How Everything Works
How Everything Works How Everything Works

Question 1160

What do engineers have to consider about waves when they are building bridges? — K
There are two answers to this question because there are two possible interpretations of the word "waves." If you mean waves in the water beneath the bridge, then naturally the engineers must plan for the forces exerted on the bridge by the moving water that flows around its surfaces. But a more interesting wave issue is waves in the bridge itself. The bridge's surface can experience waves, just as a taut rope or a long beam can have waves running through it. For example, when a heavy object drops on the surface of the bridge, a ripple heads outward along the bridge surface and doesn't stop completely until it reaches the ends of the bridge. In fact, the wave will reflect from various portions of the bridge and its effects may not disappear for many seconds after the incident that started the waves.

Most of the time, these waves aren't important and can be ignored. But occasionally some special event will cause enormous waves to begin traveling through a bridge. The classic example was the Tacoma Narrows Bridge in Washington State that collapsed in 1940 when wind-driven waves in its surface ripped it apart. The entire collapse was captured on film and is a fascinating to watch. When a large group of soldiers crosses a footbridge, they are often instructed to break step so that their rhythmic cadence doesn't excite intense waves that might damage the bridge. In general, modern bridges are engineered to dampen these waves—wasting their energy through friction or friction-like effects so that they die away quickly. While it might be fun to watch waves traveling along the surface of a bridge from a safe vantage point, you probably wouldn't want to be on a bridge when it was experiencing strong ones.


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