Damage assessment of older highway bridges subjected to three-dimensional ground motions: Characterization of shear–axial force interaction on seismic fragilities

• Influence of vertical earthquakes on seismic demand and bridge fragility.
• Validation of column shear–axial force interaction model accounting for axial force variation.
• Numerical model of older bridges typical to California region.
• Increase in the damage exceedance probability associated with shear–axial force interaction.

This study presents the influence of vertical ground motions on the seismic response and fragility of typical older highway bridges. The fragility is investigated using a set of functions that estimate exceedance probabilities of structures under seismic events. For this purpose, this study selects a typical older two-span single frame concrete box-girder bridge constructed in California in 1967. The bridge column has insufficient design details such as wide spacing of transverse reinforcement and low ratio of shear span to section depth resulting in an increased potential of shear–axial failure. The column shear response model accounts for the fluctuation in column axial force using an existing Zeus-NL material model and is validated through comparison of simulated and experimental results reported in the literature. Using this shear–axial force interaction model, a bridge system model is created in Zeus-NL to perform nonlinear time-history analyses. The response results are used to generate demand models, and component and system fragility curves of the bridge. Finally, fragility curves are compared for two bridge models with column shear model under constant axial force and with column shear model under axial force variation. The comparative results indicate that the shear model accounting for varying axial force increases the probability of exceeding the severe damage state by about 10% across the entire range of an intensity measure and reduces the median intensity measure by more than 15%. This observation highlights the importance of shear–axial force interaction in the seismic performance of older bridges.