New SR 520 bridge pontoons will be approximately 28 feet tall, 75 feet wide and 360 feet long -- as long a football field.
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One single longitudinal bridge pontoon is a little over 11,000 tons, or approximately equal to 23 Boeing 747 jets.
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Floating bridge facts
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 Web site.
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 capitol 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 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.
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.
Quick facts about the existing SR 520 bridge
What is the official name of the floating bridge?
The official name of the SR 520 floating bridge is the Governor Albert D. Rosellini Bridge - Evergreen Point.
How wide is the current bridge?
The current bridge is 60 feet wide.
How long is the floating section of the bridge?
The floating section of the SR 520 bridge is 1.42 miles (2,285 meters) long, making it the longest floating bridge in the world.
How many pontoons support the current bridge?
The current bridge is supported by 33 bridge pontoons. Each pontoon is about as long as a football field (360 feet) and 16 feet tall.
How many anchors hold the pontoons in place?
The bridge pontoons are held in place by 62 anchors attached with 2-3/16 cables. (Anchors on the pontoons that connect with the east and west approaches require thicker cables. Those cables are 2-3/4" inches thick.)
How much do the anchors weigh?
A standard achor for the SR 520 bridge pontoons weighs 77 tons, which is more than 10 male African elephants.
How deep is Lake Washington?
Lake Washington is 200 feet deep under the drawspan of the current floating bridge. The deepest point in Lake Washington is 214 feet deep.
When did the existing bridge open?
We opened the existing bridge to traffic on Aug. 28, 1963.
How long did the tolls last when the bridge opened in 1963?
We ended tolling on the existing bridge on June 22, 1979, less than 16 years after the bridge opened.
How many vehicles use the current bridge each day?
Every day, approximately 115,000 vehicles use the SR 520 bridge to cross Lake Washington.
How many vehicles did we expect to cross the existing bridge each day when we designed it?
The current bridge was designed to carry 65,000 vehicles per day.
What windspeed was the existing bridge designed to withstand?
The existing bridge was designed to withstand 50-70 mph winds.
Why do you close the bridge to traffic during severe windstorms?
We close the bridge to traffic in order to open the drawspan, which relieves pressure on the bridge from wind and wave action during windstorms.
The criteria for closing the bridge to traffic and opening the drawspan is 50 mph gusts sustained for 15 minutes. When a 40 mph gust is sustained for one minute, a warning alarm alerts crews to come to the bridge for inspection and monitoring.
As with all WSDOT bridges, our experienced crews can close the bridge anytime they deem it unsafe, or when there is a potential for damage.
You can check current conditions at the SR 520 bridge on the Web.
What repairs and retrofits have been made on the current bridge?
2006 - Replaced drawspan anchor bolts sheared off during storms earlier in the year. The anchor bolts hold the drawspan closed.
2000 - Emergency repairs to column damage.
1998 - Seismic retrofit, cable post tensioning, wave deflectors.
1997 - Pontoon bolts and crack seal.
1993 - Storm damage repair of pontoon cracks.
How many cracks in the bridge pontoons have been repaired?
Crews have repaired more than 30,000 linear feet of cracks since the 1993 Inaugural Day storm.