...
Buckling can also occur in horizontal floor elements such as beams and floors, if the floors take in plane loads (i.e have no diaphragm or semi rigid diaphragm) and have insufficient capacity, due to structural properties or small stiffness modifiers. Using rigid diaphragms and checking if the analysis converges can help rule out this as the cause of the problem.
Additional suggestions which may resolve negative-stiffness errors include:
...