Factor Of Safety

:S_design = S_yield / FoS
:S_design = S_proof / FoS

An appropriate factor of safety is chosen based on several considerations. Prime considerations are the accuracy of load and wear estimates, the consequences of failure, and the cost of overengineering the component to achieve that factor of safety. For example, components whose Failure could result in substantial financial loss, serious injury or death usually use a safety factor of four or higher (often ten). Non-critical components generally have a safety factor of two. An interesting exception is in the field of Aerospace Engineering , where safety factors are kept low (about 1.15 - 1.25) because the costs associated with structural weight are so high. This low safety factor is why aerospace parts and materials are subject to more stringent Test ing and Quality Control .

A Factor of safety of 1 implies no "overengineering" at all. Hence some engineers prefer to use a related term, Margin of Safety (MoS) to describe the design parameters. The relation between MoS and FoS is MoS = FoS - 1.

Example

In Construction Engineering the tensional stress σ is defined as σ = F / A where F is the force acting on the element and A is the Cross Section al Area . From Laboratory Testing it is known what the actual failure Tensile Stress σ_max of materials is. To find the minimum safe cross section of an element, the force acting on the element is multiplied with the safety factor γ, (its Magnitude depending on Building Code s and Regulation s). The minimum cross section is then found using A_min = F · γ / σ_max

What Factor of Safety is Not

The physicist Richard Feynman wrote about misconceptions concerning the "factor of safety" concept in The Pleasure of Finding Things Out. During his involvement with the Challenger disaster investigation, he came across the ridiculous assertion that the fatal rubber o-rings had a safety factor of 3 because, when exposed to thermal expansion and contraction, the resulting cracks only went a third of the way through them. If an engineer designs a bridge with a safety factor of 3, Feynman wrote, it means the bridge could theoretically support three times its expected load, not that the expected load would only result in it falling one-third apart.

See also

Information About ™

Factor Of Safety

Information About ^{^™}