Answer:
The reason for this warning is that tapered cross-sections cannot be designed using the methods 6.3.3 of EN 1993‑1‑1. The RF‑/STEEL EC3 add-on module designs such cross-sections using General Method 6.3.4 of EN 1993‑1‑1. However, this method is only allowed for loading in the support direction; that is, without the transverse bending moment. Therefore, the warning appears.
You can perform the stability analysis as cross-section design according to the second-order analysis, which is possible using the add‑on modules RF‑/FE‑LTB or RF‑/STEEL Warping Torsion.
As an alternative, you can neglect the transverse bending in the filter options of the "Details" dialog box or the "Stability" tab, or activate the adapted method according to Naumes in the "National Annex" dialog box, which enables transverse bending.
