Additional suggestions which may resolve negative-stiffness errors include:
Remove auto line constraints. Select all deck/shell objects in the model, then select Assign > Shell Elements > Auto Line Constraint and uncheck the Apply to Full Structure option. This will remove auto line constraints and possibly resolve instabilities.
Dear Umar,
The above suggestion works! thank you.
However, I would love to understand the engineering behind it. How line constraint is affecting the stability of the floors in P-Delta analysis when the slab stiffness modifier is reduced from 1.0 to 0.5 for instance? Do you have some sort of technical explanation to that?