Eurocode EN 1993-1-1:2005: code check using resultant forces

Question

Hello, 
 I'm looking into the code checking of a circular cross-section (class 1) using the Eurocode EN 1993-1-1:2005, and have some questions regarding how Staad.Pro does the checks: 
 1) For clause 6.2.6 (shear) there is given two utilisations, one for Z plane and one for Y plane. Couldn't this be replaced with one utilization, using the resultant shear force? 
 2) For buckling checks according to clause 6.3.3, interaction formulas 6.61 and 6.62, the individual moment components are used (M_y_Ed and M_z_Ed), as they are multiplied with interaction factors that depend on the moment distribution in each plane. I'm using annex B to calculate the interaction factors. But when comparing calculations to Staad.Pro I only get a good agreement when using the moment resultant instead. Any reason for Staad.Pro to use the moment resultant when performing this check, instead of the individual moment components? 
 Hope someone can help! 
 Best regards 
 Roar

Sudip Narayan Choudhury · Answer

1. I think this can be replaced with one shear force resultant. Currently, we do not do this for shear. But we do consider this for the moment. The moment in the two planes are resolved into one single resultant component. I will discuss this with the development. 
 
2. For Circular Hollow Sections, the AISC LRFD code has introduced the concept of using the Resultant Moments. STAAD uses the same concept for doing the EN 1993-1-1:2005 code checks. I think for sections which does not have a set of unique principle axes system, this will hold for the axial compression and bending interaction checks as well.