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SR 520 - Floating Bridge Facts

 side by side of new bridge and old

  Aerial view of the new bridge being built alongside the existing floating bridge.

Key facts: Existing and new floating bridges

 Bridge Dimensions   Existing Bridge  New Bridge
 Length  7,578 feet  7,710 feet
 Number of standard travel lanes  2 each direction  2 each direction
 Number of HOV lanes  0  1 each direction
 Bicycle/pedestrian access  No  14-foot-wide
 shared path
 Shoulder width  1 foot inside
 2 feet outside
 4 feet inside
 10 feet outside
 Roadway deck width (at midspan)  60 feet  116 feet
 Deck height above water
 (at midspan)
 13 feet  20 feet
 West navigational channel
 44 feet  44 feet
 East navigational channel
 64 feet  70 feet
 Central drawspan  Yes  No drawspan
 Life and capacity  
 Date opened to traffic  August 28, 1963  Est. spring 2016
 Existing traffic volume  70,000 vehicles/day
 (103,000 pre-tolling)
 Sustained wind speeds built to withstand  57 mph; retrofitted
 for 77 mph
 89 mph
 (100-year storm)
 Expected service life  50+ years  75+ years
 Pontoon Facts
 Number of pontoons  33  77
 Size of biggest pontoons
 (longitudinal pontoons)
 15 feet, 8 in tall
 60 feet wide
 360 feet long
 4,725 tons
 28 feet tall
 75 feet wide
 360 feet long
 11,000 tons
 Total bridge width
 (including pontoons)
60 feet  195 feet with
 stabililty pontoons;
 240 feet at cross
  Anchor Facts  
 Number of anchors (all types)  58 anchors  58 anchors
 Size of fluke anchors  33 feet wide
 16 feet, 9 in tall
 77 tons
 35 feet wide
 26 feet tall
 107 tons
 Size of gravity anchors  26 feet by 26 feet
 13 feet tall
 40 feet by 40 feet
 23 feet, 8.5 in tall
 450 tons

Common questions

Will the new bridge support light rail?
The new floating bridge is engineered to accommodate light rail in the future. The addition of light rail would require a transit analysis, additional funding, regional decision-making, a separate environmental review process, and time to conduct these steps and complete construction.

How do floating bridges float?
Floating bridges are made of large water-tight concrete pontoons connected rigidly end-to-end, upon which the roadway is built. Despite their heavy concrete composition, the weight of the water displaced by the pontoons is equal to the weight of the structure (including all traffic), which allows the bridge to float.

How are floating bridges constructed?
Individual bridge pontoons are usually built on dry land next to a waterway, then floated and towed like barges to the bridge site. They are connected to grounded approach structures on each end, starting at the edge of the floating structure and then pieced together toward the eventual bridge’s center. The pontoons are held in place by enormous steel cables generally hundreds of feet long that are connected to anchors buried deep in the lakebed. For more information and to view an example of a floating bridge under construction, visit the Hood Canal Bridge Project website.

Why is WSDOT building a floating bridge over Lake Washington as opposed to a conventional suspension bridge?
A conventional suspension bridge over Lake Washington would not work for several reasons:

  • Suspension bridges need to travel in a fairly straight line. Because SR 520 is a curved corridor, a suspension bridge would not be possible.
  • The deepest point in Lake Washington is 214 feet deep, and the bridge’s support towers would have to be approximately 630 feet in height, nearly the height of the Space Needle, to support the bridge. These massive towers would be out of character with the surroundings because it would create more noise and block views.
  • Conventional fixed bridges, such as the new bridge over the Tacoma Narrows, are expensive to build in deeper waters with soft beds, such as Lake Washington.

Where are other floating bridges?
Washington State is the floating bridge capital of the world with the four longest and heaviest floating bridges. They are the SR 520 Evergreen Point Bridge, the I-90 Lacey V. Murrow Bridge, the I-90 Homer M. Hadley Bridge, and the SR 104 Hood Canal Bridge.

In 1957, a concrete floating bridge was built across Lake Okanagan at Kelowna in south central British Columbia, Canada. Its floating length is 2,100 feet (640 meters) and its design is very similar to the Lacey V. Murrow Bridge.

The Demerara Harbor Bridge in Georgetown, Guyana, is another floating bridge. It is made of steel pontoon units and extends 6,074 feet (1,851 meters). Norway has two large floating bridges – the Bergsoeysund Floating Bridge in Kristiansund, More og Romsdal and the Nordhordland Floating Bridge. Another long-time floating bridge site is the Galata Floating Bridge in Istanbul, Turkey.

How do windstorms and waves affect floating bridges? floating bridge during a storm
Wind and wave forces are typically the controlling forces in the design of floating bridges. A major factor in wind and wave effects on floating bridges is called the fetch. The fetch is the unobstructed clear distance over the water that wind can travel to the bridge. The longer the fetch, the higher the wind and wave forces will be. In Lake Washington the critical fetch is to the southwest of the bridge, since the largest storms historically come from the southwest. Wind and wave forces cause the pontoons to bend, heave and twist, creating large stresses in the pontoons and anchor system. If a 100-year storm event were to occur, the pontoons are designed to prevent large cracks from developing that would allow water to leak in and sink the bridge.

How do earthquakes affect the floating section of the SR 520 bridge?
The floating section of the SR 520 bridge is not affected directly by ground shaking from earthquakes because is built on pontoons that are anchored to the bottom of Lake Washington. Some very deep low-frequency earthquakes can cause a seiche, or a surface wave similar to a tsunami. A seiche in Lake Washington could cause the floating bridge to bend and heave at the lake surface, adding large loads of pressure to the pontoons and anchor systems. A seiche in Lake Washington could also create an underwater landslide that could cause the pontoon anchors to slip or break.

Typically the waves from a seiche create less stress in the pontoons than wind-induced waves from a storm that occurs once every 100 years.