In near-fault regions, intense vertical seismic excitation combined with the widespread lack of tensile capacity in bridge bearings can readily induce vertical separation between piers and girders. To systematically investigate the effect of such separation-and-impact phenomena on bridge seismic performance, this study develops a finite-element model in OpenSees and conducts a fragility analysis. Bearing constraint degradation is simulated by adjusting the bearings’ horizontal mechanical parameters between connected and unconnected states. Results indicate that when the excitation period approaches the bridge’s vertical natural period, pier–girder vertical separation is readily triggered, which in turn amplifies the structure’s horizontal dynamic response and leads to substantial increases in base moment and shear in the piers. This process not only alters horizontal relative displacements between piers and girders—exacerbating the risk of bearing damage—but also affects the shear and moment capacity demands on the piers. Fragility analysis shows that vertical separation raises the probabilities of reaching severe damage and complete collapse by 22.6% and 27.5%, respectively. Parametric studies further reveal that increased bearing damping exacerbates the adverse effects of vertical separation, with response amplification under varying PGA levels rising from 19.2% to 41.3%; conversely, increased bearing stiffness reduces the critical PGA required to trigger separation, thereby increasing the overall risk of structural failure. This study clarifies the mechanism of vertical separation and collision and provides a theoretical basis for seismic design and bearing selection for beam bridges in near-fault areas.
Yang et al. (Thu,) studied this question.