This is indeed an interesting situation. Unfortunately, I cannot answer your question because I also have raised this question (not in the forums, though) and haven't found an answer yet. I have to agree that the situation seems quite odd. After reading a lot of handbooks on Eurocodes I couldn't find an answer.
If you have Ned / Npl,Rd >= 1,0 then your member is failing in compression, thus, there is no need to check interaction as cross-sections resistance is already insufficient for axial load alone.
Have a great day,
I am trying to check an existing result using of an I-cross-section subjected to combined axial compression and bending. According to the EN 1993-1-1 (2005) code it is required to check the cross-section's capacity to withstand combined effects of compression and bending first then move to member level. Formulations set out in section 6.2.9 of EN 1993-1-1 (2005).
In equation 6.36 pp 54, procedure to calculate the reduced moment about y axis (MN,y,Rd) due to the effect of compression is provided. This equation is as following,
MN,y,Rd=Mpl,y,Rd(1-(NEd/Npl)/(1-0.5*a) <= Mpl,y,Rd
where, a = ((Gross Area) - 2*(section width)*(flange thickness))/(Gross Area)
Normally for design purpose, it have to be NEd<Npl always. But for an existing result while checking if NEd>Npl the value of MN,y,Rd becomes negative which seems irrelevant. My question is how to sort this problem? Should I take ratio of NEd/Npl maximum equal to 1 or take the absolute value of the moment ignoring the negative? As the axial compression is very high the moment magnitude will be small.
Any help would be highly appreciated.