Steel Designers Manual - TheBestFriend.org

This material is copyright - all rights reserved. Reproduced under licence from The Steel Construction Institute on 12/2/2007
To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/
Steel Designers' Manual - 6th Edition (2003)
Cross-sectional dimensions and moment–rotation behaviour 375
Fig. 13.2 Local buckling of column flange
unlikely to affect significantly their load-carrying capacity when used as beams or
columns. Greater care is, however, necessary when using fabricated sections for
which the proportions are under the direct control of the designer. Also, coldformed
sections are often proportioned such that local buckling effects must be
accounted for.
13.2 Cross-sectional dimensions and moment–rotation behaviour
Figure 13.3 illustrates a rectangular box section used as a beam. The plate slenderness
ratios for the flanges and webs are b/T and d/t, and elastic stress diagrams for
both components are also shown. If the beam is subject to equal and opposite end
moments M, Fig. 13.4 shows in a qualitative manner different forms of relationship
between M and the corresponding rotation q.
Assuming d/t to be such that local buckling of the webs does not occur, which of
the four different forms of response given in Fig. 13.4 applies depends on the compression
flange slenderness b/T. The four cases are defined as:
(a) b/T £ b1, full plastic moment capacity Mp is attained and maintained for large
rotations and the member is suitable for plastic design – plastic cross-section
(Class 1).