Dynamic loads can present special problems for the bridge designer. A bridge has to be able to withstand not only the forces of normal, everyday traffic, but also unusual forces of unexpected magnitude. In California, as an example, bridges require special kinds of reinforcement to withstand possible earthquakes. The fact that engineers have not completely solved the problems presented by dynamic loads is reconfirmed from time to time. For example, during the 1989 earthquake in the San Francisco Bay area, a section of the San Francisco-Oak-land Bay Bridge collapsed, leaving a gaping hole. A freeway overpass in Oakland also failed during the earthquake, taking the lives of about two dozen motorists.
Wind gusts have been responsible for a number of bridge failures in the past. Even if wind speeds are relatively low, dynamic loads may become too great for a bridge to withstand. One reason for this phenomenon is that the bridge may begin to vibrate so violently that it actually shakes itself apart. Such was the case, for example, with the Tacoma Narrows Bridge in 1941. On November 7 of that year, with wind speeds registering only about 40 mph (25 kph), the bridge vibrated so badly that it collapsed. The actual force experienced by the bridge was considerably less than the dead and live forces for which it had been designed. But the oscillations produced by wind gusts on the day in question were sufficient to shake the bridge apart.
As a result of failures such as those in the Bay Bridge and the Tacoma Narrows Bridge, engineers have developed methods for making bridges more aerodynamically sound. For example, lighter materials arranged in geometric structures that are aerodynamically more stable are now used in bridges where earthquakes, wind gusts, or other unusually severe environmental problems can be expected.