Answer:
Since the method according to 6.3.3 of EN 1993‑1‑1 is only applicable for double-symmetric cross-sections in the case of structural components subjected to bending and compression, the add‑on module designs these components using the General Method according to 6.3.4 of EN 1993‑1‑1. This method is applicable to any single-symmetric cross-sections. In Germany, however, only I‑sections are permitted, which is why the warning appears.
You can perform the stability analysis as cross-section design according to the second-order theory, which is possible using the add‑on modules RF‑/FE‑LTB or RF‑/STEEL Warping Torsion.
As an alternative, you can also allow the General Method for non-I‑sections in the "National Annex" dialog box, but this deviates from the National Annex.
