Influences of out-of-plane buckling of non-yielding segments of BRBs on braced composite frame structure

The quasi-static test of one large-scale two-story and one-bay buckling-restrained braced composite frame(BRBCF) including concrete-filled steel tubular columns and steel beams was conducted.The structure can undergo many fully-reversed cycles without degradation of stiffness and bearing capacity,and exhibits great ductility,stiffness and retention capacities.The test frame can sustain story drifts of nearly 2% with large BRB maximum ductility demands.According to strain data analysis of key points of the structure,the formation and development of the plastic deformation of the structural system were explored.Under horizontal loading,the BRBs are the first components that yielded and dissipated energy,and played the role of protecting the main structure.The failure mode of the structure follows the order of BRBs-beams-columns.Due to frame action on BRBs,besides axial force,BRBs in the frame are also influenced by moment,which lead to buckling of non-yielding segments,significant decreases of bearing capacity and stiffness of the BRBs,and BRBs fail at last.According to the damage characteristics of the frame,design suggestions for improving the stiffness of unrestrained non-yielding segment and enlarging the length of restrained nonyielding segment were put forward.