Robust seismic response evaluation considering an uncertainty emerging in asymmetric bridges

Abstract

The robust seismic design of structural systems relies on accurately assessing peak seismic responses amidst various uncertainties. While prior studies have focused on incident direction and motion-to-motion variability of ground motions, this study identifies and characterizes a previously unrecognized uncertainty termed the Uncertainty in Asymmetric Structures (UAS) that can significantly influence seismic performance assessment. The UAS effect manifests as a systematic difference in the most critical seismic response, over all incident directions, between an asymmetric structure and its horizontal-plane mirror image. Through analytical derivation under linear-elastic assumptions, the UAS effect is shown to originate from the coupled influence of structural asymmetry, quantified by the Cross-Structural Term (CST), and bidirectional ground motion characteristics, quantified by the Cross-Modal Response (CMR). To evaluate its engineering implications, nonlinear time-history analyses are performed on five finite element models of multi-span continuous curved girder bridges with varying degrees of asymmetry. Spectrum-compatible ground motions are employed to capture motion-to-motion variability, while synthesized bidirectional ground motions are used to explore the influence of directionality. Results confirm the theoretical predictions, revealing that the UAS effect can alter critical seismic responses by up to 20 %, with its prominence increasing under greater structural asymmetry or reduced ground motion directionality. These findings introduce a new dimension of seismic input–structure interaction that has direct implications for performance-based and risk-informed seismic design.