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Steel Designers' Manual - 6th Edition (2003)
Table 16.6 Types of beam not susceptible to lateral – torsional buckling
loading produces bending about the minor axis
beam provided with closely spaced or continuous lateral restraint
closed section
x
C
Fig. 16.4 Lateral – torsional buckling
Basic design 443
vertical loading buckling out of the plane of the applied loads by deflecting
sideways and twisting; behaviour analogous to the flexural buckling of struts. The
presence of both lateral and torsional deformations does cause both the governing
mathematics and the resulting design treatment to be rather more complex.
The design of a beam taking into account lateral – torsional buckling consists
essentially of assessing the maximum moment that can safely be carried from a
knowledge of the section’s material and geometrical properties, the support conditions
provided and the arrangement of the applied loading. Codes of practice, such
as BS 5400: Part 3, BS 5950: Parts 1 and 5, include detailed guidance on the subject.
Essentially the basic steps required to check a trial section (using BS 5950: Part I
for a UB as an example) are:
(1) assess the beam’s effective length LE from a knowledge of the support conditions
provided (clause 4.3.5)
(2) determine beam slenderness lLT using the geometrical parameters u (tabulated
in Reference 2), LE/ry, v (Table 19 of BS 5950: Part 1) using values of x (tabulated
in Reference 2).
(3) obtain corresponding bending strength pb (Table 16)
(4) calculate buckling resistance moment Mb = pb ¥ the appropriate section
modulus, Sx (class 1 or 2), Zx (class 3), Zx,eff (class 4).
N