Nonlinear cyclic behavior of steel tube in concrete-filled steel tube members including local buckling

This paper presents an experimental and analytical study on the nonlinear cyclic behavior of steel tube in concrete-filled steel tube (CFST) members. An innovative CFST coupon was proposed and tested under axial loads to acquire the nonlinear cyclic response of the steel tube in CFST members including local buckling. The failure mode, hysteretic response, skeleton curves, strength and stiffness degradation of CFST coupon were studied comprehensively. The results showed that the failure modes of CFST coupon were local buckling and ductile fracture of the steel tube. The hysteretic curve exhibited negative stiffness induced by the local buckling of the steel tube. The tensile strength of the CFST coupon exhibited slight degradation throughout the whole loading process. The compressive strength decreased significantly when the local buckling emerged at the steel tube. The tensile unloading stiffness decreased slightly and the compressive unloading stiffness dropped greatly with the increasing loading level. The post-local buckling and tensile stiffness dropped linearly. The refined continuum finite element (CFE) model was proposed, which could capture the material and local geometric nonlinearities of steel tube. The nonlinear cyclic behavior of CFST coupon was comprehensively studied. Para-metric studies of the CFST coupon were conducted, considering different width-to-thickness ratios and cyclic plasticity properties of steel tube.