Seismic fragility assessment of continuous RC rigid-frame bridges with irregular geometry: insights and recommendations

This study presents a detailed seismic fragility and collapse performance assessment of geometrically irregular continuous reinforced concrete rigid-frame (CRCR) bridges based on a representative bridge in Iran. A regular configuration was first established as the reference, after which a systematic set of bridge models was generated by varying the span-length ratio, pier-height ratio, and pier skew angle to investigate their individual and combined effects on seismic response. Detailed three-dimensional nonlinear finite element models were developed in OpenSees and analyzed under a comprehensive set of far-field and near-field earthquake ground motions. Seismic fragility curves were constructed based on pier drift demand corresponding to four damage states ranging from minor damage to collapse, defined by the attainment of the complete damage state. The results indicate that geometric irregularities significantly amplify seismic vulnerability, particularly at higher damage states. Across a range of span-length ratios, bridges with pronounced pier-height irregularity exhibit up to 50-60% higher probabilities of exceeding severe damage at the same intensity level compared to the regular configuration, while the median seismic intensity associated with collapse is reduced by approximately 55% in highly irregular cases. Among the investigated parameters, pier-height irregularity was identified as the most influential factor governing fragility and collapse behavior, followed by pier skewness, whereas span-length irregularity showed a comparatively smaller effect. The resulting fragility functions and collapse metrics provide quantitative insight into the seismic performance of CRCR bridges and support fragility-based assessment and performance-based seismic design in earthquake-prone regions.