The stress in a laminate varies from layer to layer. Hence, it is convenient to deal with a simple but equivalent system of forces and moments acting on a laminate cross section. Therefore, the resultant forces and moments are obtained by integrating the corresponding stress through the laminate thickness, .

(17.3)

and

(17.4)

N is the membrane force vector,  M is the bending moment vector and n is the total number of layers. The constitutive relations are given by

(17.5)

,  and the extensional, extensional-bending coupling and bending stiffness matrix of the laminate respectively are defined as

(17.6)