Aluminium Design and Construction John Dwight

1. Fully-compact sections. The limiting values of moment for the two
checks are found by employing the plastic expressions (9.22) with S p
changed to S pm . Here S pm is a modified value of the plastic modulus,
which takes account of the inclination of mm as well as the presence
of P (Section 10.2.3).
2. Semi-compact and slender sections. The applied moment is resolved
into components M cos � and M sin � about the principal axes, the
effects of which are superposed elastically with that of P. The section
is adequate if at any critical point Q:
(9.26a)
(9.26b)
where x, y are the coordinates of Q, and the I’s relate to the effective
section. P is taken positive if tensile and negative if compressive. The
left-hand side of the equation is always taken positive.
9.7.8 Treatment of local buckling
In applying checks A–D and E–G, care is needed in dealing with
elements prone to local buckling. It was explained that the section
classification is based on the simultaneous action of P and M (Section
9.7.3). If on this basis the section comes out as fully or semicompact,
no deduction for local buckling should be made in the
determination of P c and M c for these checks, even though the section
might be regarded as slender under the action of P or M in isolation.
But if (under P+M) the section is found to be slender, P c and M c
should be found exactly as specified in Sections 9.7.4 and 9.7.6,
taking (different) effective sections each appropriate to the action
considered (P or M).
9.7.9 Eccentrically connected angles, channels and tees
Axial members of angle, channel or tee-section are often connected to
one side of a gusset-plate at their ends. Such members are subject to
end moment as well as axial force, because of the eccentricity of loading,
and may be rigorously checked using the appropriate interaction
equations. Alternatively, an easier method can be adopted as follows
without too severe loss of accuracy (based on BS.8118). The procedure
is to treat the member as an ordinary tie or strut, and make a simple
correction to allow for the eccentricity as follows:
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