Factor of Safety (FoS) and Margin of Safety (MoS)

In this blog you will learn about the Factor of Safety (FoS) and Margin of Safety (MoS) that is used in the design of Civil Engineering Structures e.g Buildings, Bridges, Over-head Tanks etc.

Factor of Safety (FoS)

It is the load carrying capacity of any structure or system beyond the expected or actual loads.

Factor of Safety (FoS) used in the design of any civil engineering structures e.g Buildings, Bridges, Over-Head tanks and other RCC, Steel, Wood and composite structures.

The Term Ultimate Stress and Allowable Stress is can be expressed as:

Ultimate Stress = Ultimate Load, Failure Load

Allowable Stress = Allowable load, Design Load

Purpose of FoS

The main purpose of FoS to take some extra loads or to resist some accidental loads by the structure that may occur during the life of that structure. So the structure designer intentionally built the structure stronger then the allowable load. The accidental loads may be due to Earthquakes, Wind, Snow or any other dead or live load that is unexpected in the structure.

Mathematical Representation of FoS

Factor of Safety (FoS) = (Ultimate Stress)/(Allowable Stress)

FoS= ðus/ðas Eq (1)


ðus= 600 N/mm2, ðas= 300 N/mm2

Put the values of ðus and ðas in the above Eq (1)

FoS = (600)/(300) = 2

FoS = 2 , mean that the structure can still take load twice of Allowable Load.

You can derive any value from Eq (1) if there is any two of them are known.

Margin of Safety (MoS)

MoS can be simply defined as it is the margin/gap provided from the allowable stress to ultimate stress. Margin of Safety (MoS) totally depends onFactor of Safety (FoS).

Purpose of MoS

The main purpose of MoS to add some extra margin to adjust or resist an accidental loads by the structure that may occur due to any reason in the structure.Through Margin of Safety (MoS) the Structural designer store some reserve capacity in the structure to take extra loads beyond the allowable load. Those Structures whose margin of safety (MoS) is equal or greater than the value one are more resistant to accidental loads. The MoS value adds the margin to structure to take some additional loads.

Mathematical Representation of MoS

Margin of Safety (MoS) = (Failure Load)/( Design Load) – 1

FoS = (Failure Load)/( Design Load)

Mos = FoS – 1

if FoS = 2

MoS = 2-1 = 1

So MoS = 1 , mean that the structure has still reserve the load carrying capacity upto 100% more than the allowable load.


Author: Engineer-Nisar

Civil Engineer

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