29 Nov 11 17:11
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?