<P> 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). </P> <P> There are several ways to compare the factor of safety for structures . 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). The difference between the methods is the way in which the values are calculated and compared . Safety factor values can be thought of as a standardized way for comparing strength and reliability between systems . </P> <P> The use of a factor of safety does not imply that an item, structure, or design is "safe". Many quality assurance, engineering design, manufacturing, installation, and end - use factors may influence whether or not something is safe in any particular situation . </P> <P> The difference between the safety factor and design factor (design safety factor) is as follows: The safety factor, or yield stress, is how much the designed part actually will be able to withstand (first "use" from above). The design factor, or working stress, is what the item is required to be able to withstand (second "use"). 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 . </P>

Necessity of partial safety factor in structural design