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D5.D.11.6 Clause 6.3.3(5) – Interaction factors kyy, kyz, kzy, and kzz

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 (λy in STAAD.Pro) and the corresponding reduction factor χy should be taken as the values from the highest values of slenderness (λ) among the flexural buckling slenderness (λy), torsional slenderness (λT) and torsional-flexural slenderness (λTF) as given in Clauses and of MS EN 1993-1-1:2005. Hence for non-doubly symmetric sections the program will calculate the critical non-dimensional slenderness as:

λ y = max { λ per Cl. λ T per Cl.


λ T = A f y N c r
N c r = min ( N c r T , N c r T F )

The MS NA or EC3 does not, however, specify a method to evaluate NcrT or NcrTF. 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 - Slenderness for torsional and torsional-flexural buckling for details.

Note: The MS NA or EC3 does not deal with angle sections specifically and therefore STAAD.Pro uses the method described in the EC3 implementation to deal with slenderness of angle sections. This is done as per cl 4.7.10 of BS 5950.

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.