Probabilistic evaluation of seismic performance of a soft-first story building retrofitted with BRBs and shear links

This study presents a probabilistic assessment of the seismic performance of a mid-rise soft-first-story building retrofitted using a hybrid strengthening strategy, addressing the limited integration of hybrid energy dissipation systems within a reliability-based framework for vertically irregular structures. The structure consists of a confined masonry system with a reinforced concrete irregular first story, where buckling-restrained braces (BRBs) are implemented at the ground level and shear-link energy dissipation devices are incorporated at the upper stories. The retrofit aims to mitigate deformation concentration at the soft story while promoting a more uniform distribution of seismic demands along the building height. The proposed methodology is based on bidirectional nonlinear time-history analyses conducted in ETABS v21, using ground motion records associated with return periods of 72, 475, and 975 years. Inter-story drift ratios are treated as random variables to account for record-to-record variability, and probabilistic models are employed to estimate exceedance probabilities and reliability-based performance metrics. The original structure exhibited low reliability levels, with β values ranging from 1.30 to 2.05 across performance levels, indicating a high probability of unacceptable performance and a pronounced soft-story mechanism. After retrofitting, the structural reliability increased significantly, reaching mean values of 7.76 and 7.65 at the immediate occupancy level, 7.43 and 7.73 at life safety, and 7.42 and 7.32 at collapse prevention for the EW and NS directions, respectively. These values greatly exceed the commonly accepted target of β = 3.5 and are associated with probabilities of unacceptable performance on the order of 10 -14 to 10 -15 . Overall, the hybrid retrofit strategy effectively eliminates the soft-story mechanism, ensures a stable redistribution of seismic demands, and significantly enhances the reliability and seismic resilience of buildings with vertical irregularities.