Aluminium Design and Construction John Dwight

Figure 9.6 Monosymmetric section. Interaction between pure flexural buckling about ss
and pure torsional buckling about S.
Figure 9.7 Typical ‘type-R’ sections, composed of radiating outstands.
Figure 9.6, and leading to a reduced failure load. Strictly speaking,
interaction between torsional and column buckling can occur very slightly
even with thick members. But the effect only becomes significant when
the section is thin.
9.6.3 ‘Type-R’ sections
In dealing with torsional buckling it is important to distinguish between
‘type-R’ sections and all others. A type-R section is one that consists
entirely of radiating outstands, such as angles, tees and cruciforms (Figure
9.7). For such members, each component element is simply supported
along the common junction, or nearly so. When such an element suffers
local buckling, it typically does so in one sweep occupying the whole
length of the member (Figure 9.8), and not in a localized buckle as for
other thin-walled shapes. This is essentially a torsional mode of
deformation, In thin type-R sections, therefore, local buckling and torsional
buckling amount to much the same thing.
In design, it is convenient to treat the buckling of type-R struts in
terms of torsion, rather than local buckling. By so doing, one is able to
take advantage of the rotational restraint that the outstands may receive
from the fillet material at the root. Double-angle (back-to-back) struts
can also be regarded as effectively type-R.
9.6.4 Sections exempt from torsional buckling
Torsional buckling is never critical for a strut with any of the sections
listed below, and need not be checked:
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