Aluminium Design and Construction John Dwight

9.7.2 Secondary bending in trusses
The members in triangulated truss-type structures, although primarily
subject to axial force, also pick up bending moments at their ends due
to joint rigidity. These ‘secondary’ moments can be significant and the
question arises as to whether to allow for them in design.
Linear elastic analysis readily enables secondary truss moments to
be computed, but the answer obtained is only valid in the early stages
of loading. When buckling is imminent, the moments at the ends of a
critical member are found to decrease and eventually change sign,
changing from disturbing moments to restraining ones. The normal
approach for obtaining the calculated static resistance of a truss member
is therefore to ignore secondary bending and just consider the axial
force. The situation is different in doing a fatigue check, when the
secondary bending stresses must be included.
The exception is when there is a significant non-concurrence of the
centroidal lines at a node, in which case the resulting eccentricity of
loading must always be considered, treated as a case of combined P
and M. Normally it is desirable to detail the truss so as to achieve
concurrence of the centroidal lines based on the gross section. However,
for thin channel-type members with very slender flange elements this
is not necessarily valid (Section 9.5.4).
9.7.3 Section classification
As in pure bending, the section must first be classified as fully-compact,
semi-compact or slender, unless it is in tension all over. Note that a
single classification is needed, corresponding to the particular combination
of P and M being applied. Again this is obtained by classifying any
individual elements that are wholly or partly in compression, the least
favourable element then dictating the classification for the section as a
whole.
In considering an element under strain gradient (Section 7.3), the
parameter � should be based on the usual assumption of flexural
behaviour (‘plane sections remain plane’). In other words, we put �=y 2 /
y 1 where y 1 and y 2 are the distances of the more heavily compressed
edge and the other edge from the neutral axis. The assumed neutral
axis should be that corresponding to the simultaneous action of P and
M, using a plastic stress pattern for the fully-compact or an elastic one
for the semi-compact check (Figure 9.15). In either case it is acceptable,
for classification purposes, to take this as the axis based on the gross
section. When P is high, it is possible for the elastic neutral axis to lie
outside the section.
In making the semi-compact check, it is permissible with an
‘understressed’ compression flange to use the same kind of relaxation as
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