Please find below reply from one of my colleague on similar query wrt to Radioss solver.
The Bending and Membrane behavior depends greatly on element's through thickness distribution.
The stress distribution across the plate thickness is a linear combination of bending and axial (membrane) stresses so if you have the distribution across the thickness of the plate the axial component is simply the average of stresses on the top and bottom and the bending stresses is the total minus the axial stresses.
Through thickness stress distribution becomes very significant when the element undergoes large deformation / rotation or being strained at large plastic strains. These behavoir will be measured / recorded by through thickness integration points
The shell element formulation in OptiStruct is based on linear deformation theory and hence do not have through thickness integration points. RADIOSS has the capability of defining through thickness integration points and hence could output Membrane and Bending stress In addition to Top and Bottom layer stresses using the below output commands: