The engineer chose a beam that will be able to withstand 10 times the load. This may sound similar, but consider this: Say a beam in a structure is required to have a design factor of 3. The design factor is defined for an application (generally provided in advance and often set by regulatory code or policy) and is not an actual calculation, the safety factor is a ratio of maximum strength to intended load for the actual item that was designed. The design factor is what the item is required to be able to withstand. The difference between the safety factor and design factor (design safety factor) is as follows: The safety factor is how much the designed part actually will be able to withstand. in the absence of highly accurate data, using the worst case configuration possible to make sure the system is adequate (to err on the side of caution).

There is a near universal push towards conservatism in the calculation of safety factors, i.e. Safety factor values can be thought of as a standardized way for comparing strength and reliability between systems. The difference between the methods is the way in which the values are calculated and compared. All the different calculations fundamentally measure the same thing, how much extra load beyond what is intended a structure will actually take (or be required to withstand). There are several ways to compare the factor of safety for structures. Many undergraduate Strength of Materials books use “Factor of Safety” as a constant value intended to be a minimum target for design (second sense).

Design Codes and Structural and Mechanical engineering textbooks often use the term to mean the fraction of total structural capability over that needed (first sense). The cause of much confusion is that reference books and standards agencies use the term factor of safety differently. Two definitions of factor of safetyĬareful engineers refer to the first sense (a calculated value) 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. The other use of FoS is a constant value imposed by law, standard, specification, contract orcustom to which a structure must conform or exceed. This is a measure of the reliability of a particular design. There are two distinct uses of the factor of safety: One as a ratio of absolute strength (structural capacity) to actual applied load.

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