Factor of safety

Factors of safety (FoS), also known as (and used interchangeably with) safety factor (SF), is a term describing the load carrying capacity of a system beyond the expected or actual loads. Essentially, the factor of safety is how much stronger the system is than it usually needs to be for an intended load. Safety factors are often calculated using detailed analysis because comprehensive testing is impractical on many projects, such as bridges and buildings, but the structure's ability to carry load must be determined to a reasonable accuracy.

Many systems are purposefully built much stronger than needed for normal usage to allow for emergency situations, unexpected loads, misuse, or degradation (reliability).

There are two definitions for 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 use (a calculated value) is generally referred to as a factor of safety or, to be explicit, a realized factor of safety. The second use (a required value) as a design factor, design factor of safety or required factor of safety. The realized factor of safety must be greater than the required design factor of safety. However, between various industries and engineering groups usage is inconsistent and confusing, it is important to be aware of which definition(s) are 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 and are realized factor of safety (first use). Many undergraduate Strength of Materials books use "Factor of Safety" as a constant value intended as a minimum target for design (second use).

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