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Steel Designers' Manual - 6th Edition (2003)
406 Columns and struts
Effectively this means that the member is assumed to be made of a steel having a
lower material strength than is actually the case. As noted above, BS 5400: Part 3
does not permit the use of columns with slender outstands.
15.4.1 Columns with slender webs
Reference 1 indicates that most UB and a few RHS, even in S275 steel, will be
slender according to BS 5950: Part 1 if used as struts. A suitable allowance for the
weakening effect of local buckling should therefore be made. Two different procedures
are given, outlined in the following example.
Consider a 406 ¥ 140 ¥ 39 UB (the UB with the highest d/t) in S275 material. This
has a d/t of 57.1 compared with the limit in Table 11 of 40 assuming the section to
be fully stressed: i.e. r2 as defined in clause 3.5.5 is equal to unity (for r2 < 1 a higher
limit is applicable).
(1) Use clause 3.6.5, limiting d/t = 40(275/pyr) 0.5
and reduced strength, p yr = ( 40 57. 1) ¥ 275
2
= 134. 8 N/mm
For L E/r y = 100, using Appendix C for p yr = 134.8 N/mm 2 and strut curve ‘a’
2
pc
= 103. 5 N/mm
Pc
= 49. 4 ¥ 103. 5 10
= 511 kN
(2) Since the web is slender, obtain Aeff from clause 3.6.2.2 using Figure 8 of
BS 5950: 2001-1.
Aeff = 4970 - 40 ¥ 6.4
= 4714 mm 2
For sections containing class 4 element(s) clause 4.7.4 gives
P c = A effp cs
in which p cs is obtained for a slenderness l(A eff/A g) 0.5
05 .
= 100( 4714 4970) = 97. 0
From Table 24, using strut curve ‘a’ and py = 275 N/mm 2 , pcs = 164 N/mm 2
\ Pc = 4714 ¥ 164 1000
= 773 kN
This compares with the value for a ‘fully effective equivalent’ of 780 kN.
2