Factor of Safety
ƒ The factor of safety is often specified in a design code or standard, such as:
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‰
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Codes often specify a minimum factor of safety
Designer’s responsibility to determine if a code or standard applies. Codes are
often specified by law. (BOCA, UBC, etc.)
Factors which affect the factor of safety:
‰ Material strength basis:
¾ brittle Materials – use ultimate strength
¾ ductile Materials – use yield strength
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American Institute of Steel Construction (AISC) – steel buildings &
bridges
American Society of Mechanical Engineers (ASME) – pressure vessels,
boilers, shafts.
American Concrete Institute (ACI)
National Forest Products Association (NFPA) – wood structures.
Aluminum Association (AA) – aluminum buildings & bridges.
Manner or loading:
¾ Static – applied slowly; remains applied or is infrequently removed.
¾ Repeated – fatigue failure may occur at stresses lower than static load
failure.
¾ Impact – high initial stresses develop.
Possible misuse – designer must consider any reasonable foreseeable use &
misuse of product.
Complexity of stress analysis – the actual stress in a part isn’t always known.
Environment – temperature, weather, radiation, chemical, etc.
More factors:
‰ Hazard of failure – what happens if the parts fail? What are the
consequences?
‰ Costs – compromises sometimes have to be made. Never compromise for
safety when danger to life and property exist.
‰ Weight – what market segment is this an issue?
‰ Market segment – how does factor of safety, N, different in these market
segments?
ƒ Truck/bus industry
ƒ Off-highway industry
ƒ Aerospace industry
ƒ
We will use the following factors of safety:
Table 3-2, Pg. 89
Manner of
Loading
Static
Repeated
Impact
Design Factor, N
Ductile Materials
Sy basis
Su basis
2
----8
--12
Brittle Materials
Su basis
6
10
15
Review of Chapter 3:
(Section 3-3 Design Normal Stress, σ d ):
¾ When designing Mechanical components the design stress (also called
allowable or working stress) must be determined so the member can be
adequately sized. The design stress is based on factor of safety, N. Factor
of safety is either stated in problem or refer to tables (i.e. 3-2) or use
engineering judgment based on above!
Equations for design stress, σ d :
σ d = Sy/N
(based on yield strength)
(3.2)
(based on ultimate strength)
(3.3)
Or
σ d = Su/N
Sy & Su are Material properties determined from previous discussion.
N is the factor of safety.
- In most s of m problems encountered, N will either be stated in the problem or refer
to tables (i.e. table 3-2) or use engineering judgment based on above! However, in
the professional world, if designing a component covered by a code, it is the
Designer’s responsibility to use appropriate code (i.e. pressure vessels, steel
structures). If not covered by code, the Designer must use good engineering
judgment to determine appropriate N.
(Section 3-3 Design Shear Stress, τd):
If failure mode is shear (i.e. member loaded in shear such as rivet shown) the designer
must design to a design shear stress given by:
P
P
τd = Sys/N
(based on yield strength in shear)
(3-6)
ƒ Methodology for determining N analogous to normal stress (i.e. previous discussion).
ƒ See table 3-4 for general guidelines for determining N based on shear stress.
ƒ Note, the shear material properties can be determined from tensile properties using
the following estimates:
¾ Sys (yield strength in shear) = 0.5 Sy (Yeild σ , tension)
¾ Sus (ultimate strength in shear) = ⊄ Su (Ultimate σ , tension)
From Table 3.5: ⊄ = .65 for aluminum, = .82 for steel
= .90 for iron/copper = 1.30 for gray cast iron
Read section 3-8 Design Bearing Stress