Development and tests of X-shaped buckling-restrained braces with rotatable middle connections

To avoid negative in-plane interaction and to further improve hysteretic behavior of X-shaped buckling-restrained braces (XBRBs), a new assembled configuration with rotatable middle connections was proposed and quasi-static tests were conducted to mainly explore the impacts of inplane gaps between restraining members and encased braces, overall and local restraint capacity of restraining members, etc. on seismic behavior of XBRBs. The diagonal BRBs (DBRBs), as well as two modified assembled XBRBs with fixed middle connections by welding, were also tested to make a direct comparison. Tests reveal that, with the same flat plates as restraining members, the out-of-plane overall flexural buckling of the DBRB with a smaller restraining ratio of 0.72 occurred earlier but the XBRB exhibited stable hysteretic curves without buckling due to middle bracing action. In-plane gaps from 1 mm to 3 mm and 5 mm have little impacts on both the interaction between the two braces and shapes of hysteretic curves for all XBRBs while inducing earlier fracture of plate braces and reducing cumulative inelastic deformation and energy dissipation capacities about 8.5 %-15.1 % and 7.4 %-9.2 %, respectively. Besides, the effects of negative interaction between two braces on the XBRBs with either rotatable or fixed middle connections, in which appropriate axial gaps were reserved between restraining members and two encased braces at lower ends, can be avoided, facilitating predictions on the hysteretic responses of XBRBs with steady tangent stiffness in applications. Both DBRBs and XBRBs with enough restraint capacity exhibited good ductility ratios from 8.9 to 12.4 and energy dissipation prior to eventual tensile failure of encased braces. In addition, out-of-plane local flexural capacity of the middle rotatable connections should be ensured, for example by adding necessary stiffeners, to further improve the hysteretic behavior and to avoid local bending failure.