Aluminium Design and Construction John Dwight

Figure 9.4 Fundamental buckling modes for a compression member.
the torsional mode never governs when the section is thick, as in heavy
gauge (hot-rolled) steel. It becomes more likely as the thickness decreases,
and for thin-walled members it is often critical, as also in light gauge
(cold-formed) steel. The checking of torsional buckling tends to be
laborious. Here we follow the treatment given in BS.8118, which is
more comprehensive than any provided in previous codes. The calculations
involve the use of torsional section properties, which may not be familiar
to some designers. Chapter 10 provides help for evaluating these.
9.6.2 Interaction with flexure
A tiresome complication with torsional buckling is that the fundamental
buckling modes often interact, depending on the degree of symmetry
in the section (Figure 9.5):
1. Bisymmetric section. The three modes are independent (no interaction);
2. Radial-symmetric section. As for 1;
3. Skew-symmetric section. As for 1;
4. Monosymmetric section. Interaction occurs between pure torsional
buckling about the shear centre S and column buckling about the
axis of symmetry ss;
5. Asymmetric section. All three modes interact.
The effect of interaction is to make the section rotate about a point other
than the shear centre S, as illustrated for a monosymmetric section in
Figure 9.5 Degrees of symmetry for strut sections: (1) bisymmetric; (2) radial-symmetric;
(3) skew-symmetric; (4) monosymmetric; (5) asymmetric.
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