Seismic Performance and Risk Assessment of RC Frames with Double-Stage Yielding Buckling-Restrained Braces

Abstract Enhancing the seismic resilience of existing reinforced concrete (RC) frame structures remains a critical challenge in earthquake engineering, particularly when actual seismic demands exceed the design fortification level. This study investigates the seismic risk-mitigation effectiveness of RC frames strengthened with a parallel double-stage yielding buckling-restrained brace (PDYBRB) system within a probabilistic seismic performance assessment framework. Two representative frame buildings are selected as case-study structures. Seismic risks associated with the design hazard level and an intensified hazard level are quantified through the integration of fragility functions and seismic hazard information. The results indicate that the installation of PDYBRBs significantly reduces the seismic fragility and risk of RC frame structures. Under the design hazard level, the collapse risk of the strengthened frames decreases from the order of 10 − 6 observed in the bare frames to approximately 10 − 8 , representing a reduction of about two orders of magnitude. When the seismic hazard level is increased by one fortification level, the collapse risks of the bare frames increase by approximately 1.5–2.1 times, whereas the strengthened frames maintain substantially lower risk levels. For example, the collapse risks of the PDYBRB-strengthened frames under the higher hazard level are about 3 × 10 − 8 and 1.5 × 10 − 7 for the five- and nine-story buildings, which remain lower than those of the corresponding bare frames designed for the original seismic hazard level. These findings demonstrate that the PDYBRB system can effectively enhance the collapse resistance of RC frames and provide an additional safety margin against seismic hazard escalation. Parametric analyses further reveal that increasing the PDYBRB design target substantially reduces severe damage and collapse risks, whereas the influence of the yield-strength ratio between the first and second stages is comparatively limited.