The DE-NA recommends the use of equations in Annex A or Annex B of DIN-EN 1993-1-1 to calculate these interaction factors. STAAD.Pro uses the method in Annex B by default. Thus the program uses Annex B for Cl.6.3.3 checks.

The DE-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.