Introduction - The Behaviour and Design of Steel Structures to EC3 - page 27

Windloadsvarygreatlyandthemagnitudesspecifiedinloadingcodesareusually
obtained by probabilistic methods.

1.7.3 Strength design

1.7.3.1 Load and capacity factors, and factors of safety

The errors and uncertainties involved in the estimation of the loads on and the
behaviour of a structure may be allowed for in strength design by using load fac-
tors to increase the nominal loads and capacity factors to decrease the structural
strength.Inthepreviouscodesthatemployedthetraditionalworkingstressdesign,
thiswasachievedbyusingfactorsofsafetytoreducethefailurestressestopermis-
sible working stress values.The purpose of using various factors is to ensure that
theprobabilityoffailureunderthemostadverseconditionsofstructuraloverload
andunderstrengthremainsverysmall.Theuseofthesefactorsisdiscussedinthe
following subsections.

1.7.3.2 Working stress design

The working stress methods of design given in previous codes and specifications
required that the stresses calculated from the most adverse combination of loads
must not exceed the specified permissible stresses. These specified stresses were
obtained after making some allowances for the non-linear stability and material
effectsonthestrengthofisolatedmembers,andineffect,wereexpressionsoftheir
ultimate strengths divided by the factors of safety SF.Thus

Working stress ≤ Permissible stress ≈ Ultimate stress
SF

(1.6)

It was traditional to use factors of safety of 1.7 approximately.
Theworkingstressmethodofaprevioussteeldesigncode[48]hasbeenreplaced
bythelimitstatesdesignmethodofEC3.Detaileddiscussionsoftheworkingstress
method are available in the first edition of this topic [49].

1.7.3.3 Ultimate load design

Theultimateloadmethodsofdesigningsteelstructuresrequiredthatthecalculated
ultimateload-carryingcapacityofthecompletestructuremustnotexceedthemost
adverse combination of the loads obtained by multiplying the working loads by
the appropriate load factors LF.Thus

( Working load × LF ) ≤ Ultimate load (1.7)

These load factors allowed some margins for any deliberate and accidental
errors,andfortheuncertaintiesinthestructureanditsloads,andalsoprovidedthe