The SW-NA recommends the use of equations in Annex A of EN 1993-1-1:2005 to calculate these interaction factors, which are used by STAAD.Pro when the Swedish NA is selected.

The SW-NA or EC3 do not deal with angle sections in specific and thus the program uses the method per cl 4.7.10 of BS 5950 for single and double angle sections to evaluate the slenderness.