SR 520 Bridge Replacement

  • Client:
    Washington State Department of Transportation (WSDOT)
  • Collaborators:
  • Project Type:
    Marine Infrastructure
  • Service Categories:
    Ocean Engineering
  • Challenge:
    Wind and wave loads analysis of one of the world’s longest floating bridges.


Western Washington is home to some of the longest floating bridges in the world. In 1983, the Washington State Department of Transportation (WSDOT) commissioned Glosten to evaluate wind and wave forces for the new I-90 bridge design and two more Lake Washington bridges. The analysis of floating bridges presents a unique challenge because the wave loads and structural deformation interact. Glosten collaborated with local civil engineers to solve this complex problem by integrating hydrodynamic analysis, wave hindcasting, and stochastic analysis with structural analysis tools.

I-90- by WSDOT


Over the following decades WSDOT recognized the need to revisit and refine the bridge analyses as wind data collection and analysis methods advanced. The State of Washington’s Bridge Division engineers still rely on Glosten’s expertise today to assess the forces on the State’s floating bridges.  We have provided WSDOT with a variety of engineering support in the years since our initial appraisal. The new SR 520 Floating Bridge opened to traffic in April 2016, and Glosten performed the wind and wave loads analysis of the new bridge ahead of construction. The SR 520 replacement bridge enhances safety with more than double the stabilizing pontoons of its predecessor and a design that can withstand a 100-year storm.

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