Aluminium Design and Construction John Dwight

Figure 7.7 shows curves of � ° plotted against �, covering the nonwelded
and edge-welded cases.
7.2.5 Very slender outstands
By a ‘very slender’ plate element we mean one of high ß that is able to
develop extra strength after the initial onset of buckling (� m > � cr ) (Figure
7.3). Expressions (7.5) and (7.7) take advantage of the post-buckled reserve
of strength in such elements. In the case of a very slender outstand, such
an approach is sometimes unacceptable, because of the change in the
stress pattern in the post-buckled state, whereby load is shed from the
tip of the outstand to its root (curve 1 in Figure 7.5). The effective minor
axis stiffness of an I-section or channel containing very slender flanges is
thereby seriously reduced, because the flange tips become progressively
less effective as buckling proceeds. Also, with the channel, there is the
possibility of an effective eccentricity of loading, as the centre of resistance
for the flange material moves towards the connected edge. Both effects
tend to reduce the resistance of the member to overall buckling.
When necessary, the designer may allow for these effects by taking
a reduced effective width for very slender outstands, based on initial
buckling (� cr ) rather than � m . This is effectively achieved by using the
following expression instead of equation (7.7):
which becomes operative when:
Non-welded outstand ß > 12.1�
Welded outstand ß > 12.9�.
(7.8)
The effect of so doing is shown by the broken curve in Figure 7.7.
Chapters 8 and 9 explain when it is necessary to use expression (7.8)
rather than (7.7).
With beams, when considering the moment resistance of a local crosssection
(Section 8.2), it is permissible to take advantage of the postbuckled
strength of a very slender outstand and work to the relevant
full line in figure 7.7 (equation (7.7)). But in dealing with LT buckling
of such a member, it may be necessary to assume a reduced effective
section based on initial buckling (� cr ). Refer to Section 8.7.6.
With compression members it is again acceptable to take advantage of
post-buckled strength, when studying failure at a localized cross-section
(Section 9.3). And when considering overall buckling of the member as
a whole, again allowance may have to be made for the loss of stiffness
when the applied stress reaches � cr . Refer to Sections 9.5.4, 9.6.9.
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