Seismic Uplift Effect at End Spans of Long-Span Rigid-Frame Bridges Subjected to Near-Fault and Far-Fault Ground Motions

Miaoziping Bridge, a long-span rigid-frame bridge, uplift appeared at the end spans in the 2008 Wenchuan earthquake. Bearings and pad stones were damaged during the earthquake. In the past, the uplift mechanism and the seismic response under near-fault and far-fault ground motions were not sufficiently recognized. Here, a refined finite-element model of the Miaoziping Bridge was established considering the initial stress state and the nonlinear characteristics of the girder (e.g., cracking of concrete). The Wenchuan strong motion records and other 40 near-fault pulse-type and 40 far-fault ground motions were selected for nonlinear time history analysis. The uplift effect was realized by considering the compressive-only behavior of bearings. The results show that the seismic responses of the bridge under Wenchuan earthquake ground motions are in good agreement with the real seismic damage observed after the Wenchuan earthquake. The excitation of the near-fault pulse-type ground motion along the longitudinal direction is the dominant factor of the uplift at the end spans. When peak ground acceleration of the near-fault pulse-type ground motion is greater than 0.3g, the end spans are prone to uplift, and the maximum uplift displacement responses can reach 50 cm. There is little uplift possibility subjected to the far-fault ground motions. When the pulse period is in the range of 1–3 s, the pulse effect equals or even exceeds the full-wave effect. The end spans can hardly uplift subjected to vertical or transverse excitation individually. There is a slight difference between the uplift displacements subjected to single longitudinal ground motions and three-directional ground motions. The time of maximum uplift displacement is delayed considering the wave-passage effect. When the wave velocity is less than 1,000 m/s, the adverse effects of the wave-passage effect may be considered.