Simplified Drift Demand Prediction of Bridges under Liquefaction-Induced Lateral Spreading

This paper develops a simplified method to quantify the effects of liquefaction-induced lateral spreading on bridge responses, expressed as the response modification factor of the column drift ratio under seismic shaking. Using the newly developed global dynamic p-y analysis procedure, nonlinear time history responses were obtained for a benchmark bridge–foundation–soil system when it was subjected to a suite of input motions under seismic shaking (nonliquefaction) and lateral spreading (liquefaction) cases. Under seismic shaking, the column drift correlated well with the peak acceleration of the nonliquefied input motion at the ground surface in addition to the dynamic characteristics of the bridge. Under lateral spreading, and due to its largely static loading nature, the column drift was related linear logarithmically to the crust layer energy content imposed on the pile foundation at bridge piers, which was a function of the cumulative absolute velocity of nonliquefied ground motion at the surface, and lateral resistances and geometric parameters of soil layers. By normalizing the column drift under the lateral spreading to that under the seismic shaking, a closed-form expression was derived for the response modification factor. Additional multipliers were identified in the proposed formula to account for different bridge designs, foundations, and soil conditions. The proposed method was validated against the simulation results for eight randomly selected bridge cases. It was demonstrated that the method can effectively estimate the column drift due to lateral spreading, which takes into account of dynamic characteristics of bridges, soil and foundation parameters, as well as ground motion variations.