SR 520 Evergreen Point Floating Bridge

For more than 50 years, a pontoon-supported bridge crossing Lake Washington was recognized as the longest floating bridge in the world. In 2016, that claim passed to a newly constructed floating bridge measuring 7,710 feet in length, or 130 feet longer than the bridge it replaced.

On April 2, 2016, a public grand opening celebration marked the opening of the $1.5 billion Evergreen Point Floating Bridge along State Route 520, which crosses Lake Washington between Seattle and the Eastside communities of Medina and Bellevue, Washington. At the grand opening, the bridge was certified as the longest floating bridge in the world by representatives from Guinness Book of World Records.

The previous floating bridge had exceeded its design life and was showing its age. The bridge pontoons had become vulnerable to windstorms, and the bridge approach roadway support columns were vulnerable to earthquakes. Constructed just north of the original four-lane floating bridge, the new structure elevates a six-lane roadway with shoulders plus a bike/pedestrian path above the floating pontoons.

The bridge is owned and maintained by the Washington State Department of Transportation (WSDOT). The original bridge was tolled upon completion in 1963, with the tolls removed in 1979. The new bridge employs time-of-day variable rate tolling to finance a portion of the overall program’s cost. Tolling was added to the existing bridge in late 2011 during construction and transferred to the new bridge when it opened in 2016. The old bridge was then decommissioned and removed.

Serving as a member of the general engineering consultant team led by HDR, Inc., WSP USA provided program and project management, tolling and financial analysis, environmental documentation, preliminary engineering, design-build procurement, design and construction oversight and quality management. One of WSP’s key roles included providing the initial toll traffic and revenue forecasts and net toll revenue projection used in the financial planning for the $4.5 billion program, which included evaluation of multiple toll scenarios through a public engagement process. The firm also assisted the owner in obtaining a $300 million Transportation Infrastructure Finance and Innovation Act (TIFIA) loan and continues to prepare annual gross—to-net toll revenue projections and other financial performance monitoring support.

The State Route 520 program had multiple construction packages and components, a primary one being the Floating Bridge and Landings Project, which had an overall approximate value of $1.5 billion. WSP was integral to the team that prepared preliminary engineering and procurement documents in a very aggressive schedule for this design-build project. The firm’s structural engineers also advised the owner to address constructability issues with the early pontoons and provided successful solutions.

Used by an average of 74,000 motorists every day, the new bridge provides two general-purpose lanes and one transit/high-occupancy vehicle (HOV) lane in each direction. It also features wider, safer shoulders that will allow vehicles to pull over in the case of a breakdown. It supports non-motorized traffic with a 14-foot-wide bicycle and pedestrian path on the north side of the bridge and was designed to accommodate future light rail if the region chooses to fund it in the future.

When project planning began, there was little doubt that a floating bridge was the most practical option. The middle of Lake Washington is 200 feet deep with poor lake bottom soil conditions. Engineers determined that building any other type of bridge would have been more difficult and costly.

The new bridge also addressed the seismic vulnerability of the old bridge, which was built at a time when less consideration was given to potential earthquake damage. The original approaches on the bridge were built on hollow piers, which are not seismically stable. Now the piers are reinforced with much stronger support columns.

The new bridge is supported by 77 pontoons, including 21 that are 75 feet wide and 360 feet long, each measuring longer than a football field.

While the previous four-lane bridge was frequently closed when wind speeds reached 35-40 miles per hour, in part due to wind waves and spray hitting the roadway on the pontoons’ deck, the new bridge is built on an elevated structure above the pontoon decks and out of reach of windstorm-driven waves and spray. It is resistant to winds up to 89 miles per hour.

The new bridge also addresses storm water concerns. With the original bridge, unfiltered storm water ran straight into Lake Washington. The new bridge has an enclosed water filtration system, so the water is cleaned before it returns to the lake.

Although most of the bridge was kept low to accommodate concerns from residents, higher sections at both ends allow for tall vessel passage without halting traffic.