Pontoon bridgeIn 1940, the first pontoon bridge of reinforced concrete in the U.S. was dedicated watched by a crowd of 2,000. Construction on the Lake Washington Floating Bridge, Seattle, Wash., had begun on 29 Dec 1938. Its total length of 34,021 feet included 25 pontoons bolted together, each having two or more 65-ton anchors making a 6,620-foot floating span attached to fixed approach spans. When Homer M. Hadley had first presented the idea of a floating concrete bridge spanning Lake Washington, people were dubious. But from his experience working in a shipyard during World War I, Hadley knew that concrete could be made to float. Building a floating bridge would be easier than trying to place piers in water 200 feet deep, with another 100 feet of soft clay on the lake bottom. The four-lane concrete highway bridge was anchored with steel cables to resist wind and waves, and hydraulic jacks to let out or take up the slack. It was the first floating draw span in the world, with a 200-foot section designed to allow vessels to pass through. Two 75-horsepower motors were used to open the span in 90 seconds. Fifty years after it was built, water from a heavy rainstorm filled the pontoons and the floating bridge sank into Lake Washington on 25 Nov 1990. |
