State Route 520 Floating Bridge and Landings Design-Build | Seattle, WA

The aging State Routh 520 Floating Bridge was vulernable to windstorms and earthquakes. WSP served as colead designer for the replacement project.


  • Washington, USA
  • United States


  • Washington State Department of Transportation (WSDOT)

Project Value

  • $586 million

After floating for nearly 50 years, the four-lane State Route 520 (SR 520) floating bridge, in Seattle, Washington, was often clogged by traffic and showed its age.

The floating bridge pontoons were vulnerable to windstorms, and bridge support columns were vulnerable to earthquakes. The bridge serves as a critical transportation link in Washington State, carrying approximately 115,000 vehicles and over 190,000 people across Lake Washington daily.

At 7,710 feet long, the new SR 520 floating bridge is the longest floating bridge in the world. The replacement bridge features six lanes, including two general-purpose lanes and one high-occupancy-vehicle lane in each direction, and a bicycle and pedestrian path on the north side. The structure was also designed for the future addition of one lane of high-capacity light rail transit in each direction. The completed floating bridge reduces travel times between Seattle and several communities to the east along SR 520, as well as significantly improves safety, as the structure was designed to resist winds of up to 89 miles per hour.