Seismic Performance Evaluation of a U-Girder Highway Bridge: A Comparative Study Under Two Ground Motion Intensity Levels Using CSiBridge

Abstract

Highway bridges are vital links in transportation networks, and large earthquakes around the world have repeatedly exposed their seismic vulnerability. This paper assesses the seismic response of the Al-Yusufiyya U-girder highway bridge using a detailed finite element analysis realized in the in the CSiBridge environment. The ground motion intensity the paper studied contains two cases as well: a low-to-moderate seismic case (Case 1: Ss = 0.3 g, S1 = 0.1 g, PGA = 0.2 g), which is the ordinary seismic hazard condition, and a high-intensity seismic case (Case 2: Ss = 2.29 g, S1 = 0.869 g, PGA = 0.954 g),which is the near-fault sever earthquake scenario. This analytical framework brings together three synergistic methodologies, namely response spectrum analysis for D/C evaluation, eigenvalue modal analysis to characterize key dynamic properties (e.g. natural frequencies and mode shapes), and nonlinear static pushover analysis following ATC-40 provisions for capacity assessment and performance point definition. Results show a striking difference of structural performance between the two loading configurations. For all three bents considered in Seismic Case 1, the D/C ratios are well below unity, confirming sufficient reserve capacity. In contrast, Seismic Case 2 produces D/C ratios well in excess of 1.0 in both transverse and longitudinal directions at all bents, showing possible failure in the event of a strong seismic excitation. Push over analysis indicates that the hybrid bridge retains a capacity of nonlinear deformation under moderate loading, but that peak lateral forces and corresponding displacements increase significantly under the high-intensity scenario. Capacity-spectrum evaluation of the performance point yields confirmation that in the moderate case the structure behaves elastically, while in the severe case demand may be at or near inelastic limits. These findings highlight the important need for seismic hazard assessment and bridge multi-level performance studies at the site-specific level of seismically active regions.