Seismic response of concentrically braced steel frames made with rectangular hollow bracing members

This paper describes an experimental study on the seismic performance of concentrically braced steel frames made with cold-formed rectangular tubular bracing members. A total of 24 quasistatic cyclic tests were performed on full size X bracing and single diagonal bracing systems. Two loading sequences were considered: a symmetrical stepwise increasing deformation sequence and a displacement history obtained from nonlinear dynamic analyses of typical braced steel frames. All specimens buckled out-of the plane of the frame and the tests were interrupted when fracture of the braces occurred in the region of highest curvature. For X bracing, the results clearly show that the effective length of the braces can be used to determine their compression strength and to characterize their hysteretic response, including energy dissipation capability. Simplified models are proposed to predict the out-of-plane deformation of the braces as a function of the ductility level. These models are then used to develop an empirical expression to assess the inelastic deformation capacity before fracture of bracing members made of rectangular hollow sections.