To study the influence of the interfacial transverse slip between concrete and the steel plates of bolted side-plated (BSP) RC beams on the structural performance, a simplified piecewise linear model for estimating the transverse slip and transverse shear transfer was proposed in analogy of the transverse shear transfer with the Winkler's model of elastic foundation, combined with previous experimental numerical studies. Thus, the functional calculation method for transverse slip and transverse shear transfer was obtained from the flexural stiffness of the RC beam and steel plates, and the shear stiffness of the bolt connection. The simplified theoretical model was also validated by test results, and it can be used for strengthening design of BSP beams.