BUILDING BIG: Databank: Garabit Viaduct

  Garabit Viaduct
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Vital Statistics:
Location: Massif Central, France
Completion Date: 1884
Length: 1,853 feet
Type: Arch
Purpose: Railway
Materials: Wrought iron
Longest Single Span: 541 feet
Engineer(s): Gustave Eiffel

In the late 1800s, a mountainous barrier blocked the railways from reaching Southern France. For years, engineers tried to figure out a way to bridge the windy Garabit Valley in France's Massif Central. Finally, one of the era's best engineers, Gustave Eiffel, came up with a brilliant solution. He built a huge wrought-iron arch in record time with just a minimal amount of material. How did he do it?

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Rather than building his bridge with thick, solid beams, Eiffel used beams with lots of holes -- holes in the shapes of triangles. Eiffel knew that if his bridge was made of thick, solid beams, it would be very heavy and the beams would rattle in the wind. But if he used a series of open triangles, called a truss, the gusty wind in the valley would blow right through them. Not only is the truss pattern lightweight; it�s very stable as well. Depending upon the position of a train on the bridge, the connecting vertical and diagonal segments are pulled into tension and pushed into compression -- forces that resist one another. A push on one segment is resisted by an opposite pull from another, all along its length. So the bridge remains strong and rigid, despite its lightness.

Here's how this bridge stacks up against some of the longest-spanning bridges in the world. (total length, in feet)