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Factor of Safety

Factors are used in all engineering design to allow for variability in loading, material, workmanship etc..which can not be define with absolute certainty, here, the strength of the material or structural element must always exceed the effects of loadings by margin of its factor of safety.

Factor of safety = 
material strength/design load

or

Ultimate stress/actual stress

NB:-

Safety factor applied at one point in design is called overall safety factor,

Applied at several points is called partial safety factors.

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