Probabilistic Fragility Assessment of Multi-Span Bridges: Anti-Symmetric Modes and Spatial Variability Using the Bouc-Wen Model

• Introduces a Kolmogorov-Smirnov-based safety factor, quantifying the impact of spatial variability in fragility assessments. • A probabilistic seismic fragility framework is developed for multi-span bridges under spatially variable ground motion. • The Bouc-Wen nonlinear structural model is utilized to enhance numerical stability • Provides engineering recommendations for optimizing seismic design of multi-span bridges based on geometrical effects and mode shape behavior. • Results show that anti-symmetric modes amplify vulnerabilities by up to 50% under Multi-Support Excitation ( MSE ). This study investigates the seismic fragility of multi-span bridges under spatially variable excitation, focusing on the role of anti-symmetric modes, geometric irregularities, and ground motion variability. A custom MATLAB code was developed to conduct over 36,000 analyses of bridge systems with varying configurations and parameters, enabling a comprehensive exploration of fragility trends. The Bouc-Wen nonlinear structural model, known for its superior numerical stability and computational efficiency, addresses the limitations of traditional finite-element methods. Conditional simulation is employed to model spatially variable ground motions, while Incremental Dynamic Analysis (IDA) facilitates the derivation of fragility curves. A novel contribution of this research is the determination of a safety factor required when spatial variability effects are ignored, offering critical insights into the risks of uniform excitation assumptions. Furthermore, the study emphasizes geometric configuration exploration through a geometry-based fragility evaluation framework, examining how variations in pier arrangement, stiffness, and symmetry affect seismic responses. Results reveal that symmetrical configurations experience amplified vulnerabilities by up to 50% under Multi-Support Excitation (MSE) due to anti-symmetric modes, underscoring the importance of probabilistic approaches to capture uncertainties. This research provides a robust framework for probabilistic seismic assessments, highlighting the significance of spatial variability and geometric dynamics in bridge design and retrofitting.