CalculiX doesn’t have true shell elements, these elements are internally expanded to solids. You can’t obtain top/bottom stresses like in other software, but you can use this setting:
If you set it to 2D, you will essentially remove bending stresses:
If OUTPUT=2D the fields in the expanded elements are averaged to obtain the values in the nodes of the original 1d and 2d elements. In particular, averaging removes the bending stresses in beams and shells.
But you will have output only from the midsurface (or top/bottom surface if you use offset). With OUTPUT=3D, you will get all the stresses.
Actually, there is one true shell element, US3, not supported in PrePoMax, but can be defined using keywords: Support of shell user element (US3)
This element provides the SNEG, SMID and SPOS output variables for the stresses at different locations (bot, mid, top), so maybe you could use that. This element has some significant limitations, though:
The shell formulation is implemented as element US3 in CalculiX and can be used for static and dynamic linear elastic (small deformations) calculations under the consideration of isotropic material properties.