Seismic design and testing of buckling-restrained braces with a thin profile

A thin-profile buckling-restrained brace (thin-BRB) consists of a rectangular steel casing and a flat steel core that is parallel to a gusset plate. A thin configuration reduces the width of the restraining member and thus saves usable space in buildings. However, deformable debonding layers, which cover the steel core plate in order to mitigate the difference between the peak tensile and compressive axial forces, provide a space for the steel core to form high mode buckling waves when the thin-BRB is under compression. The wave crests squeeze the debonding layers and produce outward forces on the inner surface of the restraining member. If the restraining member is too weak in sustaining the outward forces, local bulging failure occurs and the thin-BRB loses its compression capacity immediately. In order to investigate local bulging behavior, a total of 22 thin-BRB specimens with a ratio of steel core plate to restraining steel tube depth ranging from 0.3 to 0.7 and axial yield force capacities ranging from 421kN to 3036kN were tested by applying either cyclically increasing, decreasing, or constant axial strains. The restraining steel tube widths of all the specimens were smaller than 200mm and were infilled with mortar with a compressive strength of 97MPa or 55MPa. Thirteen of the 22 thin-BRB specimens' restraining members bulged out when the compressive core strains exceeded 0.03. A seismic design method of the thin-BRB in preventing local bulging failure is proposed in this study. Test and finite element model (FEM) analysis results suggest that the outward forces can be estimated according to the BRB compressive strength, steel core high mode buckling wavelength, and the debonding layer thickness. In addition, the capacity of the restraining member in resisting the outward forces can be estimated by using the upper bound theory in plastic analysis. Both the FEM analysis and test results indicate that the proposed method is effective in predicting the possibility of local bulging failure. Test results indicate that the proposed design method is conservative for thin-BRB specimens with a large steel core plate to restraining steel tube depth ratio. This paper concludes with design recommendations for thin-BRBs for severe seismic services.