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February 6, 2023

National Finalist: Gold Award 
Structural Systems

Photo provided by WSP USA
A view of the new post-tensioning, concrete deviator diaphragms and inspection platforms installed in the main span of the bridge inside the box girder.

Project: West Seattle Bridge rehab and strengthening  
Client: Seattle Department of Transportation 

The city of Seattle received some surprise results during a requested assessment of the heavily traveled West Seattle Bridge, indicating that the bridge was substantially degraded in cracked areas, triggering immediate closure of the bridge. 

In response to the loss of service of this critical structure, the Seattle Department of Transportation started working to stabilize the bridge. Led by WSP USA, the team conducted non-destructive testing of the bridge, designed rehabilitation measures to first stabilize the bridge and then restore service to the bridge, and implemented a structural health monitoring system. While the stabilization work was constructed, a detailed cost-benefit analysis was conducted to assess the economic and technical feasibility of further rehabilitation relative to a replacement structure. 

The cracks in the West Seattle high-rise bridge resulted from a phenomenon called “creep,” a concept that is widely debated to this day and was not fully understood when the bridge was designed in the 1980s. Creep is a manner in which permanent deformation of material occurs under constant load and temperature within the concrete structure over time.  

Many factors affect the calculation of creep that are hard to detect on an existing structure, including timing of stage construction in which the bridge element was subjected to loading, environmental factors such as humidity, and the material make-up of a non-isotropic material such as concrete. The bridge is also located in an area known to have soft, highly liquefiable soils with the potential for lateral spreading.  

Another uniqueness of the West Seattle high-rise bridge was how it was detailed, with the longitudinal bottom slab post-tensioning terminating at two discrete locations within the main span.  

To design a successful repair of the bridge, the WSP team consulted with the authors of the modified compression field theory, Dr. Michael Collins from the University of Toronto, and his associate, Dr. Evan Bentz. Through consultation, it was determined that the only way to determine the actual shear capacity of the distressed sections of the bridge was to run a non-linear analysis of the localized distressed region.  

The findings showed the capacity of the bridge was very sensitive to the amount of degradation the bridge had already gone through, which was unknown since the existing cracks had been previously epoxy injected with no records of crack widths before injection.  

Innovative data studies were utilized to assess recorded bridge thermal movements relative to AASHTO predictive models using 20 years of collected weather data. This data helped to explain the accelerated cracking that was observed and reassured the team that their structural analysis models could adequately predict the bridge movements. 

The extremely detailed assessment of the issues with the West Seattle Bridge provided critical information to a timely and cost-effective solution. The city of Seattle did not have the plans or the budget to build a new bridge. A design to repair the existing bridge, using conventional construction, was a welcome solution for the 100,000 residents that use the bridge and to the city.

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