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Steel Designers' Manual - 6th Edition (2003)
Fig. 25.3 Dispersion of load through flange
Compared with British and American sections European sections have a relatively
larger root radius and the simplification of a 1 : 2.5 dispersion does not apply.
Where there are axial stresses in a flat plate the plastic bending moment capacity
is reduced. As the dispersion depends upon the bending capacity of the flange,
when there are relatively high longitudinal stresses in the flange caused by axial load
and bending moments the angle of dispersion will be significantly reduced. To allow
for this effect the 1 : 2.5 spread should be modified to 1 : 2.5√m, where m = 1 - (f a/p y) 2
and f a = average longitudinal stress in the flange. This modification is included in the
formula in the draft Eurocode 3. 2
25.2 Stiffeners
25.2.1 General
Stiffeners 713
At connections to beams and columns web stiffeners are provided where compressive
forces applied through a flange by loads or reactions exceed the buckling resistance
of the unstiffened web, or where the compressive or tensile forces applied
through the flange exceed the local capacity of the web at its connection with the
flange. They may also be provided to stiffen the flange where it is inadequate in
bending, e.g. in a bolted tension connection.
The rules in BS 5950-1 relating to the buckling resistance of an unstiffened web
have been revised. In the 1990 version, the buckling resistance of an unstiffened web
was given by Pw = (b 1 + n 1)tp c, where b 1 and t are as defined earlier, n 1 is the length
obtained by 45° dispersion through half the depth of the section and p c is the compressive
strength obtained from Table 27(c). In BS 5950-1: 2000 clause 4.5.3.1, the
buckling resistance of the web, P x, is obtained directly from the bearing capacity P bw
and the geometry of the section. There is no longer a need to refer to the strut curve
(i.e. Table 24(c) in BS 5950-1: 2000). Three equations are presented for P x, depending
on the restraint of the flange and the location of the applied load relative to the
end of the member.