Definition
There are two distinct uses of the factor of safety: One as a ratio of absolute strength (structural capacity) to actual applied load. This is a measure of the reliability of a particular design. The other use of FoS is a constant value imposed by law, standard, specification, contract or custom to which a structure must conform or exceed.
The first sense (a calculated value) is generally referred to as a factor of safety or, to be explicit, a realized factor of safety, and the second sense (a required value) as a design factor, design factor of safety or required factor of safety, but usage is inconsistent and confusing. It is important to keep track of which of the two definitions is being used. The cause of much confusion is that various reference books and standards agencies use the factor of safety definitions and terms differently. Design codes and structural and mechanical engineering textbooks often use "Factor of Safety" to mean the fraction of total structural capability over that needed (first sense). Many undergraduate Strength of Materials books use "Factor of Safety" as a constant value intended as a minimum target for design (second sense).
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