Simulation and Estimation of Tsunami Loads on Bridge Superstructures

Abstract

This study examines the estimated tsunami loads on five California coastal bridges. The quantities of interest include the horizontal and vertical forces and overturning moment. The simulations and analysis are conducted for two stages: (1) initial impact and overtopping and (2) full inundation. The first stage starts from the time when the tsunami water free-surface elevation reaches the low chord of the bridge superstructure, the water free surface rises and reaches the top of the bridge barrier where it overtops the bridge and starts to flow on the bridge deck, and until the bridge is totally inundated. The second stage occurs when the bridge first becomes fully inundated, i.e., end of the first stage, and until the most critical events, (1) the maximum tsunami water velocity, (2) the maximum tsunami momentum flux, and (3) the maximum tsunami mass flux, have occurred. The first part of the initial impact and overtopping stage leads to a combination of lateral (horizontal) and uplift (upward vertical) forces. The maximum uplift force during Stage 1 is found to occur when the tsunami water free-surface elevation reaches the top of the bridge barrier right before the water overtops the bridge and starts to flow onto the bridge deck. It is observed that the time interval representing the initial impact of the tsunami on the bridge superstructure leads to the maximum horizontal force, downward vertical force, and overturning moment. The overall maximum uplift force is found to be in tsunami scenarios in which the bridge superstructure is totally inundated, i.e., in Stage 2, if total inundation actually occurs. Analyzing a deck-girder bridge with a failed, i.e., removed, first seaward girder shows a 15% reduction in the maximum horizontal force. The uplift force is found to be approximately 25% larger for the bridge with a failed first girder. A design procedure is proposed to compute the maximum horizontal and vertical forces on bridge superstructures based on the simulation results. Good agreement between numerical predictions and formula estimations of the tsunami forces is observed. The proposed design procedure is intended to provide estimations of tsunami loads on bridge superstructures.