Study on the restoring force model of four-limb concrete-filled steel tube laced columns

Four built-up laced columns, each consisting of four concrete-filled steel tubes (CFSTs), were tested under cyclic loading to better understand the seismic performance of such columns in a mega composite structure system. The horizontal load–displacement hysteretic curve and failure characteristics of the four-limb CFST-laced columns were obtained and studied based on the test results. The study results show that the bearing capacity and stiffness in the elastic stage of the columns decrease with the increasing slenderness ratio, while the yield displacement and ultimate displacement increase. At the same time, with the increasing axial compression ratio, the stiffness degradation rate of specimens in the failure stage increases. Based on the hysteretic curves and failure characteristics, a new trilinear hysteretic restoring force model considering rigidity degradation is proposed for the CFST-laced columns in a mega composite structure system, including a trilinear skeleton model, the law of stiffness degradation, and hysteresis rules. The trilinear hysteretic restoring force model is compared with the experimental results. The experimental data shows that the proposed hysteretic restoring force model tallies the test curves well and can be used in the elastic–plastic seismic analysis of CFST-laced columns in a mega composite structure system.