Impacts of Cascadia Subduction Zone M9 Earthquakes on Bridges in Washington State

Abstract

The Cascadia Subduction Zone (CSZ) poses a major seismic hazard to the western portions of Washington State, Oregon State, and Northern California. To plan for community resilience, it is important to understand the impacts of a large magnitude CSZ earthquake on highway bridges. Previous researchers have developed 30 sets of potential M9 ground motions at numerous locations throughout Washington State for several site classes. These motions have been combined with single-degree-of-freedom (SDOF) models to investigate bridge performance. These models are computationally efficient, however, SDOF models have limitations in predicting bridge seismic response. In particular, such models neglect the effects of bridge abutments. Typical characteristics of bridges in Washington State were identified by studying a database compiled by the Washington State Department of Transportation (WSDOT) engineers and University of Washington researchers. Based on this study, a three-dimensional, multi-degree-of-freedom (MDOF) bridge model was developed to represent typical highway bridges in Washington State. The reference bridge had three spans, a continuous superstructure, and an L-type (Seat) abutment with bearing pads and transverse shear keys. Using that model, a series of parametric studies were conducted to evaluate the effects of bridge location, site class, abutment characteristics, and properties of the columns at the intermediate supports. The simulated deformation responses of the bridges in the longitudinal direction were consistently smaller than those in the transverse direction due to the high resistance provided by the abutment and backfill soil. In the transverse direction, the largest displacement responses were calculated for the coastal cities and for site class D1. The responses tended to decrease as the distance from the coast increased. The transverse displacements increased significantly when the shear keys were removed for all locations and site classes. For bridges without shear keys, increases in column heights resulted in higher response in cities underlaid by sedimentary basins. These findings provide engineers from WSDOT an opportunity to develop cost and time-efficient prioritization bridge retrofit plans in the future.