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

Abstract

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.

Keywords: Multi-support excitations (MSE), Seismic fragility analysis, Anti-symmetric mode effects, Bouc-Wen nonlinear model, Probabilistic fragility framework