How Everything Works
How Everything Works How Everything Works

Question 206

Suspension bridges today can't oscillate like the Tacoma Bridge, can they?
Apparently not, because we've never seen or heard of one doing it. To prevent that sort of thing from happening, the bridge builders probably do two things. First, they damp all of the resonance in the bridge. By this, I mean that they introduce energy loss mechanisms that sap the energy out of all the resonant motions. For example, they could add plates that slide against one another as the bridge bends so that sliding friction will waste energy and spoil the resonant motion. Second, they make sure that there are no mechanisms for resonant energy transfer. The wind blowing on the Tacoma bridge gave it tiny pushes at just the right frequency. It oscillated the way a reed does in a musical instrument. These days, bridges are probably tested with computer modeling before they're built to make sure that they don't begin to oscillate when wind blows across them.

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