This section is from the book "Safe Building", by Louis De Coppet Berg. Also available from Amazon: Code Check: An Illustrated Guide to Building a Safe House.
If a load or strain is applied to a material and then removed, the material is supposed to recover its first condition (provided it has not been strained beyond the limit of elasticity). This practically, however, is not the case, and it is found that a small load or strain often applied and removed will do more damage (fatigue the material more) than a larger one left on steadily. Most loads in buildings
Wind Pressure and Snow.
are stationary or "dead" loads. But where there are "moving" loads, such as people moving, dancing, marching, etc., or machinery vibrating, goods being; carted and dumped, etc., it is usual to assume larger loads than will ever be imposed; sometimes going far as to double the actual intended load, or what amounts to the same thing, doubling (or increasing) the factor-of-safety, in that case retaining, of course, the actual intended load in the calculations. This is a matter in which the architect must exercise his judgment in each individual case.