SR 520 Floating Bridge
SR 520 Floating Bridge
SR 520 Floating Bridge

With the SR 520 Floating Bridge and Landings Project, the Washington State Department of Transportation replaced the existing floating bridge, also known as the Governor Albert D. Rosellini or Evergreen Point Floating Bridge, over Lake Washington. At more than 7,700 ft long, the new SR 520 Floating Bridge achieved a Guinness World Record for the longest floating bridge. Working with the design-build team, SGH served as the structural and marine engineer, as well as the engineer of record for many critical tasks on this project.

Scope | Solutions

The project team was responsible for demolishing the existing bridge, fabricating the large prestressed, precast concrete floating pontoons (mostly 360 ft by 75 ft by 28 ft), transporting and assembling the pontoons, installing anchors and anchor cables, and constructing the elevated roadway above the pontoons. Highlights of SGH’s work include the following:

  • Developed a demolition work plan for dissembling the old bridge pontoons

  • Designed a value engineering option to use a post-tensioning system for assembling and joining the floating bridge pontoons

  • Performed dynamic structural analyses to evaluate the bridge’s structural integrity during varying construction stages and storm conditions

  • Developed multiple monitoring and damage response contingency plans against accidental damages or larger-than-expected storms

  • Performed mooring, floating stability, and ballast analyses for towing, mooring, assembly, and separation of floating pontoons

  • Designed the mooring facilities and fenders for berthing and towing large concrete floating pontoons and construction vessels

  • Designed cable anchor test frames used to evaluate the structural integrity of anchor cables

  • Designed heavy lifts using floating derrick cranes used to install and remove bridge deck panels, girders, and columns

  • Evaluated means and methods for installing, removing, and relocating anchor cables on the existing and new bridges to minimize the damage risk

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