Mainshock–Aftershock Fragility Assessments of Corroded Reinforced Concrete Structures Considering FRP Retrofitting Effect

Corrosion in reinforced concrete (RC) structures increases seismic fragility by reducing strength, ductility, and bond integrity, which becomes critical in aging infrastructure. This study provides a systematic fragility comparison of intact, corroded, and FRP-strengthened structures across multiple corrosion levels under sequential earthquakes. The seismic fragility of corroded RC frames, with and without fiber-reinforced polymer (FRP) retrofitting, is investigated under both mainshock and aftershock loading conditions. A total of 508 real recorded mainshock–aftershock ground motion sequences are selected as seismic inputs to ensure the representation of earthquake demands. Nonlinear time history analyses are carried out to establish fragility curves for four limit states based on probabilistic demand–capacity relationships. The results show that corrosion significantly decreases the collapse prevention capacity (LS4), with the maximum reduction reaching about 62%. FRP retrofitting restores seismic performance to varying degrees depending on corrosion severity. For the structure with a 10% corrosion rate, FRP retrofitting enhances the collapse capacity beyond that of the intact case. For the structure with a 20% corrosion rate, FRP retrofitting recovers approximately two-thirds of the lost capacity caused by reduced ductility. The consideration of aftershock effects further increases the fragility of corroded structures, yet FRP retrofitting continues to provide improvement by reducing cumulative damage and improving deformation capacity. The study confirms that the FRP confinement effectively enhances the seismic resilience of aging RC structures and provides a practical basis for performance-based retrofit strategies under sequential earthquake events.