Utilization ratio vs. actual stresses - STAAD V8i

[Polecat]

I am analyzing an angle frame that supports antenna panels to a lattice tower.
After applying all the load cases and establishing the factored results (at 1.2D+1.6W), the member utilization table shows to be less than 1.00 for all combinations.
However, when I examine the stress table for combined axial and bending for the individual beams, there are a few members that are showing a max stress level of 39 ksi at the end of the beam. Since the input file had limited the maximum yield to be 36 ksi, why would the program show that the design utilization was OK when some individual stress points are not OK? The code check done was AISC, LRFD, FY36.
Any enlightenment would be appreciated.

http://www.spiraleng.com

 

[JoshPlumSE]

The LRFD code isn't stress based. Instead it's based on force and moments. The maximum moment in LRFD tops out at the "plastic" moment which can be considerably higher than the moment that corresponds to initial yield. The idea is that the member doesn't fail in bending until the entire cross section has yielded.

That being said most "elastic" analysis programs will report the extreme fiber stress as M*c/I even if this ends up greater than yield.

 

[Polecat]

Thanks, Josh, I think you are right on target.
I noticed in the table that listed the maximum stresses, they always indicated that it occurred at a certain corner of the angle, not across the entire cross section. Evidently, that would be the point where it would be starting to fail but hadn't yet spread to the full cross section.
Thanks for the reminder about "platic moment" design.

http://www.spiraleng.com