Precast and monolithic reinforced concrete structures with and without prestressing armature voltages are used as newly designed independent ones, and are the result of work to strengthen existing structures by in-creasing the cross section. In both cases, the cross-section of such elements is considered as a two-layer, and the resulting composite element operates under load in conditions distributed between the layers of internal forces, the mechanism and the actual value of the distribution of which depends on the physical and mechanical characteristics, the design scheme and the parameters of the contact interaction of the layers. When calculating and designing prestressed reinforced concrete precast and monolithic elements, the shear seam compliance is usually not taken into account, which complicates the analysis of the actual stress-strain state of the structure and contains a certain undisclosed potential for its rational design. One of the possible directions in solving the problem, taking into account the shift of the contact seam, is the use of variational principles of structural mechanics in the calculation of structures such as composite cross-section rods. In the framework of this work, the questions of practical applicability in the structural analysis of composite reinforced concrete precast-monolithic rod of variation principles of structural mechanics based on the method of V.Z. Vlasov – I.E. Mileykovsky in the form of displacements in combination with the stepwise-iterative method of calculation are considered. The results of numerical calculations by the proposed method are presented, which allows to take into account the specifics of the shear bonds of the precast and mono-lithic layer, to carry out practical accounting of the seam compliance, as well as to take into account the physical nonlinearity of the characteristics of materials, which allows the rational design of precast monolithic structures.