Hello,

This is the time history result of one of the reference points of the shell element(S4) in my FEM analysis. And I'm aiming at this point to find the envelope over the entire time.
However, the stress trends on the top (SPOS) and bottom (SNEG) of the shell element are significantly different, so I am not sure which one to choose.
1. Which of the two (SPOS or SNEG) values is correct?
2. If possible, I would like to know in detail the stress calculation process for SPOS and SNEG in shell elements.

Thanks for reading!