Introduction - The Behaviour and Design of Steel Structures to EC3 - page 26

in his or her knowledge of the structure and its loads. Deliberate errors include
those resulting from the assumptions made to simplify the analysis of the loading
and of the structural behaviour. These assumptions are often made so that any
errors involved are on the safe side, but in many cases the nature of the errors
involved is not precisely known, and some possibility of danger exists.
Accidental errors include those due to a general lack of precision, either in
the estimation of the loads and the analysis of the structural behaviour, or in the
manufactureanderectionofthestructure.Thedesignerusuallyattemptstocontrol
the magnitudes of these, by limiting them to what he or she judges to be suitably
small values. Other accidental errors include what are usually termed blunders.
Thesemaybeofagrossmagnitudeleadingtofailureortouneconomicstructures,
ortheymaybelessimportant.Attemptsareusuallymadetoeliminateblundersby
using checking procedures, but often these are unreliable, and the logic of such a
process is open to debate.
As well as the errors described above, there exist a number of uncertainties
about the structure itself and its loads. The material properties of steel fluctuate,
especially the yield stress and the residual stresses. The practice of using a min-
imum or characteristic yield stress for design purposes usually leads to oversafe
designs,especiallyforredundantstructuresofreasonablesize,forwhichanaver-
age yield stress would be more appropriate because of the redistribution of load
which takes place after early yielding. Variations in the residual stress levels are
not often accounted for in design codes, but there is a growing tendency to adjust
designcriteriainaccordancewiththemethodofmanufacturesoastomakesome
allowance for gross variations in the residual stresses.This is undertaken to some
extent in EC3.
The cross-sectional dimensions of rolled-steel sections vary, and the values
given in section handbooks are only nominal, especially for the thicknesses of
universalsections.Thefabricatedlengthsofastructuralmemberwillvaryslightly
from the nominal length, but this is usually of little importance, except where
thevariationinducesadditionalstressesbecauseoflack-of-fitproblems,orwhere
thereisacumulativegeometricaleffect.Ofsomesignificancetomemberssubject
to instability problems are the variations in their straightness which arise during
manufacture, fabrication, and erection. Some allowances for these are usually
made in design codes, while fabrication and erection tolerances are specified in
EN1090 [47] to prevent excessive crookedness.
The loads acting on a structure vary significantly. Uncertainty exists in the
designer'sestimateofthemagnitudeofthedeadloadbecauseofthevariationsin
thedensitiesofmaterials, andbecauseoftheminormodificationstothestructure
during or subsequent to its erection. Usually these variations are not very signif-
icant and a common practice is to err on the safe side by making conservative
assumptions. Imposed loadings fluctuate significantly during the design usage of
thestructure, andmaychangedramaticallywithchangesinusage.Thesefluctua-
tionsareusuallyaccountedforbyspecifyingwhatappeartobeextremevaluesin
loadingcodes,butthereisoftenafinitechancethatthesevalueswillbeexceeded.