HSS or box shape allowable bending in sap

[Skicat]

In using the latest version of SAP (12.0.1), I've noticed that despite the HSS or box section that I use, the allowable bending stress is always the same. I've modeled 5 frames varying only the section (from HSS12x8x5/8 to HSS2x2x1/4). In every instance, the allowable bending stress is the same. When I perform the same experiment with W shapes, the allowable bending differs as it should. Does anybody know what's going on? Thank you for your help.

 

[agostage]

The allowable stress is based on material properties. All rectangular HSS sections have fy=46ksi and would be the same for all x-sections. Not sure why W-section allowable stress would change unless you are using the section designer and the allowable stress is being reduced based on buckling modes or you're switching between A36 and Gr50.

 

[Skicat]

I've been researching and I believe the explanation is that most HSS sections are compact. SAP seems to be reducing the allowable bending stress when the sections is non-compact.

 

[agostage]

Correct, as I said, the section designer in Sap will reduce allowable stress based on buckling modes, i.e. web and flange buckling of non-compact or slender sections. This would not occur with HSS.

 

[Skicat]

Thank you.