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 βm 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 αm will be calculated from the equation given in AS 4100 clause 5.6.1.1 (a) (iii) except for cantilevers, where αm 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 βm and the moment modification factor αm will be based on real moments only.
The twist restraint factor kt 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 kl will default to 1.4 for a non-cantilever and 2.0 for a cantilever.
The lateral rotation factor kr will default to 1.0.