Estimation of Cascadia Local Tsunami Loads on Pacific Northwest Bridge Superstructures

Abstract

The time histories of Cascadia Subduction Zone (CSZ) local tsunami horizontal and vertical loads and the overturning moment on four selected Oregon coastal bridges are computed for different load scenarios. These scenarios, containing the tsunami-free surface elevation and flow horizontal velocity components, are developed using various CSZ rupture models. One of the bridges is selected to evaluate the effect of various bridge geometry characteristics on the resultant tsunami loads. Maximum tsunami horizontal and downward vertical loads are found to occur approximately simultaneously when the tsunami flow reaches the landward side of the bridge cross section and overtops the barrier. A comparison between tsunami loads on a deck-girder bridge and a box-girder bridge under identical tsunami flow condition reveals that on box-girder bridges, (1) maximum horizontal loads are slightly larger, (2) downward vertical loads are smaller, and (3) uplift loads are significantly larger. It is shown that bridge cross sections with a seaward slope (the landward side of the bridge cross section is higher than the seaward side, so the tsunami travels landward) can be beneficial in reducing the uplift force. An analysis of a deck-girder bridge with a closed railing system shows an increase in the maximum tsunami horizontal, downward vertical, and uplift loads in comparison with an identical bridge that has an open railing system. Modifications are made to a previously proposed general method to estimate the specific local tsunami loads on Pacific Northwest coastal bridges from the results of computed tsunami loads. The modified tsunami load-estimation method is shown to be reasonably accurate in estimating the tsunami loads on bridge superstructures.