Engineering & Construction


MP3 Download: Listen to Dean Bitney, a native of Mercer Island, talk about the construction of the SR 520 Bridge.

MP3 Download: Listen to Roxanne Bitney, a former SR 520 toll plaza employee, talk about working on the SR 520 Bridge.

MP3 Download: Listen to Archie Allen, Bridge Maintenance and Operations Superintendent at the Washington State Department of Transportation, talk about bridge and pontoon contruction.


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A t 7,578 feet (1.4 miles) long, the Evergreen Point Bridge is the longest floating bridge in the world. Its unique design incorporated movable pontoons and a drawspan, which allows for the passage of ships and other maritime vessels.  

The floating section of the bridge features 33 pre-stressed concrete pontoons. Interior cell walls are 6” thick, and the exterior pontoon walls are 9” thick. The largest pontoon is 360 feet long and weighs 6,700 tons. They are fastened to the lake bottom with 77-ton reinforced concrete anchors and 2 3/4 inch steel cables.

Near the Evergreen Point Bridge’s midpoint are two steel lift drawspans that flank two 16 foot long movable pontoons. The drawspans were designed to be raised 7’ 2”, which allows the movable pontoons to retract. Once opened, the span has a 200’ wide clearance for ships and other vessels. The drawspans retract into hourglass-shaped sections on both sides. 

The minimum height of the floating bridge’s roadway above water is 7.75 feet, where the roadway sits directly on the floating pontoons; the maximum roadway height is 59 feet, where the road surface is elevated above the concrete pontoons at the east end of the floating structure. The top of the railing sits about four feet above the roadway surface. Approximately 11,806,000 pounds of reinforcing steel was used to construct the bridge.  Once constructed, the bridge in its entirety weighs 114,000 tons.

Construction commenced on August 29, 1960. The construction management team made their field headquarters where the University of Washington Shell House is today.  Early discovery of deeper than expected mud near Madison Park forced more extensive dredging and backfilling but did not change the bridge design.

Seattle contractors made the bridge pontoons, superstructures and anchors at sites in the SoDo neighborhood south of downtown Seattle and on Lake Union. The pontoons were towed to and stored at Kennydale Moorage on the eastern shore of Lake Washington in Renton. By early 1963 the anchors and pontoons were in place and the floating portion of the bridge was assembled that summer and the new corridor opened for travelers in late August, 837 days after construction began.

Eastside drivers reached the bridge on new access roads at 84th Avenue NE, 104th Avenue NE, and State Highway 2A, near 116th Avenue NE.  Drivers coming from the west got on the bridge from the Montlake neighborhood at Lake Washington Boulevard and Montlake Boulevard. The bridge toll plazas were placed between 84th Avenue NE on the eastern lake shore, with seven toll booths to accommodate anticipated traffic.

On the same day (August 28, 1963) that Martin Luther King led his famous civil rights March on Washington, this new bridge was dedicated in Washington State. The "Official Opening” was attended by scores of local and national dignitaries. Governor Rosellini as Chairman of the Washington State Toll Bridge Authority presided over the dedication and ribbon-cutting ceremony.

Today, the aging floating bridge is vulnerable to failure in a severe windstorm, and the fixed bridges along the corridor do not meet current seismic standards and could collapse in an earthquake. For these reasons, the bridge is scheduled to be replaced by a new floating bridge, which is currently (2013) under construction.