Aluminium Design and Construction John Dwight

example, a reduced value might be thought desirable for extruded material
stressed transversely (across the grain).
Table 5.4 includes values for the stresses p p and p f needed in joint
design, which also depend on the member properties. See Chapter 11.
5.4 LIMITING STRESSES BASED ON BUCKLING
5.4.1 General form of buckling curves
The fourth limiting stress pb relates to overall member buckling, for
which we consider three possible modes:
LT beams in bending, lateral-torsional buckling;
C struts in compression, column (or ‘flexural’) buckling;
T struts in compression, torsional buckling.
Figure 5.5 shows the general form of the curve relating buckling stress p b
and overall slenderness parameter � for a member of given material
failing in a given mode. The ‘curve’ is shown as a scatter-band, since its
precise position is affected by various sorts of imperfection that are beyond
the designer’s control. Curve E shows the ideal elastic behaviour that
would theoretically be obtained assuming no imperfections, i.e. zero initial
crookedness, no locked-in stress, purely elastic behaviour, etc. In the case
of ordinary column buckling, this is of course the well-known Euler curve.
The range of the real-life scatter-band in relation to curve E varies according
to the mode of buckling considered. For design purposes, a curve such
as D in Figure 5.5 is needed, situated near the lower edge of the appropriate
scatter-band, with a cut-off at the limiting stress for the material.
Local buckling of a thin-walled cross-section, as distinct from overall
buckling of the member as a whole, is covered in Chapter 7. This form
of instability, which often interacts with overall member buckling, is
allowed for in design by taking a suitably reduced (‘effective’) width
for slender plate elements.
Figure 5.5 Variation of overall buckling stress with slenderness.
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