Precast reinforced concrete structures with and without reinforcement pre-stressing are used both as newly designed independent ones and are the result of work to strengthen the existing structures by increasing the section. In both cases, the cross section of such elements is considered as a two-layer section, and the resulting composite element works under load under conditions distributed between the layers of internal forces, the mechanism and actual distribution of which depends on the physical and mechanical characteristics, the design scheme and the layers contact interaction parameters. When calculating and designing prestressed reinforced concrete precast-monolithic elements, the shear seam ductility is usually not paid much attention to, which complicates the structure actual stress-strain state analysis and contains a certain undiscovered potential of its rational design. One of the possible directions in solving the problem, which takes into account the contact seam shift, is the structural mechanics variation principles use in the calculation of structures such as rods of composite section. The questions of a composite reinforced concrete precast-monolithic rod of the variation principles of structural mechanics based on the method of V.Z. Vlasov - I.E. Mileykovsky in the displacements form in combination with the step-iterative method of calculation practical applicability in the structural analysis is considered in this work,. The numerical calculations results according to the proposed method are given. This allows to take into account the precast and monolithic layers shear connections specifics, to take practical account of weld compliance, and also to take into account the material characteristics’ physical non-linearity, which contributes to the precast monolithic structures rational design.