Factor of safety

The ratio of ultimate load to the greatest allowable working load. This term is losing favor with engineers, as its use has been abused. There is no such thing as a factor of safety for a well proportioned bridge, for each member should have an intensity of working stress proportional to the character and amount of work which it has to perform. This is best accomplished by adding to the live load stress a certain varying proportion thereof to allow for the effect of impact. Same as safety factor.