Test of a full-scale two-story steel X-BRBF: Strong-axis instability of buckling restrained brace associated with out-of-plane bending of gusset connections

Abstract

This paper presents tests and finite element analyses of a two-story, buckling-restrained braced frame (BRBF) with X-bracing configuration, called X-BRBF. The BRB used in this work is composed of a core plate sandwiched by a pair of the restraining member with bolts, called a sandwiched buckling restrained brace (SBRB). The weak-axis of the SBRB core that had a rectangular plate was positioned transverse to the dual-gusset plate connection and in-plane movement of the frame. The objectives of the frame test were to evaluate: (1) the seismic performance of SBRBs with a small width-to-thickness ratio of the core plate (i.e. 3), (2) the effects of free-edge stiffeners on the gusset and BRB stability, and (3) the BRB stability associated with the lateral torsional buckling (LTB) of the beam. The two-story X-BRBF subassembly had two test phases: all gusset connections had free-edge stiffeners in Phase 1 test, and freeedge stiffeners were removed from half of gusset connections in Phase 2 test. The X-BRBF in Phase 1 test exhibited good performance up to a second-floor drift of 1.6%, but in Phase 2 test the second-floor SBRB caused by LTB of the beam showed asymmetric instability about its strong-axis of the core plate at 1.6% second-floor drift. The other SBRB with no LTB of the beam showed one-side instability about its strongaxis at 2% second-floor drift (about 2.5% interstory drift). However, the strength degradation of the X-BRBF subassembly was not observed in the strong-axis instability of SBRBs, indicating an acceptable orientation of BRBs for eliminating a typical weak-axis buckling of BRBs together with out-of-plane bending of gusset plates. The strong-axis instability of SBRBs could be evaluated by using the BRB stability concept together with the measured out-of-plane deformation of gusset plates in Phase 2 test.