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)
382 Local buckling and cross-section classification
only plastic sections are suitable: thus the designer’s choice is slightly restricted,
although no UBs and only 4 UCs in S275 steel and 7 UBs and 9 UCs in S355 steel
are outside the limits of BS 5950: Part 1 when used in pure bending. Although considerably
more sections are unsatisfactory if their webs are subject to high compression,
the number of sections barred from use in plastically designed portal
frames is, in practice, extremely small. Similarly for elastic design no UB is other
than semi-compact or better, provided it is not required to carry high compression
in the web, while all UCs are at least semi-compact even when carrying their full
squash load.
The designer should check the class of any trial section at an early stage. This can
be done most efficiently using information of the type given in Reference 2. For
webs under combined compression and bending the first check should be for pure
compression as this is the more severe. Provided the section is satisfactory no additional
checks are required; if it does not meet the required limit a decision on
whether it is likely to do so under the less severe combined load case must be made.
The economic use of cold-formed sections, including profiled sheeting of the type
used as decking and cladding, often requires that members are non-compact. Quite
often they contain plate elements that are slender, with the forming process being
exploited to provide carefully proportioned shapes. Since cold-formed sections are
proprietary products, manufacturers normally provide design literature in which
member capacities which allow for the presence of slender plate elements are listed.
If rigorous calculations are, however, required, then Parts 5 and 6 of BS 5950 contain
the necessary procedures.
When using fabricated sections the opportunity exists for the designer to optimize
on the use of material. This leads to a choice between three courses of action:
(1) eliminate all considerations of local buckling by ensuring that the width-tothickness
ratios of every plate element are sufficiently small;
(2) if employing higher width-to-thickness ratios, use stiffeners to reduce plate
proportions sufficiently so that the desired strength is achieved;
(3) determine member capacities allowing for reductions due to exceeding the
relevant compact or semi-compact limits.
Effectively only the first of these is available if plastic design is being used. For
elastic design when the third approach is being employed and the sections are
slender, then calculations inevitably are more involved as even the determination
of basic cross-sectional capacities requires allowances for local buckling effects
through the use of concepts such as the effective width technique. 1
References to Chapter 13
1. Bulson P.S. (1970) The Stability of Flat Plates. Chatto and Windus, London.
2. The Steel Construction Institute (SCI) (2001) Steelwork Design Guide to BS
5950: Part 1: 2000, Vol. 1: Section Properties. Member Capacities, 6th edn. SCI,
Ascot, Berks.