Aluminium Design and Construction John Dwight

compression flange. This is valid when the flange, although held in
position at these points, is free in terms of plan-view rotation. It is
permissible to reduce the value of l when the flange is restrained against
such rotation, although by just how much is a matter of some guesswork.
British Standard BS.8118 permits l=0.85L for the case of ‘partial’ rotational
restraint and 0.7L for ‘full’ restraint.
There are some situations where l should be taken greater than the
bay length L, namely as follows:
(a) Destabilizing loads
For a bay that carries intermediate loading (Section 8.7.3(b)) it is important
to consider at what height this loading acts. A destabilizing load is one
which effectively acts at a point X located higher than the shear centre
S (Figure 8.25). As the beam buckles, taking the load with it, the resulting
moment about S aggravates the tendency to twist. Such a load therefore
represents a more adverse condition than that of a non-destabilizing load
(acting at, or below S), and must be allowed for by increasing l. Just how
much depends on h (the height of X above S), a reasonable value being:
(8.37)
where D is the overall depth of section. When the load is applied at top
flange level in a symmetrical I-beam (h=D/2), this gives l=1.2L. Equation
(8.37) assumes that the compression flange is unrestrained against planview
rotation at the ends of the bay. When restraint against such rotation
is provided, BS.8118 permits effective lengths of L and 0.85L for the
case h=D/2, corresponding to ‘partial’ and ‘full’ restraint respectively.
(b) Beams without end-posts
It is desirable to stabilize the top flange of a beam against out-of-plane
movement over the reaction points, by fitting a properly designed end-
Figure 8.25 Lateral-torsional buckling, destabilizing load.
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