Effect of axial load and shear span on seismic performance of CFT columns reinforced with end-fixed ultra-high strength rebars

A hybrid frame with resilient members and energy dissipation devices is proposed as a solution to address the issues of the safety and post-earthquake repairability of concrete structures in earthquake-prone zones. This paper experimentally investigates the seismic performance of concrete-filled square steel tube (CFT) columns reinforced with low-bond ultrahigh-strength steel (LBUHSS) bars, which are proposed for the hybrid frames. 8 large-scale specimens with different types of LBUHSS bars, axial load ratios, and shear span ratios were tested. The results show that the use of LBUHSS bars significantly improved the seismic behavior of the CFT columns, including improving the deformation capacity and bearing capacity of the columns and controlling their post-earthquake residual displacements, especially at the large deformation stages of the columns. The cumulative damage and plastic deformation of the specimens decreased with the introduction of the steel bars. The shear-span ratio and axial loads both have a significant influence on the seismic behavior of columns. A simplified finite element analysis model was proposed and applied for a parametrical analysis. Based on the study, two simplified calculation models were proposed to predicate the peak and ultimate deformation of the reinforced CFT columns.