Effects Of Restraining Ratio On Hysteretic Performance Of Concrete-filled Steel Tube Buckling-restrained Braces

As structural members which resist lateral forces, buckling-restrained braces(BRB) do not buckle when subjected to large compressive forces, achieving the same strength in both tension and compression. BRBs have excellent energy-absorption capacity comparing to normal steel braces, thus have been widely used in seismic engineering and structure restoration. Concrete-filled steel tube buckling-restrained brace is a traditional type of BRBs, which is easy-making, economical and performance-stable. This paper designs 12 short concrete-filled BRB specimens and 12 long ones to investigate the effects of constraining ratio on hysteretic performance of concrete-filled steel tube BRBs with plate and cruciform cores.Short specimens, which include one group of BRBs with cruciform cores and three groups of plate core BRBs of different steel tube thicknesses, were cyclically loaded under the MTS electro hydraulic servo loading system. Specimens’ ductility and loading capacity was analyzed, and their energy dissipation capacity was evaluated by area of hysteretic curve, energy dissipation ratio and equivalent viscous damping coefficient. Except a few specimens failed due to the cracking of the welding in end plates during the tests, all other BRBs finished the whole loading process without showing degradation of stiffness and strength, exhibiting good ductility and energy dissipation capacity.Specimens of medium and large lengths were tested using a self-made reaction frame and hydraulic jacks. Specimens of medium and large lengths both include BRBs with plate and cruciform cores. The tension and compression capacity of BRBs with the same cores increased with brace lengths, because the extension of contact areas between concrete and core braces caused more friction. Furthermore, BRBs with medium and large lengths had fuller hysteretic curves and indicated better energy dissipation capacity than short BRBs mainly due to the differences of loading devices and loading speeds. The strain measured by strain gauges showed that the stress level of steel tubes increased with restraining ratio decrease, the steel tubes of the longest BRBs with plate cores yielded in the middle part when the BRBs was compressed 1/70 length of the braces.FE models of BRBs were built in the Abaqus software. Hysteretic curves produced by FE models were similar to those of the experiments both in the tension and compression capacity, though BRBs with plate cores agree better than those with cruciform cores in hysteretic curve shapes. Stress legend showed that the largest strain occured in the ends of the braces where core section changes and in the middle of outer steel tubes. The results of the simulation showed that under large restraining ratios hysteresis curves varied little when different steel tube thicknesses were applied.