Experimental Qualification of a Long Buckling-Restrained Brace

Long buckling-restrained braces (BRBs) pose unique challenges, as they generate greater cumulative friction forces, pose increased risk of core binding and may be subjected to significantly larger compressive overstrength forces than shorter BRBs with equal capacity. Previous tests of BRBs with slender core plates have produced irregular hysteresis loops with compressive adjustment factors that increase rapidly over the final cycles, making it difficult to reliably assess the compressive overstrength. Recent analytical and numerical studies have shown this behavior to be consistent with core binding resulting from friction-induced compressive strain amplification. While design methods have been proposed to size the debonding gaps and prevent core binding, these have not yet been experimentally validated in large scale testing. This paper presents the quasi-static qualification testing of a long BRB with a slender core plate (25 mm thick, 9.4 m yield length). The strong-axis debonding gaps were specially designed to accommodate the predicted compressive strain amplification and associated Poisson expansion at the core ends. This successfully prevented core binding and resulted in excellent performance, with a smooth hysteresis and compression adjustment factor of less than 1.1 at 2% strain. The validated analytical method was then used to compare the required debonding gaps for short vs long BRBs, illustrating how the core length affects the design and performance of BRBs.