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Steel Designers' Manual - 6th Edition (2003)
378 Local buckling and cross-section classification
stress system to which the plate element under consideration is subjected. If, in addition
to the moment M, an axial compression F is applied to the member, then for
elastic behaviour the pattern of stress in the web is of the form shown in Fig. 13.6(a).
The values of s1 and s2 are dependent on the ratio F/M with s2 approaching sy, if
F is large and M is small. In this case it may be expected that the appropriate b
limits will be somewhere between the values for pure compression and pure
bending, approaching the former if s2 ª sy, and the latter if s2 ª-s2. A qualitative
indication of this is given in Fig. 13.7, which shows Mu as a function of d /t for three
different s2/s1 ratios corresponding to pure compression, s2 = 0 and pure bending.
If the value of d /t is sufficiently small that the web may be classified as compact or
plastic, then the stress distribution will adopt the alternative plastic arrangement of
Fig. 13.6(b).
For a plate element in a member which is subject to pure compression the loadcarrying
capacity is not affected by the degree of deformation since the scope for a
change in strain distribution as the member passes from a wholly elastic to a partially
plastic state, as illustrated in Fig. 13.4 for pure bending, does not exist.
The plastic and compact classifications do not therefore have any meaning; the only
decision required is whether or not the member is slender, and specific values are
only required for b3.
stress due to compression F
stress due to moment M
stress due to compression F
stress due to moment M
(a)
(b)
a2 =
a1 =
H
Fig. 13.6 Stress distributions in webs of symmetrical sections subject to combined bending
and compression. (a) Semi-compact, elastic stress distribution. (b) Plastic or
compact, plastic stress distribution