Local buckling of thin-plate elements - The Behaviour and Design of Steel Structures to EC3 - page 129

1 2 3 4 Section classification

Beams W el f y

M p

Beams M p

N y , M y

N Rd , M Rd ∝1/

Columns

N y

Not to scale

0 1 2 3 = b / t

Flange outstands 9 10 14

Simply
supported
flanges

33 38 42

Figure 4.28 EC3 section resistances.

while inelastic moment redistribution takes place in the beam. Class 2 sections
satisfy

λ 1 ε ≤ b / t ≤ λ 2 ε

(4.60)

inwhich λ 2 ε istheappropriateClass2slendernesslimitgiveninTable5.2ofEC3
(somevaluesof λ 2 ε areshowninFigures4.28and5.33).Theselimitsareslightly
greater than those implied in equations 4.12 and 4.13 for the strain-hardening
buckling of inelastic plates.
Class 3 elements are able to reach first yield, but buckle locally before they
become fully plastic. Class 3 elements satisfy

λ 2 ε ≤ b / t ≤ λ 3 ε (4.61)

in which λ 3 ε is the appropriate yield slenderness limit given in Table 5.2 of EC3
(somevaluesof λ 3 ε areshowninFigures4.28and5.33).Theselimitsareclosely
related to equations 4.18 used to define the effective widths of flange plates in
uniform compression or modified from equation 4.37 for web plates in bending.
Class 4 elements buckle locally before they reach first yield, and satisfy

λ 3 ε< b / t . (4.62)

Beam cross-sections are classified as being Class 1, Class 2, Class 3, or Class 4,
depending on the classification of their elements. AClass 1 cross-section has all
of its elements being Class 1. A Class 2 cross-section has no Class 3 or Class 4
elements and has at least one Class 2 element, while a Class 3 cross-section has
no Class 4 elements and at least one Class 3 element.AClass 4 cross-section has
at least one Class 4 element.