The design concept for lateral-torsional buckling analysis according to standards for steel or aluminum structures is based on the assumption that the y-axis of the cross-section corresponds to the major axis. Therefore, it is necessary to apply Iy > Iz. This logic has been adopted in RFEM. Thus, it is not allowable in RFEM (although possible from a mechanical point of view) to perform the lateral-torsional buckling analysis for cross-sections with the major axis “z”.
This problem can occur, for example, with standardized flat steel cross-sections from the RFEM library. A simple solution in this case is to use the parametric rectangular cross-section. Here, you can define the external dimensions as required, and thus always fulfill the condition Iy > Iz.
Depending on the modeling, the z-axis can also be the major axis for RSECTION cross-sections. In this case, simply rotate the cross-section in RSECTION by 90°.