Friday, August 3, 2007

The drama of bridges

In 1883, the Brooklyn Bridge became New York’s first major suspension bridge, at the time the longest in the world. It’s one of 2,027 bridges in New York City alone. Today 144,000 vehicles cross it daily.

There was great drama in the making of the Brooklyn Bridge. The bridge’s architect John Roebling never saw it completed: he died from an accident during construction. Graft and corruption were rampant. Dozens of workers died building the bridge, many of the bends, which was not well understood at the time. And during its opening, 12 people were trampled to death when someone yelled that the bridge was collapsing.

In all there are nearly 600,000 bridges in the US, and they usually come in four flavors:

Arch bridges, which the Romans built thousands of years ago out of stone;

Beam bridges, where a horizontal beam is supported at either end;

Truss bridges, like the one that collapsed in Minnesota, use triangles of steel to create a cantilevered central span; truss is actually more a type of construction and may be used in arch and beam bridges; and

Suspension bridges like the Brooklyn Bridge, are by far the longest, capable of spanning over a mile. They also have some interesting math: When cables hang from a suspension bridge before the descender cables are attached to the bridge deck that holds the roadway, they form a shape known as a catenary. What’s a catenary? A car driven with square wheels will ride smoothly when driven over a road with regular inverted catenary curves. After the bridge deck is attached, the cables then trace a parabola.

Before he died John Roebling insisted that the Brooklyn Bridge be made six times stronger than necessary, just in case. He wasn’t being hyperbolic; that’s why it’s still standing today. They just don’t make ‘em like they used to.

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