Resonance

Resonance (a vibration in something caused by an external force that is in harmony with the natural vibration of the original thing) is a force which, unchecked, can be fatal to a bridge. Resonant vibrations will travel through a bridge in the form of waves. A very famous example of resonance waves destroying a bridge is the Tacoma Narrows bridge, which fell apart in 1940 in a 40-mph (64-kph) wind. Close examination of the situation suggested that the bridge's deck-stiffening truss was insufficient for the span, but that alone was not the cause of the bridge's demise. The wind that day was at just the right speed, and hitting the bridge at just the right angle, to start it vibrating. Continued winds increased the vibrations until the waves grew so large and violent that they broke the bridge apart.

When an army marches across a bridge, the soldiers are often told to "break step." This is to avoid the possibility that their rhythmic marching will start resonating throughout the bridge. An army that is large enough and marching at the right cadence could start a bridge swaying and undulating until it broke apart.

In order to mitigate the resonance effect in a bridge, it is important to build dampeners into the bridge design in order to interrupt the resonant waves. Interrupting them is an effective way to prevent the growth of the waves regardless of the duration or source of the vibrations. Dampening techniques generally involve inertia. If a bridge has, for example, a solid roadway, then a resonant wave can easily travel the length of the bridge. If a bridge roadway is made up of different sections that have overlapping plates, then the movement of one section is transferred to another via the plates, which, since they are overlapping, create a certain amount of friction. The trick is to create enough friction to change the frequency of the resonant wave. Changing the frequency prevents the wave from building. Changing the wave effectively creates two different waves, neither of which can build off the other into a destructive force.