Aluminium Design and Construction John Dwight

Figure 8.20 Lateral-torsional buckling: (a) buckling occurs in a single bay over the whole
length (L) of the beam, no lateral support being provided by the load; (b) buckling
occurs in one of the bays (L 1 or L 2 ) between points of lateral support. S=lateral support.
the minor axis (‘side moment’), the combined effect of the two
components should be allowed for by using the interaction equation
given in Section 8.7.9
If effective lateral support is provided only at the end reaction points,
with the compression flange otherwise free to deflect sideways, buckling
is investigated by treating the whole span as a single unit or bay (Figure
8.20 (a)). Alternatively if one or more lateral supports are provided
along the length of the span, then the stability of each bay between
such points of support must be checked (Figure 8.20(b)). In this case,
the designer should also establish that these points really do provide
lateral support to the compression flange, if necessary by employing a
properly designed plan-view bracing system. The checking procedure
we give is broadly based on BS.8118.
8.7.2 Basic check
In order to check against possible failure by LT buckling, it is necessary
to ensure that the following requirement is met in every bay between
points of effective lateral support:
(8.30)
where: M – =equivalent uniform moment in the bay considered,
S=plastic modulus of gross section about major principal axis,
p b =LT buckling stress (Section 8.7.4),
� m =material factor (Section 5.1.3).
8.7.3 Equivalent uniform moment
M – is the magnitude of an assumed uniform moment that would have
the same effect, in terms of buckling, as the actual pattern of moment
on the bay considered. In a preliminary check, the designer can
conservatively take M – equal to the greatest moment arising in the bay.
For a more accurate value, it is necessary to consider the shape of the
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