Seismic tests of thin-profile buckling-restrained braces

A series of thin-profile buckling-restrained brace (Thin-BRB) component tests using the rectangular steel casings were conducted in the Taiwan National Center for Research on Earthquake Engineering. A flat steel core plate parallels to the gusset plate is adopted as the energy dissipation segment. This configuration minimizes the width of the steel tube and increase usable space in the buildings. However, the local bulging failure must be prevented. The steel core plate high mode buckling wave crests contact with the restraining member and create outward force on the steel tube surface when the BRB is in compression. The local bulging failure could occur easily if the restraining member is too weak to sustain the outward force. The BRB loses its compression capacity after the local bulging failure occurs. The purposes of this study are to investigate local bulging failure behavior and provide the recommendations for seismic design of Thin-BRBs. Fourteen Thin-BRB specimens with yield force capacities range from 337 kN to 3184 kN were tested using axial cyclic increasing loadings. All the specimens' steel tube widths are smaller than 200 mm. Seven specimens bulged out when the compressive core strains exceeded 0.03. Test results suggest that the outward forces can be estimated based on the high mode buckling wave length and the debonding layer thickness. According to the finite element analyses, the outward force spreads through the mortar and acts on the inner surface of steel tube. A rectangular contact surface can be approximated according to the steel core plate width and the mortar thickness. In order to avoid the local bulging failure, the steel tube wall must be thick enough to resist the outward force. Based on the finite element analyses and the experimental observations, it is found that the capacity of the steel tube can be estimated by using the upper bound theory in plastic analysis. Test results indicate that the proposed method in computing the outward force and the steel casing tube capacity can be used to predict the possibility of local bulging failure. This paper concludes with the recommendations for seismic design of Thin-BRBs.