The Evergreen Point Floating Bridge, also known as the 520 Bridge and officially the Governor Albert D. Rosellini Bridge, carries Washington State Route 520 across Lake Washington from Seattle to its eastern suburbs. At 7.710-feet long it is the largest floating bridge in the world, as well as the widest.
The original Evergreen Point Floating Bridge, also named for state governor Albert D. Rosellini, opened on August 28, 1963, carrying the four-lane State Route 520. Due to increased traffic generated by rapid growth of the Eastside area, bridge replacement was explored as early as 1969.
The original Evergreen Point Floating Bridge was designed before the implementation of modern earthquake engineering standards, with vulnerabilities in its hollow support structures that could have failed in a major earthquake.
Planning of the replacement bridge started in 1997 with a cross-lake study conducted by the state Department of Transportation. The study followed several others in the late 20th century to find solutions to traffic on the SR 520 floating bridge, with most proposals rejected after heavy opposition from communities on both ends of the bridge.
The final environmental impact statement for the project was issued in 2011, allowing for construction of the pontoons to begin the following year.
Construction of the bridge deck, beginning with the eastern approach in Medina, began in March 2012. Despite delays from two construction accidents, the bridge was completed in 2015.
The new bridge was designed to be more stable in stronger winds and raised the bridge deck much higher above the surface of the lake than the old bridge. Unlike the original floating bridge, where the road surface is directly on pontoons connected end-to-end, the new bridge featured pontoons laid north–south, perpendicular to the direction of vehicular traffic, and a road surface on a platform raised 20 ft above the water. This design now includes shoulders and a protected pedestrian and bicycle path across the viaduct.
The floating bridge is laid atop 77 concrete pontoons that float above the water and are secured by 58 anchors to the lake bottom. All of the pontoon designs include watertight compartments that are remotely monitored to detect leaks that might cause catastrophic failure. If the pontoon is breached, an alarm will sound inside the maintenance building. From there, a pump can be lowered into the chamber and controlled from the deck above.
The bridge deck is elevated 20 ft above the pontoons, which forms a lower deck that allows for maintenance vehicles to access the pontoons without disrupting traffic.
The new bridge was dedicated on April 2, 2016, in a ceremony presided over by Governor of Washington Jay Inslee and attended by an estimated 40,000 to 50,000 people. The ceremony also included a community fun run and walk on the bridge, and a bicycle ride hosted by the Cascade Bicycle Club on the bridge and the Interstate 5 express lanes the following day. As part of the opening ceremonies, the bridge was certified as the world's longest floating bridge by Guinness World Records, at 7,708.49 ft long; the bridge took the record from the previous Evergreen Point Floating Bridge, which is 130 ft shorter in length.
In April 2017, the bridge project was awarded the 2017 Grand Conceptor Award from the American Council of Engineering Companies (ACEC).
Cover image by the Washington State Department of Transportation is licensed under CC BY-NC-ND 2.0. Information courtesy of Wikipedia.