stress at upper face and lower face for shell

[jsboy]

I am interested to know the flow in Nastran to calculate the stress at upper face and lower face for shell element. Here, I am only talking about bending state because stress at membrane state is same throughout thickess direction. In my understanding by reading FE book, it is, first get stress resultant which is actually moment Mx, and using equation, sigma=(Mx/I)*z, to obtain the stress at upper and lower. Please throw some light on that.