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Steel Designers' Manual - 6th Edition (2003)
Torsional and flexural-torsional buckling 411
actual weld
slenderness X
Pc = Aepc (clause 10.6.1.1)
in which Ae is the effective area defined as
Ae = SKc(
khAc) for members other than CHSs
20 N/mm2 reduced curve
Fig. 15.5 Modified strut curve for welded sections used in BS 5950: Part 1
where Kc allows for loss of effectiveness in slender plate elements, determined from
Figure 36, k h allows for the presence of holes, and A c is the net area; and
Ae = Ac
D
for CHSs for which
t
Ê py
ˆ
Á ˜ £ 50
Ë 335¯
È
D
Ae = AcÍ1.
15 -0.
003
Î
t
Ê py ˆ ˘
Á ˜
Ë 355¯ ˙
˚
D
for CHSs for which
t
Ê py
ˆ
Á ˜ > 50
Ë 355¯
pc is obtained from a set of curves (Figure 37) similar to Fig. 15.3.
Selection of the appropriate column curve is made using a simpler selection table
than that of Part 1 of BS 5950. Essentially it distinguishes between curves on the
basis of the ratio of radius of gyration to distance from neutral axis to extreme fibre,
apart from heavy sections and hot-finished SHS, which are universally allocated to
the lowest and highest curves respectively.
As noted in the example of 15.4.1, for class 4 sections the slenderness l may be
reduced in the ratio of the square root of the effective to the gross area.
15.6 Torsional and flexural-torsional buckling
In addition to the simple flexural buckling described in the previous section, struts
may buckle due to either pure twisting about their longitudinal axis or a combination
of bending and twisting. The first type of behaviour is only possible for
centrally-loaded doubly-symmetrical cross sections for which the centroid and shear