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

Abstract

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.