Forces which are derived from moving loads such as wind, earthquakes, machinery, vehicles, trains, cranes, and hoists. Analysis techniques which take into account the vibrations of the structures are required for loads which are repeated many times, such as machinery in motion, and produce harmonic motions of equal amplitude and constant frequency (cyclic loading); loads such as the motion produced by earthquakes (random motion); and varied loads, such as that of the wind, which produce gusts or short-duration impulses.
Repeated loads applied to a structural member can cause failure by fracture of the material. This fracture can occur at various stress levels depending upon the amplitude or acceleration of the motion, frequency, and duration. Often the stress level is below that of the design level for statically applied loads, and is referred to as the fatigue strength of the material for that application. See Stress and strain