Compression members - The Behaviour and Design of Steel Structures to EC3 - page 62

3.5 Design of compression members

3.5.1 EC3 design buckling resistance

Real compression members may be analysed using a model in which the initial
crookednessandloadeccentricityarespecified.Residualstressesmaybeincluded,
andtherealisticstress-strainbehaviourmaybeincorporatedinthepredictionofthe
loadversusdeflectionrelationship.ThuscurveCinFigure3.12maybegenerated,
and the maximum load N max ascertained.
Rationalcomputeranalysesbasedontheabovemodellingarerarelyusedexcept
inresearch,andareinappropriatefortheroutinedesignofrealcompressionmem-
bersbecauseoftheuncertaintiesandvariationsthatexistintheinitialcrookedness
and residual stresses. Instead, simplified design predictions for the maximum
load (or resistance) are used in EC3 that have been developed from the results of
computer analyses and correlations with available test data.
A close prediction of numerical solutions and test results may be obtained by
usingequations3.11and3.12whicharebasedonthefirstyieldofageometrically
imperfect member. This is achieved by writing the imperfection parameter η of
equation 3.12 as

η = α(λ − 0.2 ) ≥ 0 (3.21)

in which α is a constant (imperfection factor) which shifts the resistance curve as
shown in Figure 3.13 for different cross-section types, proportions, thicknesses,
buckling axes, and material strengths (Table 6.2 of EC3). The advantage of this
approach is that the resistances (referred to as buckling resistances in EC3) of a
particular group of sections can be determined by assigning an appropriate value

1.2

Curve d

1

Curve c

0.8

Curve b

0.6

Curve a

0.4

Curve a 0

0.2

0

0 0.5 1 1.5 2 2.5

Generalised slenderness = N /

y N cr

Figure 3.13 Compression resistances of EC3.