Fragility of irregular RC buildings under mainshock-aftershock sequences

Abstract Reconnaissance studies following the 2011 Tohoku and 2023 Kahramanmaraş sequences documented that buildings with moderate mainshock damage collapsed under weaker aftershocks, yet single-event assessment frameworks do not account for this vulnerability in irregular reinforced concrete (RC) buildings. This study investigates the seismic fragility of plan-irregular and vertically irregular RC frame buildings under mainshock-aftershock sequences through incremental dynamic analysis. Three four-story RC structures classified as irregular per EN 1998-1 are analyzed in both original and regularized configurations, the latter obtained by modifying the column layout while preserving lateral stiffness to separate irregularity and sequence effects. The analytical campaign comprises 5,040 nonlinear time history analyses using 28 records from the 2011 M w 9.0 Tohoku sequence with fiber-based plasticity models incorporating degrading constitutive laws. Maximum likelihood lognormal fragility functions are derived at three FEMA 356 performance levels for four scenarios of irregular and regular configurations under mainshock-only and full-sequence excitation. Aftershock loading reduces median life-safety fragility capacity by 16–19% for the irregular configurations and by 8–10% for regular counterparts. The interaction term I, defined as the excess median capacity reduction beyond the sum of individual irregularity and sequence contributions, takes values of − 0.08 g to − 0.10 g at the life-safety level and is statistically distinguishable from zero for all three structures. Story-level Park-Ang damage indices reveal severe cumulative damage at stories where peak interstorey drift remains below the life-safety threshold, undetected by peak-drift metrics. The reported fragility parameters are specific to the far-field Tohoku sequence and require validation against other tectonic environments.