The recovery of important bridges after a strong earthquake has been the main objective in the recent provisions of seismic design codes. The required recoverability is the returning of these bridges to their original functions following a major earthquake. In the present study, column residual deformation and post-yield stiffness are applied as seismic performance indices to investigate the required recoverability of reinforced concrete (RC) bridge columns. Using these indices, the aim of this study was two-fold. First, the recovery viability of 13 RC bridge columns reinforced with ordinary steel bars (OSBs), available from the current literature, was scrutinized. Although the examined columns successfully achieved their theoretical strengths and were able to maintain their capacities up until high drift ratios were reached, the recoverability limit corresponded to the column lateral drift ratio, which fluctuated more or less around two. The effects of the potentially influential design parameters on the required recoverability were also studied; the study showed that the axial load ratio is the only factor resulting in a clear effect on the residual deformations, where columns tested under a compression axial load of 24% of the column axial strength could be recoverable almost until 3% lateral drift. Second, to enhance and control the post-earthquake recoverability of RC bridges, steel fiber composite bars (SFCBs) are proposed here as innovative longitudinal reinforcements for RC bridge columns. Different reinforcement ratios of basalt fibers and carbon fibers were used in the production process of the new rebars. Analytical and experimental studies on concrete bridge columns reinforced with SFCBs were conducted. The results of this study demonstrated that the hybridized fibers with the longitudinal reinforcement of RC columns work in parallel with column materials. Specifically, replacing OSBs with SFCBs did not cause any significant changes on the yield stiffness of conventionally reinforced columns, and when the deformation became large, positive and stable post-yield stiffnesses were evident. The study also indicated the beneficial effect of conventional reinforcement replacement with steel basalt fibers composite bars (SBFCBs) as an alternative to merely increasing the amount of reinforcement. In this case, the required strength can be fulfilled with the existence of post-yield stiffness, and the final residual displacements can be substantially reduced.