D5.D.6.6 Clause 6.3.3(5) – Interaction factors kyy, kyz, kzy, and kzz

The French NA recommends the use of equations in Annex A of NF EN 1993-1-1:2005 to calculate these interaction factors. STAAD.pro uses the method in Annex B for design per EC3 (without National Annex). Therefore, the method in Annex A has been added into the program.

Note: The NA mentions that this method can be extended to singly symmetric I-Sections (symmetric about the minor axis) if the elastic properties are used instead of the plastic properties. However, since STAAD does not have a provision to specify such sections, this case will not be considered for this implementation.

The NA also mentions that torsional flexural buckling needs to be taken into account in case of mono symmetric sections. This is taken into account based on the method given in the NCCI document SN001a-EN-EU: Critical axial load for torsional and flexural torsional buckling modes. Refer to D5.D.6.7 Clause 6.3.1.4 - Slenderness for torsional and torsional-flexural buckling

The NA also recommends a lower limit as given below for the term C_mi,0 in Table A.2 of Annex A:

C_{m i, 0} \geq 1 - \frac{N_{E d}}{N_{c r, i}}