Seismic Performance Levels of Midrise Steel Moment Frames Considering Residual Lateral Displacement and the Effects of Near‐ And Far‐Field Aftershocks

Steel moment frames, despite their high ductility, can experience significant residual drifts and cumulative damage under mainshock (MS)–aftershock (AS) sequences, potentially compromising postearthquake functionality and reparability. This study evaluates the seismic performance of midrise steel moment frames under AS effects by simultaneously analyzing the structure using the maximum relative lateral displacement (MD) and maximum residual relative lateral displacement (RD) indices. The results demonstrate that the RD index is particularly effective in providing a more precise prediction of failure probability across various performance levels. The fragility of the studied frame was assessed under both simultaneous engineering demand parameters (EDPs) and a single index, with the probabilities of reaching different performance levels calculated for both scenarios. Relative probability coefficients were determined for various damage levels under the MS and AS effects, across different peak ground acceleration (PGA) intensities in near‐ and far‐field ASs. These coefficients indicate that the largest discrepancies between single‐index and simultaneous‐index results occur in low‐intensity earthquakes. As PGA increases under AS conditions, this discrepancy diminishes and approaches zero. Additionally, as the structure reaches higher performance levels due to ASs, the relative coefficient increases, whereas it approaches unity at lower performance levels. Notably, for PGA values below 0.5 g in near‐field aftershocks and below 0.7 g in far‐field ASs, the MD index alone can effectively assess structural resilience. Furthermore, at identical PGA levels, the RD index has a greater influence on achieving the target performance level in far‐field aftershocks compared with near‐field ASs.