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Repairing the world’s largest floating bridge

Photo: COWI


​COWI Marine North America designs a 585-ton floating cofferdam for the repair of the floating bridge across Lake Washington.

The SR-520 bridge crosses Lake Washington, linking Seattle and its neighboring cities to the east. Approximately 115,000 vehicles and more than 190,000 people cross it every day, and it is the largest floating bridge in the world.

Having been in operation for nearly 50 years, age is starting to take its toll on SR-520. The floating bridge pontoons are vulnerable to windstorm, and the four-lane road is often clogged by heavy traffic. 

Therefore, the State of Washington is replacing the existing bridge with a new six-lane floating bridge, to which light rail may be added in the future.

“This was a fast-track project. We were hired by Kiewit-General-Manson Joint Venture, the contractor to the bridge, to recommend a repair plan for the floating bridge pontoons in November 2012. By February 2013, the repair plan was completed and approved by the owner, Washington Department of Transportation. The cofferdam method was selected to repair two of the four floating pontoons,” says Sam Yao, Design Manager at COWI MARINE North America.


A unique project

The cofferdam has an estimated dry weight of 585 tons and fabrication was completed on a barge outfitted with a launch rail system. In addition to design of the floating cofferdam, COWI MARINE North America provided on-site inspection of the extent of cracking, design of repairs utilising a FRP strengthening system, and on-site review of repairs.

“A floating cofferdam is unique in many aspects. As Design Manager, I see the biggest challenge in developing a cost-effective design of a floating cofferdam that can meet the unique requirements for marine operations – including launching the cofferdam off a barge and sealing off against a floating concrete pontoon – and withstand a five-year storm event in its installed position,” says Yao.

COWI Marine North America has designed cofferdams for over 80 years, first as a heavy marine construction contractor and as an engineering consultant since the 1970s.

The opening of the new bridge is scheduled for 2016.​

LAST UPDATED: 16.09.2016