LRFD or ASD? The saga continues... 12

[vmirat]

I have a project being designed for us by another contractor. It involves a simple support bracket system for HVAC duct work. The structural engineer did calcs using AISC ASD method.

Whenever I see ASD used on design, I usually ask why, out of professional curiosity. Here was their response:

"Designing a large structure with large quantities of steel one should use LRFD to take advantage of the cumulative weight savings. That is not the case here, so ASD was used for simplicity and speed."

I'm wondering if we are schooling new engineers in both methods for this reason? I didn't take this any further, but I wonder how they decide the break point for LRFD vs. ASD.

 

[ToadJones]

The commentary seems to indicate the "Fcr" from section F12 is the elastic buckling stress

 

[271828]

Yes, but that wasn't my point. My point is the lack of clarity because they're using Fcr to indicate an idealized (theoretical only) elastic buckling stress in various places in Ch. F.

There are three kinds of buckling stresses in the AISC Spec: idealized elastic buckling stresses (like Fe in Ch. E), design level elastic buckling stresses (like Fcr=0.877Fe in Ch. E, adjusted to fit data, etc.), and inelastic buckling stresses (like Fcr=Fy*0.658^(Fy/Fe) in Ch. E).

Whether we're talking about flexural buckling, lateral-torsional buckling, local buckling, or any other kind of buckling, these three usually exist, and have for a long time. They need to be kept straight, or the provisions will be confusing.

Idealized elastic buckling stresses should always have an "e." Design level buckling stresses can be inelastic or elastic, and should always have a "cr" subscript.

Ch. E is clear on this. Ch. F isn't, and I think that's an impediment. It's not the issue in your particular example, but I think it's a problem in general.