Skip to content
Wed, Oct 14, 2026 / Ideas for a clearer worldINDEPENDENT INSIGHTS · PRACTICAL TOOLS · EXPLAINERS
Sign in
EXPLOREArtificial IntelligencePersonal FinancePythonStartupsMechanical EngineeringDigital MarketingCybersecurity
Factor of Safety / August 16, 2021

Factors of Safety

Factors of Safety – FOS – are important for engineering design Factors of Safety – FOS – are a part of engineering design and can for structural engineering typically be expressed asContentsFactors of Safety – FOS – are important for engineering designExample – Structural Steel Column in a BuildingTypical overall Factors of SafetyGeneral recommendationsFactor of Safety related […]

By · August 16, 2021 · 3 min read

Factors of Safety – FOS – are important for engineering design

Factors of Safety – FOS – are a part of engineering design and can for structural engineering typically be expressed as

Contents

FOS = Ffail / Fallow                                   (1)

where

FOS = Factor of Safety

Ffail = failure load (N, lbf)   

Fallow   = allowable load (N, lbf)     

Example – Structural Steel Column in a Building

Due to buckling the failure load of a steel column in a building is estimated to 10000 N. With a safety factor FOS = 5 – the allowable load can be estimated by rearranging (1) to

Fallow = Ffail / FOS                    (1b)

Fallow = (10000 N) / 5

         = 2000 N

Typical overall Factors of Safety

Typical overall Factors of Safety:

EquipmentFactor of Safety
– FOS – 
Aircraft components1.5 – 2.5
Boilers3.5 – 6
Bolts8.5
Cast-iron wheels20
Engine components6 – 8
Heavy duty shafting10 – 12
Lifting equipment – hooks ..8 – 9
Pressure vessels3.5 – 6
Turbine components – static6 – 8
Turbine components – rotating2 – 3
Spring, large heavy-duty4.5
Structural steel work in buildings4 – 6
Structural steel work in bridges5 – 7
Wire ropes8 – 9

Design Factors of Safety are often published in technical standards but there is no dedicated standard to the subject.

Note that for statutory items such as cranes and pressure vessels FOS are specified in the design codes.

General recommendations

ApplicationsFactor of Safety
– FOS – 
For use with highly reliable materials where loading and environmental conditions are not severe and where weight is an important consideration1.3 – 1.5
For use with reliable materials where loading and environmental conditions are not severe1.5 – 2
For use with ordinary materials where loading and environmental conditions are not severe2 – 2.5
For use with less tried and for brittle materials where loading and environmental conditions are not severe2.5 – 3
For use with materials where properties are not reliable and where loading and environmental conditions are not severe, or where reliable materials are used under difficult and environmental conditions3 – 4

In general there is a linear connection between load and stress and the factor of safety can within mechanical engineering for normal stress be modified to

FOS = σfail / σallow                            (2)

where

σfail = failure normal stress (N/m2, psi)

σallow = allowable normal stress (N/m2, psi)   

FOS for shear stress can be expressed as

FOS = τfail / τallow                            (3)

where

τfail = failure shear stress (N/m2, psi)

τallow = allowable shear stress (N/m2, psi)   

Be aware that in some cases there may not be a linearity between applied load and stress.