Aluminium Design and Construction John Dwight

Figure 8.23 Sections covered in Table 8.1.
1. Conventional I and channel sections
For the sections covered by Figure 8.23 an acceptable estimate of � can
be found using the following formula:
where: D=depth of section,
T=flange thickness=t for sections LI and LC,
y =l/ry �
l=effective buckling length of compression flange,
ry =radius of gyration about minor axis for gross section.
(8.32)
The two coefficients may be safely taken as X=1.0 and Y=0.05.
Alternatively, they can be calculated from the empirical formulae given
in Table 8.1, leading to a better estimate for �. Note that these formulae
become unreliable when used outside the ranges indicated.
2. Other sections
For � these should be found from the general expression:
(8.33)
where M cr =elastic critical moment, S=plastic modulus of the gross section,
and E=modulus of elasticity=70 kN/mm 2 .
M cr is the elastically calculated value of uniform moment at which
LT buckling would occur in the bay considered, in the absence of any
imperfections such as initial crookedness, HAZ softening, locked-in stress,
etc. For symmetrical sections (Figure 8.24), M cr may be calculated from
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