I am modeling a 5 story steel moment frame. One of my diaphragms is failing with P-Delta. I have went through the trouble shooting process of turning off P-Delta and running dead load, which is running fine. I can run wind and seismic without P-Delta as well but when I turn it on, my diaphragm is unstable for dead load only (and the other load cases). I have checked the modeler and I have no integrity/data check warnings or errors. I also sized up my steel to ensure it was rigid enough. Are there any other solutions that would point to this issue? I circled the diaphragm in question in the last picture. The other diaphragms shown are not on the same level/story.
The diaphragms of that level have a flimsy lateral system, especially in torsion I suspect. When we do our stiffness reduction it goes to zero. If you can't justify lumping the masses to another level then you are probably stuck with using B1 and B2 factors in the design in lieu of P-Delta, or adding additional lateral stiffness.