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SR 520 Bridge Facts

SR 520 bridge height
High capacity transit and the future SR 520 bridge
SR 520 bridge lane configuration and width information
SR 520 pontoon facts
Floating bridge facts
Quick facts about the existing SR 520 bridge

SR 520 bridge height

What will be the height of the new SR 520 floating bridge?

In the January 2010 supplemental draft environmental impact statement, we evaluated a new SR 520 floating bridge that would be approximately 30 feet above the water.

Based on community feedback, we lowered the height of the floating bridge to approximately 20 feet, included in the preferred alternative that we announced in April 2010.

Comparison graphics of the existing floating bridge and the new floating bridge are below.

Existing SR 520 bridge

New SR 520 floating bridge with approximately 20-foot bridge height

What will the navigational clearances be on the new floating bridge?

The new west navigational channel of the SR 520 floating bridge will have a clearance of 44 feet (the same as the existing bridge) and the new east navigational channel will have a clearance of 70 feet (the same as the clearance on I 90’s east channel bridge).

You can find photo renderings of the new navigational channels on our Flickr page.

High capacity transit and the future SR 520 bridge

What is the current plan to accommodate future high capacity transit on the new SR 520 floating bridge?

In 2008, a transit strategy was developed by WSDOT, King County Metro, and Sound Transit, in cooperation with the University of Washington. The plan is outlined in the SR 520 High Capacity Transit Plan, published in 2008.

The proposed transit strategy for the SR 520 corridor includes a network of bus rapid transit lines that will be operational when the new corridor opens to traffic. There will be opportunities for rail-to-bus transfers at the future Montlake Multimodal Center, which will include the Sound Transit U-Link line, located near Husky Stadium and the UW Medical Center.

In addition to bus rapid transit on SR 520, voters in the Puget Sound region approved Sound Transit 2 in November 2008. This funding package included operating light rail across Lake Washington on I-90 and over 100,000 additional transit service hours to further develop and support bus rapid transit in the SR 520 corridor.

Will the new SR 520 floating bridge be able to accommodate light rail?

Yes. WSDOT engineers have designed the new SR 520 floating bridge so that additional supplemental pontoons could be added in the future to support the weight of light rail. As shown in the graphics below, light rail could be accommodated in the future by converting the transit/HOV lane to light rail, or adding more width to accommodate light rail in each direction in addition to the 4+2 general-purpose and transit/HOV lanes.

Adding light rail to the SR 520 floating bridge would require analysis of transit connections and routes, additional funding, regional decision-making, and a separate environmental review process.

How much will it cost to add light rail to the floating bridge?

First there is the cost of time. The state Attorney General’s office and the Federal Highway Administration estimate that an additional two years of time would be necessary to conduct the required environmental analysis. This would be essential since the current analysis assumes the four general-purpose and two transit/HOV lane configuration and does not address light rail.

Secondly, it would cost between $150 million and $200 million to construct the 30 additional pontoons and to install them on the floating bridge, along with the 77 pontoons required for the six-lane alternative. There would be other costs associated with the bridge deck expansion and other infrastructure, including rail lines.

Bridge layout diagrams

The planned SR 520 bridge
With two general-purpose lanes and one transit/HOV lane in each direction.
 Planned SR 520 floating bridge

Potential SR 520 bridge configurations that accommodate light rail

Scenario 1: With light rail displacing the transit/HOV lanes.
 Potential SR 520 floating bridge with light rail in transit/HOV lanes
Scenario 2: With light rail in addition to the transit/HOV lanes.
 Potential SR 520 floating bridge with addition of light rail
Note: Each scenario would require WSDOT to construct an additional 30 pontoons to support the weight of light rail. Adding light rail to the SR 520 floating bridge would also require analysis of transit connections and routes, additional funding, regional decision-making, and a separate environmental review process.

Pontoon diagrams

 SR 520 pontoon diagram for six lane roadway
Pontoon diagram for the proposed SR 520 bridge, with two general-purpose lanes and one transit/HOV lane in each direction.

Total pontoons: 77
Bridge deck width: 116 feet

 Potential SR 520 floating bridge pontoon configuration to accommodate light rail
Pontoon diagram for an SR 520 bridge that includes 30 additional pontoons to support the weight of light rail. Additional pontoons are noted in orange.  

Total pontoons: 107
Bridge deck width: 116-150 feet


SR 520 bridge lane configuration and width information

What is the current lane configuration of SR 520?

Existing SR 520 bridge
Typical width: 60 feet

The current configuration across the SR 520 bridge includes:

  • Two general-purpose lanes in each direction, with an 11-foot inside lane and a 12-foot outside lane.
  • One 2-foot outside shoulder in each direction.
  • One 1-foot inside shoulder in each direction.
  • One 2-foot median barrier.
  • A 3-foot maintenance sidewalk on the south side of the bridge.

What are WSDOT’s plans for the lane configuration of the new SR 520?

Future SR 520 floating bridge cross section

Typical width: 116 feet

As required by legislation and analyzed in the SDEIS, WSDOT will construct a six-lane SR 520 corridor from I-5 in Seattle to SR 202 in Redmond. The 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.

How does WSDOT plan to build pontoons to replace the SR 520 bridge?

In 2009, we requested bids from contractors interested in building pontoons for the new SR 520 floating bridge. As part of the contracting process, we required the contractor to design pontoons that would accommodate three potential configuration scenarios, including a possible option for high capacity transit that could be pursued in the future by the region.

Three pontoon construction scenarios:
  1. Rapid replacement of four-lane capacity in case of catastrophic failure. A four-lane bridge with two 11-foot-wide lanes in each direction for general-purpose traffic.

This action is analyzed in the Pontoon Construction Project Draft Environmental Impact Statement, released in May 2010.
  2. New six-lane floating bridge. A six-lane bridge as described above, with two 11-foot-wide lanes in each direction for general-purpose traffic and a 12-foot-wide lane in each direction for a transit/HOV lane. Also adds a 14-foot bicycle and pedestrian path and safety shoulders.

This action is covered in the I-5 to Medina: Bridge Replacement HOV Project Supplemental Draft Environmental Impact Statement released in January 2010.
  3. Future high capacity transit. We have planned to accommodate two potential high capacity transit scenarios:

1. A six-lane bridge for light rail or dedicated bus rapid transit.

2. Displace the transit/HOV lane in each direction to accommodate potential future light rail or dedicated bus rapid transit.

Either high capacity transit scenario would require additional regional decision-making and an environmental review process in the future. Lane and shoulder width configurations would be determined as part of a future regional decision-making process.

What are WSDOT’s current pontoon construction plans?

In January 2010, WSDOT awarded a design-build contract to Kiewit-General Joint Venture to design and build enough pontoons to replace the existing four-lane bridge (as described in the first scenario listed above). This work includes a total of 23 large pontoons and 10 smaller stability pontoons. These pontoons will be stored in the Grays Harbor area until they are needed either for the rapid replacement of the facility as a result of a catastrophic failure or for the planned SR 520 bridge replacement. The design-build contract does not include towing pontoons to Lake Washington or construction of the new roadway.


SR 520 pontoon facts

New SR 520 bridge pontoons will be approximately 28 feet tall, 75 feet wide and 360 feet long -- as long a football field. 

Pontoon length compared to a football field

One single longitudinal bridge pontoon is a little over 11,000 tons, or approximately equal to 23 Boeing 747 jets. 
Pontoon weight compared to airplanes

Pontoon anchor cable compared to Orca whales


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 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 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 1.44 miles (7,578 feet) 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 anchor 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.