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Discover SR 520 floating bridge key facts and common questions. Want more details? Read the SR 520 floating bridge online booklet (pdf 6.8mb).
The new SR 520 floating bridge opened to traffic in April 2016.
Bridge Dimensions |
Old Bridge |
New Bridge |
Length | 7,578 feet | 7,708.5 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) |
6.5 feet | 20 feet |
West navigational channel clearance |
44 feet | 44 feet |
East navigational channel clearance |
64 feet | 70 feet |
Central drawspan | Yes | No drawspan |
Life and capacity |
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Date opened to traffic | August 28, 1963 | April 11, 2016 (westbound); April 25, 2016 (eastbound) |
Existing traffic volume | 70,000 vehicles/day (103,000 pre-tolling) | NA |
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 |
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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 pontoons |
Anchor Facts |
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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 132 tons |
40 feet by 40 feet 23 feet, 8.5 in. tall 450 tons |
Does 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.
Why did WSDOT build 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:
Where are other floating bridges?
Washington state is the floating bridge capital of the world, with four of the five longest floating bridges. They are the SR 520 Gov. Albert D. Rosellini (Evergreen Point) Bridge (7,708 feet), the I-90 Lacey V. Murrow Bridge (6,620 feet), the SR 104 Hood Canal Bridge (6,521 feet), and the I-90 Homer M. Hadley Bridge (5,811 feet).
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, with a design very similar to the Lacey V. Murrow Bridge.
The Demerara Harbor Bridge in Georgetown, Guyana is the world’s fourth-longest floating bridge (6,074 feet). It is made of steel pontoons. Norway has two large floating bridges – the Bergsoeysund Floating Bridge in Kristiansund, More og Romsdal and the Nordhordland Floating Bridge.
How do windstorms and waves affect floating bridges?
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.