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Steel Designers' Manual - 6th Edition (2003)
376 Local buckling and cross-section classification
Fig. 13.3 Rectangular hollow section used as a beam
(b) b 1 < b/T < b 2, full plastic moment capacity Mp is attained but is only maintained
for small rotations and the member is suitable for elastic design using its full
capacity – compact cross-section (Class 2).
(c) b 2 < b/T £ b 3, full elastic moment capacity My (but not Mp) is attained and the
member is suitable for elastic design using this limited capacity – semi-compact
cross-section (Class 3).
(d) b 3 < b/T, local buckling limits moment capacity to less than My – slender crosssection
(Class 4).
The relationship between moment capacity Mu and compression flange slenderness
b/T indicating the various b limits is illustrated diagrammatically in Fig. 13.5.
In the figure the value of Mu for a semi-compact section is conservatively taken as
the moment corresponding to extreme fibre yield My for all values of b/T between
b 2 and b 3. This is more convenient for practical calculation than the more correct
representation shown in Fig. 13.4 in which a moment between My and Mp is
indicated. Since the classification of the section as plastic, compact, etc., is based on
considerations of the compression flange alone, the assumption concerning the web
slenderness d/t is that its classification is the same as or better than that of the flange.
For example, if the section is semi-compact, governed by the flange proportions,
then the web must be plastic, compact or semi-compact; it cannot be slender.
If the situation is reversed so that the webs are the controlling elements, then the
same four categories, based on the same definitions of moment–rotation behaviour,
are now determined by the value of web slenderness d/t. However, the governing
values of b 1, b 2 and b 3 change since the web stress distribution differs from the pure
compression in the top flange. Since the rectangular fully plastic condition, the triangular
elastic condition and any intermediate condition contain less compression,
the values of b are larger. Thus section classification also depends upon the type of