Seismic Performance Testing Of Composite Frame Structure With Buckling-restrained Braces

Based on the pseudo-static test of two-storey single-span concrete-filled steel tube (CFST) frame with buckling-restrained braces (BRBs), it was researched that the mechanical behavior and seismic performance of concrete-filled steel tube frame with BRBs under cyclic low-cycle load are excellent in the aspects such as damage character, loading capacity, hysteretic performance, ductility, energy dissipation, degradation of strength and stiffness; and the mechanical characteristics of BRBs and their contribution to the seismic performance of the structure are also investigated. Because the bending resistance of the BRBs with crisscross crosssection unrestrained segment is relatively low, a new kind of BRBs with I crosssection unrestrained segment is brought forward to increase the bending stiffness of the unrestrained segment BRBs. The hysteretic performance of the new BRBs is tested. According to the damage character of the specimens, design suggestions are put forward. All the researches above indicate that the mechanical behavior and seismic performance of CFST frame with BRBs are good, which is a new kind of double lateral-resistance structure system with high efficiency. BRBs with I crosssection unrestrained segment have reasonable conformation and good performance. The contents of this article are as follows:(1) Pseudo-static tests on two two-storey single-span CFST frames with BRBs and a CFST frame in the same size are done. It is found that the lateral stiffness, ultimate bearing capacity and dissipation capability of the CFST frame are significantly improved when installed with the BRBs. The structure is also of great ductility, stable later stiffness and sustainable later bearing capacity.(2) According to strain data analysis of key points of the structure, The formation and development of the plastic hinge of the structural system are explored. Under horizontal load, the BRBs are the first components that yield and dissipate energy, and play the role of protecting the main structure. The damage mode of the structure follows the order of BRBs—beams—columns, which belongs to the mechanism of beam-hinge failure. After the yielding of BRBs, the CFSTs continue to increase the bearing capacity of the structural system, which is the second effective seismic defence line.(3) The connections of BRBs with CFST frame are bolted joint and welded joint, both of which play the intended function perfectly, and the performance of both structure are of little difference. Besides axial force, BRBs in structures are also influnced by moment, which leads to significant decrease of bearing capacity and stiffness of the BRBs.(4) Four BRBs with I crosssection unrestrained segment are designed to study their hysteretic behavior. The transformation of the unrestrained segment's section can significantly improve the moment-resistance capacity of the unrestrained segment, avoiding buckling instability effectively due to sudden enhancement of stiffness. The effect of ratio of width to thickness of the core unit on the mechanical behavior and energy-dissipation capacity of BRBs is also studied. A suggestion for design improvement that using circular transition at the junction between rectangle section segment and I crosssection unrestrained segment is proposed.