The World's Longest Floating Bridge: SR 520 in Washington

The Evergreen Point Floating Bridge in Washington State

Evergreen Point Floating Bridge
Paul A. Souders/Corbis Documentary/Getty Images

The Evergreen Point Floating Bridge, or what is commonly known as the 520 Bridge, stretches across Lake Washington from Seattle to Medina holds the title of the World Longest Floating Bridge.

The bridge was rebuilt and reopened on April 2, 2016. The original bridge was 7,500 feet long, while the new 520 Bridge is 7,708.49 feet long. The original bridge was replaced due to age and was replaced by a more modern floating bridge.

The Pontoons

The bridge is built over pontoons, just like temporary bridges or military crossings. A pontoon contains over 58 reinforced concrete anchors that will sink into loose materials on the bottom of the water body to hold the pontoons in place.

The 520 Bridge end anchors are drilled directly into the ground, which keeps the roadway from swaying. A typical pontoon measures 360 feet long, 75 feet wide and 28 feet tall; however only six feet of the World Longest Floating Bridge is above water.

These pontoons are so large that external vibrators were used to consolidate concrete instead of using internal vibrators. Furthermore, extra-fluid concrete was used to produce better results while pouring concrete. The pontoons were floated to the project site as part of a separate $586 million contract.

Design and Engineering

The SR-520 Bridge road surface has also been elevated to 20 feet above the water surface, providing extra space for utilities, keeping splashing water from hitting vehicles.

The bridge is designed to withstand over 90 miles-per-hour winds thanks to design that distributes its weight uniformly over a larger area.

Below the bridge is a fire protection system, electrical piping, smart transportation communication system, fiber optic wiring and a flood protection device to alert when pontoons are flooded.

The bridge was also designed to accommodate a light rail system with only minor improvements to the pontoons.

World Longest Floating Bridge: Key Facts

The Washington State Department of Transportation (WSDOT) lane configuration across the new SR 520 Bridge includes:

  • Two 11-foot general-purpose lanes in each direction
  • One 12-foot 3+ transit/HOV lane in each direction
  • One 10-foot outside shoulder in each direction
  • One 4-foot inside shoulder in each direction
  • One 2-foot median barrier
  • A 14-foot bicycle/pedestrian path on the north side

Other key facts from the World longest floating bridge are:

  • Governor Albert D. Rossellini Bridge is the official name of the bridge.
  • The floating section of the SR 520 Bridge is 7,578 feet long.
  • The bridge is supported by 77 bridge pontoons.
  • The SR 520 bridge pontoons are held in place by 58 anchors secured to the lake bottom.
  • The deepest point in Lake Washington is 214 feet.
  • More than 115,000 vehicles use the SR 520 Bridge to cross Lake Washington on a daily basis.
  • The existing bridge was designed to withstand 50-70 mph winds, while the new bridge will hold up to 92 mph winds.
  • Pre-stressed concrete pontoons were part of the original bridge design, each pontoon measuring approximately 360 feet long, 60 feet wide and 14.9 feet deep.
  • Washington State is the floating bridge capital of the world with the four longest and heaviest floating bridges.

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