In this research a theoretical study has been carried out on the behavior and strength of simply supported composite beams strengthened by steel cover plate taking into consideration partial interaction of shear connectors and nonlinear behavior of the materials and shear connectors. Following the procedure that already has been adopted by Johnson (1975), the basic differential equations of equilibrium and compatibility were reduced to single differential equation in terms of interface slip between concrete slab and steel beam. Furthermore, in order to consider the nonlinear behavior of steel, concrete and shear connectors, the basic equation was rearranged so that all terms related to materials are isolated in the equation from the main variable (interface slip). The exact solution was obtained by considering appropriate boundary conditions according to load types and location. A computer program has been written using MATLAB R2013a to simplify the process of computation of section properties where the load applied iteratively from zero to ultimate capacity of the beam, and the results are compared with available experimental results which show good agreement.
As the composite section reaches its ultimate capacity in bending and lower flange start yielding due to excessive loading, cover plate are furnished in order to increase load carrying capacity of beam. In the process of strengthening, using of cover plate as a percent of the area of lower flange of steel section equal to 41%, 82% and 164% will increase the beam carrying capacity by 15%, 30% and 43% respectively; also using the same above mentioned area of cover plate will reduce the central deflection by 59%, 72% and 80% respectively.
