How to Build the World’s Longest Floating Bridge

The floating bridge on Seattle?s Route 520 already has the distinction of being the longest one in the world. Now crews are preparing to overhaul the bridge and make it even longer. If you?re wondering how they?ll get a quarter-million tons of concrete to float, we found out. By Tim Newcomb

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Hulking engineering projects built with 230,000 tons of concrete generally don?t float. Washington state, however, knows a little about how to make so much mass actually buoyant.

Washington owns the four longest floating bridges in the world. The longest, State Route 520, connects Seattle to points east over Lake Washington, which dips to 214 feet deep. That means a straight-line suspension bridge is out of the equation, and a suspension bridge in water that deep would require a bridge tower the height of Seattle?s Space Needle. Conventional bridges have proven too expensive to build in deep waters with soft beds, so floating a bridge over Lake Washington pencils out as the only possible solution to carry the roughly 115,000 daily vehicles.

Now the Route 520 bridge is getting even longer. And as Washington State?s Department of Transportation (WSDOT) starts reconstructing the world?s longest floating bridge, originally built in 1963, it has spent years developing a new bridge design and even a specialized concrete and concrete-pouring method, setting a new standard for floating bridges worldwide. The new bridge, which should open by 2014, will extend the span from 7578 feet (about 1.5 miles) to 7710 feet, and feature 77 concrete pontoons serving as the foundation for a six-lane, 116-foot-wide bridge deck.

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