EUROCODE COMPRESSION RESISTANCE OF STEEL HOLLOW SECTION (CHS & BOX)

Question

Hi, 
 I'm wondering how does STAAD calculates compression resistance of Hollow Sections based on Eurocode Requirements, anyone can explain to me? I've been looking (extensively) for previous threads about this topic but not able to find exactly what i am looking for. 
 Based on Eurocode 1993-1-1, under section 6.2.4, Compression resistance of sections under Class 1,2 & 3 should be determined using equation 6.10 Ncrd = (Afy/ymo) and for class 4 cross sections Ncrd = (A(eff)fy/ymo) equation 6.10. Say i have Class 3 Cross section, gross area is = 42240 mm^2 , Fy = 275 N/mm^2 and Ymo = 1.0, by direct subs in eq 6.10, my Ncrd = 11616 kN, but STAAD gives only 8024.3 kN? I have gone thru Eurocode Resistance checks but found nothing for any reduction factor for compression resistance checking only. 
 Hope you can share some lights from this, (see attached image). Thank You. 
 
 Mac

rtully14 · Accepted Answer

The code requires you to perform two checks, the first is the cross-section resistance which depends on the cross-section classification which is the equations you note (6.10 & 6.11), the second is the buckling resistance which is the result you may be seeing. Can you confirm what section you are evaluating or provide me with your Staad.Pro file and I will check the results?

Modestas · Answer

I noticed that you are trying to design a custom box section. In such case, please use the design parameter GST with appropriate value to set a proper section type for STAAD.Pro. STAAD.Pro designs custom sections as I sections by default, that's why appropriate GST parameters is needed for other section types. If this still does not help, please send us your hand calculations for further investigation. Thanks!