Aluminium Design and Construction John Dwight

The determination of l involves a considerable degree of judgment
(i.e. guesswork) by the designer—as in steel. It is found as follows:
l=KL (9.7)
where L=unsupported buckling length, appropriate to the direction of
buckling. K may be estimated with the help of Figure 9.3.
9.5.4 Column buckling of struts containing very slender outstands
For a column containing very slender outstands (Section 7.2.5), the designer
must know whether it is permissible to assume an effective section that
takes account of the post-buckled strength of these. There are two possible
procedures:
1. Method A is effectively the same as that given in BS.8118. p 1 (expression
(9.5)) is based on an effective section that ignores post-buckled strength
in such elements, using expression (7.8) to obtain � o . But in finding
�, it bases r on the gross section. It is further assumed that the member
is under pure compression (no bending) when the applied load aligns
with the centroid of the gross section.
2. Method B employs an effective section which takes advantage of
post-buckled strength in very slender elements, with � o found from
expression (7.7). This effective section is employed for obtaining both
A e and r. The member now counts as being in pure compression
when the applied load acts through the centroid of the effective section.
Method B thus employs a higher buckling curve (higher p 1 ), but enters
it at a higher �. Method A is found to be the more favourable for the
majority of cases, but method B becomes advantageous if the member
is short (low �).
9.6 TORSIONAL BUCKLING
9.6.1 General description
There are three fundamental modes of overall buckling for an axially
loaded strut [27]:
1. column (i.e. flexural) buckling about the minor principal axis;
2. column buckling about the major axis;
3. pure torsional buckling about the shear centre S.
Figure 9.4 shows the mid-length deflection corresponding to each of
these for a typical member. The mode with the lowest failure load is the
one that the member would choose.
Torsion needs to be considered for open (non-hollow) sections. Because
the torsional stiffness of these is roughly proportional to thickness cubed,
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