Popular Mechanics Teaches You How to Build a Floating 520 Bridge

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Did you know that each of the 58 anchors that hold the floating 520 Bridge "can weigh as much as 10 male African elephants"? Neither did I until I read today's piece in Popular Mechanics.

The engineering mag profiled Seattle's floating bridge--the longest in the world--focusing on the redesign that will, when completed in 2014, make it longer, wider, taller and generally more awesome.

A sample:

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.

As one might expect, building the world's largest floating bridge is a complicated affair. Concrete pontoons can crack, underwater support structures can erode, waves can crash over the side.

But with a 200-foot-plus deep lake, building the bridge in any kind of conventional way just won't work. All this, of course, is known to most locals here in Seattle.

But PopMech, as it's so famous for doing, manages to illuminate many of the most interesting details about the redesign.

For example:

--77 concrete pontoons will support the new bridge, each about 360 feet long, 75 feet wide and as heavy as 23 Boeing 747s.

--It took $2.8 million worth of testing to come up with the salt-water-resistant fly ash and microsilicia concrete mixture that is used on the bridge.

--The new bridge will be raised seven feet higher from the water's surface than it is now, making able to stay open during wind storms up to 92 MPH.

The whole thing is fascinating and I can't wait to see the Discovery Channel's inevitable Build It Bigger episode about the bridge.

 
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