Seismic risk and failure modes assessment of steel BRB frames under earthquake sequences

Buckling-Restrained Braces (BRBs) are characterized by steady and nearly symmetric hysteretic loops, providing large energy dissipation capacity under strong earthquakes. These devices are designed to sustain a specified maximum ductility demand and, if not properly designed, may fail due to excessive inelastic deformations. Moreover, their low post-yielding stiffness may lead the structure to large residual inter-story drifts at the end of the earthquake motion, and the cumulative ductility demand due to repeated plastic excursions may lead to low-cycle fatigue failure of the device core. The risk of reaching either of these failure modes is exacerbated when considering multiple earthquakes. Although BRBs are designed to function as a fuse element, there is a lack of consensus on the criteria for replacement, particularly when large residual deformations are not observed. Recent studies have suggested that BRBs can withstand several loading cycles before developing low-cycle fatigue rupture; thus, the decision to replace a BRB after a single ground motion may be overly conservative. The present study investigates the likelihood of BRBs reaching these failure modes within a stochastic framework that considers the probability of occurrence of multiple earthquakes during the structure’s lifetime. For this purpose, two steel Moment Resisting Frames (MRFs) retrofitted with BRBs are numerically modeled in OpenSees and subjected to the cumulative demand from hazard-consistent multiple earthquake sequences. The demand values are compared with multiple capacity models for low-cycle fatigue in the BRB core, as well as conventional limits for residual drifts and other failure modes. The outcomes of this study suggest that the risk of developing low-cycle fatigue in BRBs is negligible, even when multiple ground motions are considered, while other failure modes are significantly more likely to occur, particularly when the structures are subjected to pulse-like ground motions.