THE BRIDGE THAT BROKE

Figure 14.1a shows the Normandy Bridge under construction in France. When designing bridges, engineers must take into account the possibility of the wind causing a build-up of stationary waves, which may lead the bridge to oscillate violently. Famously, this happened in October 1940 to the Tacoma Narrows Bridge in Washington State, USA. High winds caused the bridge to vibrate with increasing amplitude until it fell apart (Figure 14.1b).
Did you know that the Tacoma Narrows Bridge fell apart because its natural frequency of oscillation matched the thumping frequency of the swirling wind? Do a web search for a videoclip of this momentous event.

Figure 14.1: a A suspension bridge under construction. b One that failed – the Tacoma Narrows Bridge.

The waves we have considered so far in Chapters 12 and 13 have been progressive waves; they start from a source and travel outwards, transferring energy from one place to another. A second important class of waves is stationary waves (standing waves). These can be observed as follows. Use a long spring or a plastic toy spring. A long rope or piece of rubber tubing will also do. Lay it on the floor and fix one end firmly. Move the other end from side to side so that transverse waves travel along the length of the spring and reflect off the fixed end (Figure 14.2). If you adjust the frequency of the shaking, you should be able to achieve a stable pattern like one of those shown in Figure 14.3. Alter the frequency in order to achieve one of the other patterns.

You should notice that you have to move the end of the spring with just the right frequency to get one of these interesting patterns. The pattern disappears when the frequency of the shaking of the free end of the spring is slightly increased or decreased.