Aluminium Design and Construction John Dwight

Table 7.1 Classification of elements under uniform compression—limiting ß-values
Note: where p ° is in N/mm 2 .
7.2.2 Limiting values of plate slenderness
Table 7.1 lists limiting values of ß f and ß s needed for the classification
of elements under uniform compression (see Section 7.1.4). These have
been taken from BS.8118 and are expressed in terms of the parameter
� given by:
(7.3)
where the limiting stress p ° is measured in N/mm 2 . This parameter is
needed because ß f and ß s depend not only on the type of element, but
also on the material properties: the stronger the metal, the more critical
is the effect of buckling. Note that e is roughly equal to unity for 6082-
T6 material or equivalent.
7.2.3 Slender internal elements
For a slender non-welded internal element under uniform compressive
strain, the typical form of the stress pattern at collapse is as shown by
curve 1 in Figure 7.4, with the load mainly carried on the outer parts
of the plate. For design purposes, we approximate to this by taking an
idealized stress pattern 2, comprising equal stress blocks at either edge
which operate at the full stress p ° with the material in the middle regarded
as ineffective. The width of the two stress blocks is notionally adjusted
to make their combined area equal to that under curve 1. For a nonwelded
element, therefore, the assumed effective section is as shown in
diagram N with equal blocks of width b e1 and thickness t.
A modified diagram is needed if the element contains edge welds.
First, the effective block widths must be decreased to allow for the
adverse effects of weld shrinkage (locked-in stress, greater initial outof-flatness).
Secondly, an allowance must be made for the effects of
HAZ softening. Diagram W shows the effective section thus obtained
for a welded plate having the same ß. The block widths b e1 are seen to
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