Optimization of Sa(T1)-Based Combined Ground Motion Intensity Measure Using Simulated Annealing Algorithm in Seismic Fragility Analysis of RCS Frame Structures

This study presents a seismic fragility analysis of reinforced concrete column–steel beam (RCS) frame structures using an enhanced version of the Park–Ang damage model. The applicability of various seismic intensity measures (IMs) in fragility assessment was evaluated. Furthermore, a two-parameter IM was refined through simulated annealing optimization. Initially, the damage evolution of the structure under both near-field and far-field ground motions was investigated using a modified Park–Ang model tailored for RCS systems from the literature. Subsequently, seismic fragility was assessed through multiple stripe analysis, developing fragility curves for distinct damage limit states under the two ground motion types. The effectiveness of 22 different IMs was then examined across these limit states. A two-parameter IM that accounts for the softening period was identified as particularly effective in capturing ground motion uncertainty. This measure was further optimized by applying simulated annealing to minimize the record-to-record variability (βRTR), targeting its period coefficient (n) and weighting factor (α). Finally, the enhanced IM’s sufficiency and scaling robustness were validated. Results indicate that near-field ground motions induce considerably more severe damage in RCS frames compared to far-field motions, with damage concentrating in lower stories. The optimized IM achieved reductions in βRTR ranging from 8.7% to 38.1% across different damage states.