Kemin Jia, Chengshun Xu, Zhichao Lai, Junfeng Jia, Jia Song, Xiuli Du
ABSTRACT The coupled influence of horizontal and vertical ground motions (VGMs) commonly leads to complex dynamic soil‐structure interaction behavior. However, the mechanisms governing the VGM effects on the seismic demand of pile group‐supported bridges in liquefiable sloping sites remain insufficiently understood. This study therefore aims to elucidate the influence mechanisms of VGMs on the transverse dynamic response of pile group‐supported bridges in liquefaction‐prone inclined deposits, while evaluating the practical effectiveness of the conventional 2/3‐ratio rule for VGM input. First, an efficient finite‐element (FE) computational framework was established to effectively simulate the seismic response of the pile‐soil‐bridge systems in liquefiable inclined soils under combined horizontal (H) and vertical (V) ground motions. Subsequently, comparative analyses were conducted on the system's seismic responses under horizontal excitation alone, simultaneous H‐V excitation, and horizontal excitation with a 2/3‐ratio rule vertical components. Finally, the cross‐correlation technique was employed to examine the mechanistic influences of kinematic and inertial effects on critical seismic demands of pile group‐supported bridges. The research findings demonstrate that VGMs significantly exacerbate pore pressure oscillations, while substantially amplifying seismic demands and damage potential in pile‐soil‐bridge systems. This is particularly evident in the increased risk of shear failure at the pier bottom and pile head. The code provision specifying VGMs as 2/3 of horizontal components considerably overestimates their effects on pile group‐supported bridge behavior. Cross‐correlation analysis reveals that inertial effects predominantly control pier curvature and shear force. In contrast, kinematic interaction primarily governs pile curvature and shear force, with superstructure displacement mainly driven by ground displacement.