Say you have an HSS round section that has Mx and My bending with some minor axial load (Equation H1-1b condition).

If you use the equation in H, and enter the numbers for Pu, Mx and My you arrive at a specific unity value - and if less than 1.0 you are considered safe.

One of our software programs didn't match our hand check of the unity but was much smaller.  We then discovered that the program first combined the Mx and My into a single Mr (resultant value) of the two moments - since the pipe has no true major or minor axis but is symmetrical all around.

Using this Mr we could arrive at the program's unity value.

The AISC specification doesn't seem to address this - the AISC Hollow Structural Sections Connection Manual, however, does say this:

"For biaxial flexure of round HSS that are laterally unbraced along their length and with end conditions such that the effective length factor K is the same for any direction of bending, the design is permitted to be based upon a single resultant moment Mur, where   
      Mur = Square Root[ (Mux)^2 + (Muy)^2 ]

The AISC manual doesn't address this which makes me wonder how you can do this without the steel spec allowing it - and also does this apply to pipe sections (vs. HSS section) as well?

Just curious - any thoughts or knowledge on this issue?

 

I don't think the code defines what the strong axis or weak axis of a pipe section is. Therefore, you should be able to use some engineering judgement to determine how this should be defined.

The flexural strength is the same in all directions for a pipe, so it really shouldn't matter how you define the local axes. You should be free to use whatever definition makes the most sense to you.  

Very odd, however, that this isn't at least mentioned somewhere in the commentary.  The HSS manual did make this very clear, so at least it was clearly codified once.

 

I agree with JoshPlum, the HSS manual made it very clear and thus codified.  It is very interesting though that AISC-360 is different.  Personnally I believe it was just overlooked during the developement of the 13th Edition Manual.  The 13th was the first manual to include not only ASD and LRFD design methods, but also incorporated the HSS manual.  Previous manuals did not mention pipe and HSS in chapter H commentaries, however the 13th, and now the 14th do include these sections.  I would use Mur and still sleep at night knowing you have a safe structure.