The Evergreen Point Floating Bridge, also known as the SR 520, at 7,708.5 feet long is the world’s longest floating bridge. Located on Lake Washington, USA, it connects the city of Seattle with the city of Bellevue, was opened in April 2016, and replaced the old bridge that had lived out its life. The State of Washington contains multiple floating bridges of varying sizes as this typology is most suitable to environmental and soil conditions of the area.
Floating bridges are quite distinct from other more traditional types of bridge construction as they are placed on pontoons or barges that essentially float on top of the water as opposed to being rigidly anchored into the soil below, a methodology developed from temporary pontoon bridges used by the military. The depth of Lake Washington and the soft ash soil underneath it made it impractical to construct a traditional bridge and thus this floating bridge was the solution.
The SR 520 floating bridge is laid atop 77 concrete pontoons that float above the water and are secured by 58 anchors to the lake bottom. Land based bridges are usually governed by seismic loads however this floating bridge was designed in accordance with the wind and wave forces around it. The floating bridge was designed to withstand a 100-year storm, defined as a storm having 98 mph winds and 6-foot waves.
The new SR 520 floating bridge is unique in that the entire roadway is elevated above the pontoons over the full length of the bridge. The benefit of the elevated roadway is two-fold. First, it keeps vehicles above the lake’s wave spray and splash that occur during large storm events. Second, it provides a maintenance corridor below the elevated roadway that allows maintenance staff access to the pontoons. The bridge currently has six vehicular lanes and a 14-foot wide bicycle and pedestrian path on the north side. It was also pre-designed for future widening that would allow for the addition of two light rail train lines down the center. The design also includes a new 12,500 square foot office/maintenance facility building that is a two-story facility, including office spaces, freight elevator, passenger elevator, conference room, kitchen area, and shop area.
