Development of a New Analytical Approach for Earthquake-Induced Hydrodynamic Forces of Elevated Pile-Cap Foundations Submerged in Water

Elevated pile-cap foundations are widely used in long-span sea-crossing bridges for their adaptability to varying water depths and complex seabed terrains. Accurate assessment of earthquake-induced hydrodynamic forces on these foundations is vital for evaluating the seismic resilience of bridges. Given this fact, theoretical and ${\gamma }_s$-based methods for calculating the hydrodynamic forces on elevated pile-cap foundations submerged in water, considering pile-cap-group interaction, are first derived based on radiation wave theory and the cross-sectional area ratio ${\gamma }_s$ between the pile group and pile cap, and verified by comparing the results obtained from the existing computational fluid dynamics (CFD) investigation. Subsequently, a simplified analytical model of an elevated pile-cap foundation considering the hydrodynamic effect is developed for accurately predicting the seismic responses. Finally, the simplified analytical model is programmed in OpenSees for further validation against the underwater shaking table test of a pile-supported bridge tower conducted by previous studies. The results demonstrate that these methods accurately predict the hydrodynamic forces on elevated pile-cap foundations, with maximum errors of less than 3.6% and 5.3% for the theoretical and ${\gamma }_s$-based methods, respectively, compared to CFD results. It is also found that neglecting or overestimating the pile-cap-group interaction can lead to significant errors, making its proper consideration essential during seismic analyses. In addition, the simplified analytical model can accurately capture the seismic responses of the bridge tower, providing a highly effective and efficient method for the seismic performance assessment of sea-crossing bridges and other offshore structures.