Local buckling behavior of steel angle core members in buckling-restrained braces: Cyclic tests, theoretical analysis, and design recommendations

The local stability of buckling-restrained braces (BRBs) is one of the most concerned issues. However, few discussions were conducted on the BRBs using section steel or welded cruciform core members, and the currently proposed methods concentrated on the design of casing only. In this study, incremental cyclic tests were conducted on eight BRB specimens using steel angle core members. Key parameters included the core width-to-thickness (b/t) ratio and the gap between the core and the casing. The test results show that two types of local buckling modes of the core were observed, while no local failure of the casing induced by local buckling was found. Significant local buckling and cyclic deterioration behavior were observed for the specimens with the b/t ratio of around 8.5 when the axial core strain reached 1.4%, causing 13% reduction in energy dissipation. Local buckling waves tended to concentrate near the core ends with enlarged buckling amplitudes due to the friction and uneven gap. The ratio between buckling amplitude and buckling half-wave length is found to be a governing parameter for BRB seismic performance. This ratio is recommended to be smaller than 0.04 to ensure stable cyclic behavior. The analytical model for the local stability of steel angle core members is proposed and the inelastic local buckling critical load is obtained. The analytical results show good agreement with experiment. Three re-defined general requirements for BRB local stability design are proposed, and the design recommendations for the steel angle core members are provided. Several implications of this study to other BRB configurations are presented finally.