Dear Jesús,
I’ll just clarify that the calculation of the ultimate strength would ideally be based on the geometry and material of the cross section. The calculation is quite simple for simplified cross sections such as cylinders made of isotropic material, but requires special cross-sectional analysis for complex cross sections and composites. See, e.g., the discussion here: Mlife - User Defined Distribution - #15 by Jason.Jonkman. In this case, if you’re analyzing the fatigue of the blade root but change the design of the blade mid-span (without changing the design of the blade root), the ultimate strength of the blade root would not be influenced by the design change.
In conceptual design, often the 1D beam properties are known, but the cross sectional geometry and material are not known (such is the case for the NREL 5-MW wind turbine). In this case, one can model the turbine in OpenFAST, but can’t compute the ultimate strength for the fatigue post-processing. In this situation, we have assumed that the ultimate strength can be derived by scaling up the ultimate load found from OpenFAST simulations (assuming that the cross section is designed sufficiently to not fail when the ultimate load from OpenFAST is reached), as is done in the report I linked above.
I hope that helps.
Best regards,