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Dear members, Using ETABS 16.2 version, when setting the stiffness modifiers for my slabs (shell-thin) to 1.0, the run goes smooth without warring and I will get reasonable mode periods. However, when I modify the m11, m22, and m12 stiffness to 0.5 for my slabs and run the model again, I got some negative stiffness eigenvalues in my P-Delta analysis and no modal analysis is made! When I tried 0.9 instead of 0.5, no warnings! Tried 0.8, warnings came back! By the way, there is no warnings and or errors when perform "check model". Everything is ok. Attached is my ETABS file for your reference. Please advice if you have faced similar problem? ETABS Model.rar
