Cyclic behavior of concrete-filled square tubular columns with stiffened plastic hinge region

Abstract

The seismic behavior of concrete-filled square steel tube (CFST) columns is greatly influenced by the local-buckling of steel tubes within the plastic hinge region. This paper focuses on a novel stiffener that strengthens the potential plastic hinge regions to enhance the local-buckling resistance of steel tube, hence improving the overall seismic performance of the CFST columns. To verify its effectiveness and investigate the hysteretic behavior of the stiffened CFST (referred to as SCFST) columns, three CFST and three counterpart SCFST columns were fabricated and tested under cyclic lateral load while subjected to constant axial load. Furthermore, flexural capacities calculated using representative design equations in current codes as well as a newly proposed method were compared with the experimental results. Calculation method for the effective strengthening length of SCFST was presented. The test and analytical results showed that: (1) this stiffeners could effectively delay the local buckling of steel tube and change its local buckling modes; (2) the stiffener significantly enhanced the lateral load capacity of SCFST columns with improvement of 14%, 15%, and 38% comparing with the counterpart CFST columns; (3) the thinner the steel tubes, the more significant the effect of the stiffener on the ultimate lateral capacities; (4) the design equations prescribed in current codes tended to underestimate the ultimate flexural strength of SCFST columns with a discrepancy between 22% and 60%; (5) the proposed calculation method taking account of the effect of the stiffener, could more accurately predict the flexural strength of SCFST columns than current design equations with an improved discrepancy ranging between 7% and 32%.