Seismic fragility assessment of scoured bridges under coupled horizontal-vertical ground motions

River-crossing bridges in seismic regions are increasingly exposed to flood-induced scouring and earthquake shaking during their service life. Although vertical ground motions are known to affect the response of bridge structures, the specific interactions with the boundary condition changes introduced by flood-induced scour have not been studied sufficiently. In this study, a probabilistic fragility assessment framework for scoured bridges is developed by comparing the cases of coupled horizontal and vertical excitations with those of horizontal-only excitations. A three-dimensional nonlinear finite element model with soil–structure interaction is established in OpenSees, and probabilistic seismic demand models are constructed using paired horizontal and vertical records across multiple scour depths. The results indicate that considering the vertical component generally increases the estimated seismic fragility, with effects that depend on the PGA level, scour depth, and damage state. Under slight and moderate damage conditions, coupled excitations produce only marginal alterations in fragility, suggesting that horizontal excitations primarily influence damage initiation. At higher damage levels, the effect of vertical excitations is more pronounced because of the acceleration of post-yield stiffness and strength degradation, thereby increasing the deformation demand for a given PGA. In the case of deep scouring, the mechanism is amplified, thereby making this effect even more significant. Record-to-record variability also governs the vertical contribution, indicating that constant vertical scaling rules may underestimate the tail behavior relevant to the reliability. Overall, omitting vertical excitation can lead to non-conservative estimates of the exceedance probability for scoured bridges, particularly after the initial yielding. The framework supports the explicit consideration of coupled horizontal and vertical excitations when assessing the vulnerability of scoured bridges in multi-hazard environments.