Seismic fragility analysis of continuous rigid-frame bridges considering site liquefaction and hydrodynamic effects

Previous studies have shown that site liquefaction and hydrodynamic forces significantly increase the seismic risk of bridges; however, research on bridge seismic fragility that couples both effects remains limited. This study developed a simulation method in OpenSees that accounts for both effects and validated it through centrifuge shake table tests. A seismic fragility analysis method was then established for continuous rigid-frame bridges under coupled conditions, and a fragility influence index that quantifies the severity of seismic fragility via the area under the fragility curve was proposed. Numerical simulations for a continuous rigid-frame bridge revealed that the effect of water depth depends on soil relative density, whereas soil density exerts an influence independent of water depth. Both site liquefaction and hydrodynamic forces can aggravate bridge damage, with liquefaction having a more pronounced effect than hydrodynamic forces, particularly on piles, where damage amplification is most evident. Compared with the dense sand site, the maximum overall influence index, obtained by weighting the fragility influence index for minor damage, moderate damage, extensive damage, and complete failure, reaches 0.829 for the loose sand site. For the 40 m water depth condition, the maximum overall influence index relative to the no-water case is 0.357.