The paper considers the methodology of constructing the strength area of a rectangular reinforced section. The notion of “section strength area” is used in the calculations of structures according to the limit equilibrium. The peculiarity of the strength region is that inside the strength region the section works in the elastic stage, and at its boundary it transits to the limit state with the possibility of unrestricted plastic deformation. The equations describing the boundary of the section’s strength region are often called yield conditions. In this paper, it is assumed that the material of which the section is made and the reinforcing material deform according to the ideal elastoplastic body law. Thus, the deformation diagrams of the materials are described by the Prandtl diagram. The material, of which the section is made, has different tensile and compressive yield strengths. The reinforcing material has the same tensile and compressive yield strengths. In deriving the equations describing the boundary of the section strength area, it was assumed that the bending moment and the longitudinal force applied in the center of the rectangle act in the section. Given that the section may have asymmetrical reinforcement, different equations are used to describe the upper and lower boundaries of the strength area. In order to construct the strength area, it is necessary to solve the optimization problem of finding the extreme value of the moment taking into account the constraints (equations and inequalities) for a given value of the longitudinal force. The analysis of results obtained in this way for a symmetrically reinforced cross-section made it possible to propose a simpler technique for constructing the strength area of a rectangular reinforced cross-section without solving the optimization problem.