Nib asked
How do bridges not fall down?
A bridge never fights its weight, it hands it down a path to the ground as squeezing and stretching its parts can take, and engineers also test how wind will move it.
7 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1the puzzle
A bridge looks like a road held up by almost nothing, yet it carries trucks all day. It works because every bit of weight is handed along a path of beams, arches or cables until it reaches the ground.
A bridge stays up by giving its weight a path to the ground.
check yourselfWhat does a bridge have to do with the weight of its traffic?
2a beam
Lay a beam across two supports and put a load on it. The beam bends a little, and the bending squeezes the top of the beam (compression) and stretches the bottom (tension). The farther apart the supports, the weaker a simple beam becomes, which is why beam bridges stay short or add trusses.
A loaded beam is squeezed on top and stretched underneath.
check yourselfWhich side of a bending beam is stretched?
3an arch
An arch is curved so that the weight on it flows along the curve as a squeeze, with no stretching at all. The two ends push outward as well as down, so the abutments, the solid supports at each end, hold the feet in place and stop them splaying apart.
An arch carries its load as a squeeze and the abutments stop its feet spreading.
check yourselfWhat stops an arch's feet from spreading outward?
4a cable
A suspension bridge hangs its deck from big cables. The cables are stretched tight, in tension, and pass the pull to the towers and to anchorages at each end. The towers are squeezed from above by the weight, and the anchorages hold the cable ends so they cannot pull in.
A suspension bridge's cables are stretched, its towers are squeezed, and anchorages hold the ends.
check yourselfAre the cables of a suspension bridge squeezed or stretched?
5Tacoma, 1940
The Tacoma Narrows Bridge in Washington state opened on July 1, 1940. Its main span was 2,800 feet. On November 7, 1940, in a wind of about 40 miles an hour, its deck began to twist violently and the bridge fell.
A bridge on paper can still fail: Tacoma Narrows fell four months after opening.
check yourselfHow long did the Tacoma Narrows Bridge last?
6not resonance
Many books say the wind pushed the bridge in time with its natural rhythm, which is resonance. Engineers now know that is not what destroyed it. The deck was thin and flexible, and its shape made the wind feed energy into a twisting motion that grew by itself. That self-feeding twist is called aeroelastic flutter.
Tacoma Narrows fell to flutter, a self-feeding twist of the deck, not simple resonance.
a common mix-upIt is a popular myth that resonance from the wind's rhythm destroyed the Tacoma Narrows Bridge. It was aeroelastic flutter.
check yourselfWhat fed the twisting of the Tacoma Narrows deck?
7the keepsake
Every bridge gives its weight a path to the ground: beams bend, arches squeeze, cables stretch and towers are squeezed. And because of Tacoma Narrows, engineers test how a bridge will move in the wind, using models in wind tunnels, before it is built.
A path for every load, in parts that can take it, plus wind testing.
A bridge never fights its weight; it hands it along, squeezed here and stretched there, all the way down to the ground.