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Steel Designers' Manual - 6th Edition (2003)
Chapter 13
Local buckling and
cross-section classification
by DAVID NETHERCOT
13.1 Introduction
The efficient use of material within a steel member requires those structural properties
which most influence its load-carrying capacity to be maximized.This, coupled
with the need to make connections between members, has led to the majority of
structural sections being thin-walled as illustrated in Fig. 13.1. Moreover, apart from
circular tubes, structural steel sections (such as universal beams and columns, coldformed
purlins, built-up box columns and plate girders) normally comprise a series
of flat plate elements. Simple considerations of minimum material consumption
frequently suggest that some plate elements be made extremely thin but limits
must be imposed if certain potentially undesirable structural phenomena are to
be avoided. The most important of these in everyday steelwork design is local
buckling.
Figure 13.2 shows a short UC section after it has been tested as a column. Considerable
distortion of the cross-section is evident with the flanges being deformed
out of their original flat shape. The web, on the other hand, appears to be comparatively
undeformed. The buckling has therefore been confined to certain plate elements,
has not resulted in any overall deformation of the member, and its centroidal
axis has not deflected. In the particular example of Fig. 13.2, local buckling did not
develop significantly until well after the column had sustained its ‘squash load’ equal
to the product of its cross-sectional area times its material strength. Local buckling
did not affect the load-carrying capacity because the proportions of the web and
flange plates are sufficiently compact. The fact that the local buckling appeared in
the flanges before the web is due to these elements being the more slender.
Terms such as compact and slender are used to describe the proportions of the
individual plate elements of structural sections based on their susceptibility to local
buckling. The most important governing property is the ratio of plate width to plate
thickness, b, often referred to as the b/t ratio. Other factors that have some influence
are material strength, the type of stress system to which the plate is subjected,
the support conditions provided, and whether the section is produced by hot-rolling
or welding.
Although the rigorous treatment of plate buckling is a mathematically complex
topic, 1 it is possible to design safely and in most cases economically with no direct
consideration of the subject. For example, the properties of the majority of standard
hot-rolled sections have been selected to be such that local buckling effects are
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