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how do bridges not fall down
A bridge is a road standing on almost nothing, yet trucks roll over it all day. The secret: every bit of weight has a path down to the ground.
The simplest path is a beam across two supports. Loaded, it bends: the top gets squeezed, the bottom gets stretched. The farther apart the supports, the weaker it is.
An arch dodges the bending. Its curve turns the road's weight into a squeeze along the arch, and the abutments at each end stop the feet spreading.
A suspension bridge does the opposite: it hangs its deck from stretched cables. The towers get squeezed, and anchorages at the ends hold the cables.
But the right shape can still fail. The Tacoma Narrows Bridge, 2,800 feet across, opened in July 1940 and fell four months later in a wind of about forty miles an hour.
The common story says wind shook it in rhythm, resonance. Not so. Wind and the thin, flexible deck fed each other, a self-powered twisting called flutter.
So a bridge stays up by giving every load a path, squeezed or stretched where it can take it, and by testing how wind will move it, in wind tunnels.
A bridge never fights its weight; it hands it along, squeezed here and stretched there, all the way down to the ground.