Experimental investigation of behavior of steel frames with y-shaped concentric bracing

Concentric bracings composed of three members arranged in y-shaped geometry have been traditionally used to provide openings in braced bays. However, using common single gusset plates in y-braced frames leads to single curvature flexure and out of plane buckling of braces accompanied by low hysteretic energy dissipation. In order to explore and improve the behavior of y-braced frames, a research program including experimental tests was conducted at BHRC1 structural engineering laboratory. Specimens presented in this paper include four full-scale frames with y-bracings of different geometries and cross sections. Quasi-static cyclic loading was increasingly applied until yielding and failure occurred in the specimens. The results show that out-of-plane buckling with single curvature in braces can be replaced by in-plane, double curvature buckling through appropriate detailing of cross sections and connections. These sections have larger radius of gyration for out of plane buckling of bracing members. Hysteretic energy dissipation and damping of y-bracing are increased due to inelastic flexural deformation of brace members. Energy dissipation capacity of y-braced frames with new details is comparable with the traditional X bracing. Based on these findings, two-bay y-braced frames were designed to carry the same lateral load as X-braced frames. The seismic performance of these frames was compared using nonlinear static procedures and found to be similar.

► Four full-scale y-braced frames with different geometries were tested. ► By appropriate detailing, in-plane buckling can be induced in y-bracings. ► Energy dissipation of y-braced frames is comparable to that of X bracing. ► The seismic performance of specimens is assessed by nonlinear static procedures. ► Seismic performance of y-braced frames is similar to that of X-braced frames.