This paper investigates the bonding between adherents in the reinforcement of steel structures by using FRP materials. The interface behaviour between the steel and the FRP strips and in particular the interaction between the plastic deformation and the debonding process is investigated. This paper first illustrates a numerical procedure to evaluate the bending moment and shear force diagram through the application of the well-known principle of virtual work. The stress migration due to plastic deformation of statically undetermined steel structures is also outlined. A method based on cross-sectional equilibrium and strain compatibility is then proposed to predict the FRP axial stress and interface shear stress of the steel beam reinforced by FRP materials. Finally, the adhesive layer shear stress concentration in critical sections and failure conditions are evaluated through numerical examples
