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7. There are two defnitions or the actor o saety: One as a ratio o absolute

strength (structural capacity) to actual applied load, this is a measure o the

reliability o a particular design. The other use o Fo is a constant !alue imposed

by law, standard, specifcation, contract or custom to which a structure must

conorm or e"ceed.

 The frst use (a calculated !alue) is generally reerred to as a actor o saety or,

to be e"plicit, a reali#ed actor o saety. The second use (a re$uired !alue) as a

design actor, design actor o saety or re$uired actor o saety. %owe!er,

between !arious industries and engineering groups usage is inconsistent and

conusing, it is important to be aware o which defnition(s) are being used. The

cause o much conusion is that !arious reerence boo&s and standards agencies

use the actor o saety defnitions and terms di'erently. esign codes and

structural and mechanical engineering te"tboo&s oten use Factor o aety to

mean the raction o total structural capability o!er that needed. *any

undergraduate trength o *aterials boo&s use Factor o aety as a constant

!alue intended as a minimum target or design (second use).+alculation

 There are se!eral ways to compare the actor o saety or structures. ll the

di'erent calculations undamentally measure the same thing: how much e"tra

load beyond what is intended a structure will actually ta&e (or be re$uired to

withstand). The di'erence between the methods is the way in which the !alues

are calculated and compared. aety actor !alues can be thought o as a

standardi#ed way or comparing strength and reliability between systems.

 The use o a actor o saety does not imply that an item, structure, or design is

sae. *any $uality assurance, engineering design, manuacturing, installation,

and end-use actors may inuence whether or not something is sae in any

particular situation.

esign actor and saety actor

 The di'erence between the saety actor and design actor (design saety actor)

is as ollows: The saety actor is how much the designed part actually will be

able to withstand (frst use rom abo!e). The design actor is what the item is

re$uired to be able to withstand (second use). The design actor is defned or

an application (generally pro!ided in ad!ance and oten set by regulatory code

or policy) and is not an actual calculation, the saety actor is a ratio o ma"imum

strength to intended load or the actual item that was designed.

  esign load being the ma"imum load the part should e!er see in ser!ice.

/y this defnition, a structure with a FO o e"actly 0 will support only the design

load and no more. ny additional load will cause the structure to ail. structure

with a FO o 1 will ail at twice the design load.

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2.