The MS NA recommends that the method in Annex A or Annex B of MS EN 1993-1-1:2005 can be used to calculate the interaction factors for Cl. 6.3.3 checks in the case of doubly symmetric sections. STAAD.Pro uses the equations in Annex B of MS EN 1993-1-1:2005 to calculate these interaction factors for doubly symmetric sections..

However, for non-doubly symmetric sections, the MS NA gives the option of using Annex B with some modifications as given in the NA. (Cl. NA-3.2 of the MS NA). The MS NA requires additional checks to be done to check for the maximum allowable values of λ and X to be used in equations 6.61 and 6.62 of MS EN 1993-1-1:2005.

As per the MS NA, for non-doubly symmetric sections, the slenderness about the weak axis (λ_y in STAAD.Pro) and the corresponding reduction factor χ_y should be taken as the values from the highest values of slenderness (λ) among the flexural buckling slenderness (λ_y), torsional slenderness (λ_T) and torsional-flexural slenderness (λ_TF) as given in Clauses 6.3.1.3 and 6.3.1.4 of MS EN 1993-1-1:2005. Hence for non-doubly symmetric sections the program will calculate the critical non-dimensional slenderness as:

where

The MS NA or EC3 does not, however, specify a method to evaluate N_crT or N_crTF. Hence, the program uses the method specified in the NCCI document SN001a-EN-EU: Critical axial load for torsional and flexural torsional buckling modes to calculate these. Refer to D5.D.11.7 Clause 6.3.1.4 - Slenderness for torsional and torsional-flexural buckling for details.

Clause NA 3.2 of the MS NA also requires that “Where the section is not an I Section or a hollow section and is a class1 or class 2 section, it will be treated as a class 3 section for the purposes of this clause”. Hence for all Class 1 or Class 2 cross sections that are not I, H, SHS, RHS or CHS sections, the elastic properties will be used for the purposes of 6.3.3 checks.