We have a project with two internal mezzanines (therefore no wind load), and we are in SDC A, so we would like to handle their seismic lateral through full-height column bending of the columns that run through the mezzanine. We tried turning all of the columns that go through the mezzanine to lateral columns, but got P-Delta errors even using the maximum size W14 column. This was with a rigid diaphragm.
We tried changing the diaphragm to semi-rigid, but when we looked at the deflected shape it showed the diaphragm bending as if it were a moment frame with the lateral columns.
Is there a correct way to model this situation in RAM? I saw a response from Seth Guthrie on eng-tips from several years ago (https://www.eng-tips.com/viewthread.cfm?qid=306557) where he addresses this and says "Note, these types of structures tend to be very sensitive to P-Delta amplification and I usually recommend no P-Delta analysis, at least in the preliminary design."
Can we be confident that everything is being designed correctly if we turn P-Delta off? And this is not for a preliminary design, this is for Construction Documents, so any advice you can give would be very helpful.
Thank you.
I would run it as a rigid diaphragm with no exposure and no P-Delta, at least initially. If things look OK and deflections are small, then try with P-Delta, but our P-Delta global-stiffness reduction method can blow up for models with offset, partial levels. The alternative is B1 and B2 factors to use in the steel design mode.
P.S. You may want to specify the period or use T-Ta, since the isolated mezzanines likely have a long, unconservative first and second mode shape. See also