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Factors of Safety

Factors of safety (FOS) are an important part of engineering design used to account for uncertainties. The FOS is defined as the failure load divided by the allowable load. A steel column example shows a FOS of 5, giving an allowable load of 2000N. Typical FOS values range from 1.3-20 depending on the component and material. FOS can also be expressed as the failure stress divided by the allowable stress. The FOS accounts for uncertainties in materials, loading conditions, and manufacturing.

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351 views3 pages

Factors of Safety

Factors of safety (FOS) are an important part of engineering design used to account for uncertainties. The FOS is defined as the failure load divided by the allowable load. A steel column example shows a FOS of 5, giving an allowable load of 2000N. Typical FOS values range from 1.3-20 depending on the component and material. FOS can also be expressed as the failure stress divided by the allowable stress. The FOS accounts for uncertainties in materials, loading conditions, and manufacturing.

Uploaded by

Denny1990
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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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 as

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:

Factor of Safety
Equipment
- FOS -

Aircraft components 1.5 - 2.5

Boilers 3.5 - 6

Bolts 8.5

Cast-iron wheels 20

Engine components 6-8


Factor of Safety
Equipment
- FOS -

Heavy duty shafting 10 - 12

Lifting equipment - hooks .. 8-9

Pressure vessels 3.5 - 6

Turbine components - static 6-8

Turbine components - rotating 2-3

Spring, large heavy-duty 4.5

Structural steel work in buildings 4-6

Structural steel work in bridges 5-7

Wire ropes 8-9

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

Factor of Safety
Applications
- FOS -

For use with highly reliable materials where loading and environmental conditions are not severe and where weight
1.3 - 1.5
is an important consideration

For use with reliable materials where loading and environmental conditions are not severe 1.5 - 2

For use with ordinary materials where loading and environmental conditions are not severe 2 - 2.5

For use with less tried and for brittle materials where loading and environmental conditions are not severe 2.5 - 3

For use with materials where properties are not reliable and where loading and environmental conditions are not
3-4
severe, or where reliable materials are used under difficult and environmental conditions
Factor of Safety related to Stress
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.

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