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Here are the basics of a floating bridge: Crews build watertight concrete pontoons connected end-to-end, and then place the roadway on top (there are also supplementary stability pontoons off to the side). The weight of the water displaced by the pontoons is equal to the weight of the structure and vehicles, allowing the bridge to float. Once the pontoons are in the water, crews will drop 58 anchors, made of reinforced concrete and attached to the pontoons via steel cables nearly 3 inches thick. A typical anchor can weigh as much as 10 male African elephants; they will sink into the loose material on the bottom of the lake to hold the pontoons in place. At each end of the bridge, the anchors will be drilled directly into the ground. Altogether, this system keeps the roadway from swaying.



But the science behind a floating bridge doesn’t start with displacement. It starts with the concrete mix. A typical pontoon measures 360 feet long, 75 feet wide and 28 feet tall. It’s as heavy as 23 Boeing 747s, but has to float. Not only that, only about 6 feet of the pontoon will stay above water. The rest will be pushed below the surface, subject to the wear and tear of salt water. (Editor's note: The pontoons are being built in salt water, but their destination, Lake Washington, is fresh water.)