# Lateral torsional buckling resistance (Member moment capacity) (Columns: AS 4100)

For beams with major axis bending, a Lateral Torsional Buckling (LTB) check is required, except in the following circumstances:

- when the segment critical flange is continuously restrained for LTB, or
- when bending exists about the minor axis only, or
- when the section is a CHS, or
- when the segment length satisfies the relevant limit given in clause 5.3.2.4 of AS 4100

In the latter case, when calculating the limiting LTB length, the ratio β_{m}
will be taken as -0.8 if the segment has major axis bending induced by transverse
load within its length, and the ratio of end moments otherwise.

The LTB resistance (member moment capacity) check is performed according to AS 4100 clause 5.6

Note that the moment modification
factor α_{m} will be calculated from the equation given in AS 4100 clause
5.6.1.1 (a) (iii) except for cantilevers, where α_{m} will be 0.25 if the
free end moment is greater than the ignore forces major moment, and 1.0 otherwise.

The design moment may include
eccentricity moment - see Moment Capacity (Section): Eccentricity Moments - but note
in particular that the ratio β_{m} and the moment modification factor
α_{m} will be based on real moments only.

The twist restraint factor k_{t} will be determined by consideration of the
LTB cross-section restraints at either end of the segment, per Table 5.6.3(1) of AS
4100

The load height factor k_{l} will default to 1.4 for a non-cantilever and 2.0
for a cantilever.

The lateral rotation factor k_{r} will default to 1.0.