Lake Washington which is between us and Seattle hosts three of the longest and heaviest floating bridges in the world. These bridges sit on top of concrete pontoons. I just hope the people building these things were paying attention in class and did all their calculations correctly.
Floating bridges are used because the bottom of Lake Washington is so soft that conventional bridges are too expensive to build; supporting towers would have to be 600+ feet tall.
The image shows the old 520 bridge and the new one under construction. At 7,710 feet, the new 520 bridge across the lake is the longest floating bridge in the world. It replaces a bridge that was 53 years old. The new bridge floats on 77 twenty eight foot high concrete pontoons. Each pontoon weighs 11,000 tons (yet it still floats)! The floating bridge is designed to handle a dramatic drop in water level. If the Ballard Locks failed the lake could drop 20 feet. Check out this article for more details and see what had to be built to help ducks landing in storm water collection spaces.
This article explains how the whole concept works.
Click here to read about the time part of the old bridge sank due to problems with the concrete pontoons. Scary!
More pictures and information here
- Own work Flickr copy: SR 520 Floating Bridge and replacement
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