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For double symmetric I profile, we don't have to perform any lateral torsional buckling check if NEd/Npl,Rd ≤ 0.15 and the conditions provided in Table 6 SIA263 are satisfied. For any other case, a LTB check must be perform.
Calculations described in Annex B for I,U and PPL can be applied to T sections only if the flange is subjected to compression. Otherwise, as for section not supported by SIA263 in the LTB check, we use prescriptions given in EC3-ENV Annex F. Those rules allow us to determine an elastic critical moment for lateral torsional buckling for symmetrical (formula F.2 EC3) and non symmetrical (formula F.1. EC3) sections around the minor axis.
In the case of I, U, PPL and, T only with compression in flange, characterised by a reduced area or not, we have to determine before any calculation irc, defined as the radius of gyration of a section comprising the compression flange plus 1/3 of the compression web area, taken about an axis in the plane of the web.
Haunched sections (I+Ivar, Iw+Plvar, Iw+Iwvar, Iw+Ivar, I+Iwvar) and composed rail sections (Iw+rail, Iwn+rail, I+rail, I+2PL+rail, I+PL+rail, I+2L+rail, I+Ud+rail) are considered as equivalent asymmetric I sections.
For advanced Lateral-torsional buckling analysis, see "Annex D: Use of diaphragms ".