The previous chapter Calculation According to Second-Order Analysis explains how to enter such a calculation, taking into account the global imperfections. If you were to activate the Steel Design add-on now, you could use the internal forces calculated with it to perform an equivalent member design or a design according to the general method. However, both methods are unsuitable for the design of stair stringers, because the stringers have a U-shaped cross-section and are subjected to both compression and bending loads.
The following overview shows which procedures can be used in which cases. The equivalent member method can only be carried out for double-symmetrical cross-sections in the case of a combined load from compression and bending, while the general method according to the German National Annex may only be applied to I-sections.
The only option in RFEM to consider the lateral-torsional buckling even with asymmetrical cross-sections is the warping torsion analysis with 7 degrees of freedom. The internal forces are calculated according to the second-order analysis, taking into account the global and local imperfections. A cross-section analysis is then carried out, which replaces the stability analysis.