Aluminium Design and Construction John Dwight

distance of its centroid from the neutral axis. For a relatively small longitudinal
weld, there is no need to have a very precise value for y. The above approach
may also be used when bending is not about an axis of symmetry, except
that the lost areas now affect the location of the neutral axis.
10.3 ELASTIC FLEXURAL PROPERTIES
10.3.1 Inertia of a section having an axis of symmetry
The inertia I (second moment of area) must generally be determined
about an axis through the centroid G of the section.
To find Ixx about an axis of symmetry Gx we split up the area on one
side of Gx into convenient elements (Figure 10.4(a)), and use the
expression:
(10.7)
where: I Exx =an element’s ‘own’ inertia about an axis Ex through its centroid
E parallel to Gx, A E =area of the element, and y E =distance of E from the
main axis Gx. The summations are made for the compression material
(C) only.
Figure 10.6 shows various common element shapes, for which the
value of I Exx and the position of E may be obtained by using the expressions
in Table 10.1.
When bending does not occur about an axis of symmetry, but about
one perpendicular thereto (Figure 10.4(b)), we have first to locate the
neutral axis, i.e. find the position of the centroid G. To do this, we select
any convenient preliminary axis XX parallel to the axis of bending. The
distance Y G that the neutral axis lies above OX is given by:
Figure 10.4 Elastic symmetric bending.
Copyright 1999 by Taylor & Francis Group. All Rights Reserved.
(10.8)