The development of the composite section is of great importance in structural design, while the aim is to reduce weight and deflection in a way that does not affect the composite section's strength and durability. This paper studies the computational analysis of the composite section where the reinforced concrete was replaced with Autoclaved Aerated Concrete (AAC) in the compression zone. The theory of first principles of concrete, together with Hook's law, were adopted to find flexure, shear, equivalent modulus of elasticity, effective moment of inertia, and deflection. Results were discussed and investigated using the finite element models developed by ANSYS WORKBENCH software. It was concluded that the proposed analytical model could efficiently provide the exact solution for the composite section.