Failure Modes of Shear Keys in Girder Bridges Subjected to Girder–Bearing Separation and Collision Under Bidirectional Seismic Excitations

This study investigates the influence of vertical separation between the girder and bearings under bidirectional seismic excitations on the failure modes of shear keys. A continuum dynamic model for a two-span girder bridge was established and solved using the transient wave eigenfunction expansion method. The dynamic response of structural vertical separation and horizontal collision under bidirectional seismic excitations was systematically analyzed. The results indicate that the vertical seismic component induces separation at the girder–bearing interface, which significantly alters both the magnitude and contact location of the horizontal collision force on the shear keys. This phenomenon is most pronounced when the predominant period of the ground motion approaches the structure’s vertical natural period, drastically amplifying the girder’s horizontal seismic response. Consequently, the failure mode of shear keys exhibits an evolutionary sequence: with increasing seismic amplitude, it transitions from flat shear to inclined shear, and ultimately to flexural failure. Crucially, this dynamic coupling effect simultaneously amplifies the horizontal collision force and reduces the shear keys’ load-bearing capacity. Under the most unfavorable collision conditions, the limit of the horizontal collision force can reach up to 5.12 times greater than its design bearing capacity. This study reveals that neglecting vertical seismic excitation severely underestimates the actual failure risk of shear keys, underscoring the critical need to consider this coupling effect in the seismic design and performance evaluation of bridges in high-intensity zones.