I have a 4 story building that is "L-shaped". Currently I have moment frames from the foundation to the roof in both wings and I've even tried having some down the middle. I followed the Ram Frame tutorial as best as I could (no pictures), but I seem to have some serious mistake in my model. When I run the Analysis, I receive no warnings. When I do the steel check for the generated load combinations, I have failures from the 3rd floor to the foundation. The columns and beams fail with the C2-3 formula: Pnt > Pe2x. No matter what column size I select, I still have this error. The top floor works fine.
I am clueless as to what to do here. This showed up when I selected to have P-delta checked in the analysis. If I take it out, I can make things work just fine. Did I miss something easy during the modeling phase?
All beams and columns are fixed at each level and to the footings. The floor diaphragms are 1.5" deck with 4.5" additional concrete at each floor and a standard roof deck on the top. The roof is semi-rigid and the floors are rigid. I did notice that the K value for my columns after analysis is still 1, but I can't change that - it's calculated for me. I thought for sure when I changed my columns to an enormous size, that surely I could receive some results.
Thanks,
Lance
I read up on Stability Analysis And Design in the Steel Manual. I found that I was analyzing the structure as a Direct Analysis method and when I switched over to Effective Length Method, the errors went away. I'm not sure what the Direct method is doing differently (aside from adding in a little load for column sway), but for some reason it is giving my model some serious trouble.
There might not be anything wrong with your model, this just happens under some circumstances. There is a fairly lengthy explanation of how this check is performed in the manual Section 3.2.1 AISC 360 (ASD and LRFD) and beyond, and I see you’ve already discovered our Stability Analysis technote. Note, the diaphragm type and the number of columns outside of the diaphragm also have an effect on these calculations.
In the cases I have seen, the problem manifests when a lateral force in one direction, let’s say the X direction, results in a little bit of shear in the opposite Y direction. Calculation of B2Y in those cases can explode and then the whole story is penalized. Note, in our code we have some minimum thresholds implemented to avoid these problems, but they are quite small.
The simplest work-around for such a situation, in my opinion, is to analyze all the X direction loads first (+ gravity) and then design all the X direction frames. The run the analysis for the Y direction loads only and evaluate the Y direction frames. For cases where orthogonal effects or angled wind loads are required, you might need extra runs.
Using P-Delta analysis in lieu of B2 is the other way to go.
Answer Verified By: Lance Landusky