The sign convention used in the plate section is defined in this section. Notice that unlike some publications on this subject, the loading term p(x, y) points to the same direction as the z-axis. Please refer to the classical plate theory for more details.
The loading P and deflection w in the z direction indicated are positive
For the sign of moment resultants, a moment caused by a positive stress with a positive arm is considered positive and the subscript of the stress is used to denote this moment.
Therefore, positive Mx points toward the y direction under the right-hand rule.
Positive directions for bending moments.
Positive directions for shear forces.