Aluminium Design and Construction John Dwight

Figure 7.2 Basic element types: internal (left) and outstand (right).
its longitudinal edges, an outstand at only one (Figure 7.2). In some
lightgauge steel codes, these have been confusingly referred to as
‘restrained’ and ‘unrestrained’ elements.
An element may be subjected to uniform compression (see Section
7.2), as when it forms part of a compression member or of a horizontal
compression flange in a beam. Or it may be under strain gradient as in
a beam web (see Section 7.3). Elements are usually in the form of a
plain flat plate. However, it sometimes pays to improve their stability
by adding a stiffener, in which case they are referred to as reinforced or
stiffened elements (see Section 7.4).
7.1.3 Plate slenderness parameter
The ability of an element to resist local buckling depends on the plate
slenderness parameter ß which is generally taken as:
(7.1)
where b is the flat transverse width of the plate, measured to the springing
of any fillet material, and t its thickness, ß does not depend on the
length a (in the direction of stress) because this has no effect on local
buckling resistance, unless a is very small, i.e. of the same order as b.
7.1.4 Element classification (compact or slender)
In order to classify the cross-section of a member, we first classify its
individual elements, excluding any that may be wholly in tension. The
most adverse classification thus obtained then defines that for the member
as a whole.
For any given type of element there is a critical slenderness ß s such
that local buckling failure occurs just as the applied stress reaches the
limiting value p o for the material. An element having ß < ß s is said to
be compact, since it is fully effective and able to reach p o without buckling.
If ß > ß s the element will buckle prematurely and only be partially
effective, in which case it is referred to as slender.
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