Where the centroid plane of a slab changes elevation, there is an interaction of in-plane and out-of-plane forces. The interaction of the two sets of forces is simple and is defined purely by moment and force equilibrium. A simple centroid step is shown in elevation view below:

F_x’ = F_x

V_xy’ = V_xy

V_xz’ = V_xz

M_y’ = M_y - F_x·d

M_xy’ = M_xy - V_xy·d