Analytical Framework for System‐Level Fragility Surfaces of Reinforced Concrete Bridges Under Mainshock‐Aftershock Sequences

Conventional single intensity measure (IM) fragility approaches with simplified component dependency assumptions overlook the progressive damage accumulation and evolving inter‐component correlations that govern system behavior under mainshock‐aftershock (MSAS) loading. This paper presents an analytical framework for developing dual‐IM fragility surfaces for reinforced concrete (RC) bridge systems, explicitly accounting for progressive deterioration and vulnerability under sequential ground motions. The framework employs a bivariate probabilistic seismic demand surface relating cumulative hysteretic energy‐based engineering demand parameters (EDPs) to both mainshock and aftershock intensities and extends the joint probabilistic seismic demand model (JPSDM) to incorporate inter‐component correlation under sequential excitation. The framework is demonstrated on a representative multi‐span RC I‐girder bridge, which shows that MSAS sequences substantially amplify system‐level seismic vulnerability, with conditional median intensity values decreasing by 16%–29% across damage states relative to the mainshock‐only case. The inter‐component correlation structure evolves between the two excitation phases, reflecting the progressive loss of the intact‐state inter‐component dependence structure due to mainshock‐induced damage, underscoring the importance of accounting for evolving interdependencies in sequential fragility assessment. The proposed framework provides a scalable and statistically consistent basis for performance‐based seismic evaluation of bridges under sequential earthquakes, emphasizing the importance of incorporating aftershock effects into fragility and risk assessments for resilience‐oriented design.