Design and Behaviour of Buckling Restrained Braces

Abstract eng:
Buckling restrained braces (BRBs) have become a common and economic form of concentric bracing over the past decade. Used as the primary lateral load resisting system in high seismic areas, BRBs exhibit near balanced hysteresis loops, allowing for effective energy dissipation. Two forms of BRBs are commonly used, (i) a composite BRB composing of a steel core, surrounded by a concrete restraining medium and steel hollow section casing, and (ii) an all-steel BRB of a steel core surrounded by a steel restraining section(s). By restraining the steel core laterally along its length, the steel core can develop equal compression and tension capacity, and also suppress local buckling throughout the full brace length. Although simple in theory, BRBs are composed of five elements, three of which are defined segments along the core length; a yielding core, non-yielding restrained core, non-yielding unrestrained core, unbonding layer and restraining mechanism. The design and influence of these five elements is crucial in design of BRBs, a small imperfection or geometric change can significantly influence how the BRB behaves, in some cases in an explosive way. The purpose of this research is to understand the influence and contribution each element has in BRB design and also the sensitivity of variations in the detailing of these parameters to overall response. This paper outlines the outcome of part one of a three part program addressing the design, sensitivity and behaviour of composite and all-steel BRBs as members and within a structural frame. Part one considers the effects the transition region and outer casing stiffness have on the overall brace performance and design. It was found that for smaller transition gradients, core local failure within the outer casing dominated, the compression strength was maintained at larger displacements and the maximum compression strength occurred after few loading cycles. For specimens with thinner outer casings (and subsequently lower stiffness) local buckling was sustained, rather than in-plane rotation at the connection. The compression strength was also maintained over larger displacements and a number of unstable cycles could be sustained before failure. Specimens with casings of a thickness equal to or greater than the steel core thickness fractured prematurely.

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