M

fz.Sd fz

in which x is the reduction factor for flexural buckling of the free flange, obtained from 6.2.1(2)P using buckling curve a (imperfection factor a = 0,21) for the relative slenderness Xfe given in (2).

(2) The relative slenderness Xfe for flexural buckling of the free flange should be determined from:

xi with:

lfz is the buckling length for the free flange from (3) to (7);

zfe is the radius of gyration of the gross cross-section of the free flange plus 1/6 of the web height, about the z - z axis.

(3) For gravity loading, provided that 0 < R < 200, the buckling length of the free flange for a variation of the compressive stress over the length L as shown in figure 10.4 may be obtained from:

where:

La is the distance between anti-sag bars, or if none are present, the span L of the purlin;

and t/j to tj4 are coefficients that depend on the number of anti-sag bars, as given in table 10.2.

[Dotted areas show regions in compression] Figure 10.4: Varying compressive stress in free flange for gravity load cases

Table 10.2: Coefficients m

Number of anti-sag bars per span

Vi

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