Assessment of Washington State Bridges for Post-Earthquake Mobility and Recovery Planning

Abstract

A 2019 Department of Homeland Security report assessing the regional resiliency of Western Washington state to a Cascadia Subduction Zone earthquake predicted widespread and high levels of bridge damage. Because of this report, the state’s emergency response and recovery plan is heavily reliant on air transportation modes to provide resources to the affected populations. However, surface transportation modes, if available, are better suited for moving large volumes of resources and serving as critical lifelines for impacted regions of the state. The goal of this study was to improve the prediction of non-functional, partially functional, and functional bridges to assist in post-earthquake emergency planning. This research project focused on determining the functionality of bridges along the routes that connect to the main WSDOT critical lifeline corridors of I-5/405 and I-90. WSDOT has active research projects that are focusing on the resiliency of the I-5/405/90 lifeline routes but are not directly assessing the functionality of the routes connecting into it. Without these secondary routes, the post-earthquake mobility of the state will be reduced, emergency management plans will be difficult to enact, and long-term recovery will be impaired.

This study used updated bridge models, based on a database of important characteristics of selected bridges across Washington state, and 60 synthetic Cascadia Subduction Zone earthquake ground motions to improve predictions of likely loss of functionality for bridges along the secondary state routes of 101, 12, 16, and 3. Results of these analyses were that very few, if any, bridges are expected to be completely destroyed, resulting in no functionality post-earthquake. Bridges inland of the coast and outside of the sedimentary basins of Seattle, Port Angeles, and Tacoma were predicted to remain fully functional post-earthquake. Lastly, bridges located along the coast are likely to sustain damage requiring repair but should be partially functional within 3 to 6 months post-earthquake, based on estimates from the Washington Bridge Design Manual. These results considered only multi-span bridges supported by piers with reinforced concrete columns. Single-span bridges were omitted but are expected to remain functional. Other bridges would need to be evaluated on a case-by-case basis, and liquefaction was omitted but could cause significant bridge damage.