The effectiveness of the numerical methods GMNIA and MNA/LBA proposed in EN1993-1.6 for the design of steel shells is studied for cylindrical steel shells with an unreinforced or reinforced rectangular cutout chamfered elliptically at the four ends and for the corresponding shells without cutout. Moreover, another design method proposed in the literature and denoted here as MNA/GNA, which is based on a modified slenderness, is also evaluated. GMNIA is considered as the most reliable analysis type, provided that a judicious choice of shape and amplitude of initial imperfections is made. Thus, GMNIA results are used as basis for comparison, except for shells without cutout where the EN1993-1.6 normative strengths could serve the same purpose as well. For shells without cutout it is found that the modified slenderness gives similar results to the corresponding results of the conventional slenderness definition. In the case of unreinforced cutout the modified slenderness gives better results, thus the use of MNA/GNA is recommended. However, in the case of reinforced cutout the GMNIA results are approximated better by employing the conventional slenderness, thus MNA/LBA is more appropriate.