Aluminium Design and Construction John Dwight

plates (if fitted), p v =limiting material stress for the web material in
shear, p v1 =buckling stress without tension-field action (Figure 8.11), and
v 2 =tension-field parameter (Section 8.3.6).
The section of the end-post must be adequate to resist M and V
simultaneously, when checked as in Section 8.4.
8.7 LATERAL-TORSIONAL BUCKLING
8.7.1 General description
When the compression flange of a deep beam is inadequately stabilized
against sideways movement, there is a danger of premature failure due
to lateral-torsional (LT) buckling, before the full moment resistance of
the cross-section has been reached (Figure 8.19). The compression flange
deflects sideways, dragging the tension flange with it, thus producing
a buckling mode that is a combination of lateral and torsional deformation
[15, 27].
As a very rough guide, it may be assumed that LT buckling will not
be critical if lateral supports to the compression flange are provided at
a spacing less than 40�ry , where ry is the minor axis radius of gyration
of the section, and �=�(250/po ). Nor will it be a factor for hollow shapes,
such as box sections, unless these are very deep and narrow. Generally,
it is found that LT buckling becomes serious for deep non-hollow shapes,
especially when the metal is thin.
If it is possible that LT buckling might be a critical factor, the adequacy
of any proposed design may be checked using the procedure given below.
The basic method considers the case of an applied moment acting about
the major principal axis of the section. If there is also some moment about
Figure 8.19 Lateral-torsional buckling.
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