Development of a Guideline for Estimating Tsunami Forces on Bridge Superstructures

摘要

The Pacific Northwest is vulnerable to seismic events in the Cascadia Subduction Zone (CSZ) that could generate a large tsunami that could devastate coastal infrastructure such as bridges. In this context, this paper describes the development of a guideline for estimating tsunami forces on bridge superstructures along the Oregon Coast. A multiphysics based numerical code is used to perform numerical modeling of tsunami impact on full-scaled bridge superstructures of four selected bridges Schooner Creek Bridge, Drift Creek Bridge, Millport Slough Bridge, and Siletz River Bridge located on Highway 101 in the Siletz Bay area on the Oregon Coast. Two different types of bridge superstructure, deck-girder and box sections, are developed in the case of the Schooner Creek Bridge to study the effect of geometry of bridge cross-section. The results show that tsunami forces on box section superstructures are significantly higher than the forces on deck-girder sections; therefore, the box section design might not be appropriate to be used in a tsunami run-up zone. Moreover, numerical simulation of a deck-girder bridge with rigid rails and with open rail spacing, subjected to identical tsunami loads, was performed to examine the effect of rails on tsunami forces. The results suggested that horizontal and vertical tsunami forces on bridges with rails are larger than those on bridges with open rail spacing, up to 20% and 15%, respectively. These numerical results are finally incorporated into the mathematical formulations from the existing literature to develop a simplified method for estimating tsunami forces on bridge superstructures. Appropriate empirical coefficients for bridge superstructures under tsunami loads were evaluated based on an average value of the scattering data from the numerical results. The developed guideline is intended to be used as a preliminary guidance for design only as it did not account for uncertainties; thus, an appropriate load factor must be included in the calculations. A previous analysis of tsunami forces on the Spencer Creek Bridge on the Oregon Coast is revisited to examine the applicability of the guideline developed in the present work.