Lateral torsional buckling resistance, clause 4.3 (Beams: BS 5950)
For beams that are unrestrained over part or all of a span, a Lateral torsional buckling (LTB) check is required either:
- in its own right, clause 4.3 check,
- as part of an Annex G check,
- as part of a combined buckling check to 4.8.3.3.1, 4.8.3.3.2 or 4.8.3.3.3, (see Member buckling resistance, clause 4.8.3.3.1, Member buckling resistance, clause 4.8.3.3.2, and Member buckling resistance, clause 4.8.3.3.3, respectively)
This check is not carried out under the following circumstances:
- when bending exists about the minor axis only,
- when the section is a CHS or SHS,
- when the section is an RHS that satisfies the limits given in Table 15 of BS 5950-1: 2000.
For sections in which LTB cannot occur (the latter two cases above) the value of Mb for use in the combined buckling check is taken as the full moment capacity, Mcx, not reduced for high shear in accordance with clause 4.8.3.3.3 (c), equation 2 (See Member buckling resistance, clause 4.8.3.3.3).
Effective lengths
The value of effective length factor is entirely at your choice. The default value is 1.0 for “normal” loads and 1.2 for “destabilizing loads”. Different values can apply in the major and minor axis.