Lateral torsional buckling resistance, Clause 4.3 (Columns: BS 5950)

For columns 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 clause 4.8.3.3.2 or 4.8.3.3.3, (See: Member buckling resistance, Clause 4.8.3.3.2, and Member buckling resistance, clause 4.8.3.3.3).

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 M_b for use in the combined buckling check is taken as the full moment capacity, M_cx, not reduced for high shear in accordance with clause 4.8.3.3.3 (c), equation 2 (See: Member buckling resistance, Clause 4.8.3.3.2).

Destabilising loads are excluded from Tekla Structural Designer, this is justified by the rarity of the necessity to apply such loads to a column. If such loads do occur, then you can adjust the “normal” effective length to take this into account although you can not achieve the code requirement to set m_LT to 1.0.

Effective lengths

The value of effective length factor is entirely at your choice. The default value is 1.0.