Unseating Failure of Bridge Spans under Seismic Excitations Considering Inelastic Pier and Foundation Motions

The unseating failure of bridge spans is examined by using the simplified 3 degree-of-freedom system, which retains the characteristics of the bridge motions including the inelastic pier and foundation motions. The inelastic pier is modeled by adopting the hysteresis loop which is obtained from the force-displacement curve by the moment-curvature relation based on the pier sections. The foundation is assumed to move in two directions, which are translational and rotational. Various peak ground accelerations are applied to the bridge system to see the effects of the nonlinearity and foundation motions on the global response behaviors. The unseating failure is investigated in the stochastic perspective by examining the probabilistic properties of the maximum displacement from the ensemble consisting of 400 time histories of the bridge system responses. As the peak ground acceleration increases, the effect of foundation motions is found to increase. It is suggested that under strong earthquakes, foundation motions, particularly the rotational motion should be considered in determining the bridge responses. The unseating failure increases dramatically as the pier height increases, indicating that care should be taken in the decision of the support length against unseating events.