Data-Driven Assessment of Post-Earthquake Bridge Functionality and Regional Mobility

Abstract

The seismic performance of bridges is essential to post-earthquake mobility, as bridges are relied upon to carry goods and people into and out of urban centers after natural disasters. A 2019 Department of Homeland Security (DHS) report assessing the regional resiliency of Western Washington state to a Cascadia Subduction Zone (CSZ) earthquake predicted widespread and high levels of bridge damage. A primary objective of this study was to create an improved prediction of non-functional, partially functional, and functional bridges that will assist in post-earthquake emergency planning. The Washington State Department of Transportation (WSDOT) bridge database was expanded to include site class, properties of abutments and foundations, and additional bridges. Properties of bridges in the database were used to define the parameters for a parametric study on Western Washington bridges subjected to Magnitude 9.0 CSZ ground motions. Detailed multi-degree-of-freedom bridge models were developed with OpenSees. Models were formulated for a suite of representative bridges and used to conduct nonlinear time history analyses for synthetic ground motions that had been generated in previous studies. Results from the model analyses were used to provide a more detailed understanding of the likelihood of bridge damage and the likely service levels post-earthquake. Bridge response was limited in the longitudinal direction because of stiffness provided by the abutments and backfill soil. In the transverse direction, shear keys and bearings were found to limit the lateral deformation in columns as a result of participation of the bridge deck. The majority of bridges in the WSDOT inventory have shear keys and bearings and were predicted to be in full service following a CSZ earthquake. WSDOT has been retrofitting older bridge columns with steel jackets since 1991, and this retrofit has been shown to enhance ductility. Bridges without shear keys and bearings should be prioritized for retrofit. Shorter period bridges near the coast and longer period bridges in locations with sedimentary basins were also identified as being more prone to damage. Bridge functionality after a CSZ earthquake is likely to be considerably better than anticipated by the 2019 DHS report. Some bridges may require repair, but bridges are likely to remain useable for emergency vehicles and post-earthquake response. These conclusions were reached within the scope of the study, with several limitations noted in the report that will require further investigation.