What is Factor of Safety of Structures?

Factor of Safety (FOS) is an important design parameter used in the design of engineering structures. In structural engineering FOS is expressed as the ratio of Failure load to the allowable load. Therefore,

FOS = Ffail / Fallowable

Hence, factor of safety can be defined as the load carrying capacity of the structural system beyond its actual or allowable load carrying capacity. It is a answer to the question , "How much stronger the structure is than what is required".

The Dickinson Press


Features of Factor of Safety of Structures (FOS)


FOS is the basckbone of all the structures and safety equipment. For more reliability, every structure is built more stronger than what is required. This is to tolerate any unexpected extra load. This extra capacity is factor of safety. 

Also Read: What is difference between Characteristic Compressive Strength and Target Mean Strength? 

How to Determine Factor of Safety (FOS)?


The parameters used by different engineering materials to determine FOS are different. Brittle material use ultimate strength and ductile material use yield strength to determine FOS. Yield strength therefore determine the FOS until the start of deformation. Ultimate strength determines the FOS until failure.

In case of ductile materials, yield and ultimate strength is used to determine FOS. While brittle materials only calculate ultimate safety factor as the yield and ultimate values are close.


The factor of safety of the structure demanded choose the material used. If the building structure requires high FOS, a ductile material is used.


What is Realized Factor of Safety?


Required safety factor or the design factor is the constant value imposed by the law or the standards. This is the minimum factor value to be satisfied. 
Realized Safety factor is the ratio of structural capacity to the actual applied load. It is developed based on the reliability of the design.




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