Response spectrum - SRSS - negative values

Dear all, I’m running a spectrum analysis with Ansys, and I found a stranger behaviour in final results. I modeled a simple column, 30x30x300 cm, subject to its own load. I run a modal analysis, and then a response spectrum, using 4 shape modes and a SRSS combination mode. I analysed Ansys results: as regard the deformation and the normal stress, Ansys results correspond with manually ones; as regard principal stress, on the contrary, I don’t obtain the same results. Moreover, I noticed that Ansys gives not only positive but also negative values, that is impossible according to the definition of the SRSS combination method itself. I tried also to repeat the analysis using another combining method (CQC) but the problem still exists. What can be the cause? Thankyou.


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