Aluminium Design and Construction John Dwight

CHAPTER 10
Calculation of section
properties
10.1 SUMMARY OF SECTION PROPERTIES USED
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The formulae in Chapters 8 and 9 involve the use of certain geometric
properties of the cross-section. In steel design these are easy to find, since
the sections used are normally in the form of standard rollings with
quoted properties. In aluminium, the position is different because of the
use of non-standard extruded profiles, often of complex shape. A further
problem is the need to know torsional properties, when considering lateraltorsional
(LT) and torsional buckling. The following quantities arise:
(a) Flexural S Plastic section modulus;
I Second moment of area (inertia);
Z Elastic section modulus;
r Radius of gyration.
(b) Torsional St Venant torsion factor;
IP Polar second moment of area (polar inertia);
H Warping factor;
X Special LT buckling factor.
The phrase ‘second moment of area’ gives a precise definition of I and Ip .
However, for the sake of brevity we refer to these as ‘inertia’ and ‘polar
inertia’.
10.2 PLASTIC SECTION MODULUS
10.2.1 Symmetrical bending
The plastic modulus S relates to moment resistance based on a plastic
pattern of stress, with assumed rectangular stress blocks. It is relevant
to fully compact sections (equation (8.1)).
Firstly we consider sections on which the moment M acts about an axis
of symmetry ss (Figure 10.1(a)), which will in this case also be the neutral
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