Impact of Cascadia Subduction Zone Earthquake on the Seismic Evaluation Criteria of Bridges

Peter Dusicka, Portland State University


ODOT project page:                                                                       

Under our current seismic design code, bridge designers could be under-designing our bridges to levels below what would be expected directly from the Cascadia Subduction Zone (CSZ) event. In other words, our bridges may not be designed to high enough or safe enough levels in some areas of the state. The seismic risk used for defining the code based design criteria is characterized in hazard maps by ground or spectral acceleration values. These values are obtained from different sources of ground shaking, with each source having different probability of occurrence and a different distance to the bridge site. The contribution from each source is probabilistically combined to obtain shaking intensity across the state for equal probability of occurrence. The Cascadia Subduction Zone (CSZ) significantly contributes to the equal probability shaking intensity for the western part of the state. When considered deterministically, earthquake from CSZ can have acceleration values that are above those obtained probabilistically. There is a need to determine the impact of this demand difference on the seismic assessment of existing bridges for safety. In addition to considering safety criteria, current seismic retrofit approach also considers serviceability criteria. The acceleration levels considered by Oregon for serviceability are higher than used in the FHWA Seismic Retrofit Manual. The CSZ zone also contributes to the equal probability serviceability level shaking intensity, which for the serviceability case could result in over-design to meet elastic performance. This is significant because recently completed TS&L projects aimed at estimating the costs of Phase II seismic retrofit showed that the retrofit was driven by satisfying strength in serviceability criteria rather than deformation in the safety criteria. There is a need to determine the impact of the CSZ demand on the cost of seismic retrofit triggered by the serviceability criteria.

Project Details

Project Type:
Project Status:
End Date:
March 31,2016
UTC Grant Cycle:
non-UTC project