Coupled flexural-shear design of R/C beams strengthened with FRP

The ultimate strength of R/C beams retrofitted in flexure and shear by means of externally bonded fiber reinforced polymers (FRP) has attracted the attention of many researchers due to the particularities highlighted by a wide set of experimental results. In fact, an increase of the external reinforcement area does not always lead to the expected increase of the beam load capacity, due to the interaction of flexural and shear behaviour within the discontinuity regions of the strengthened element. In this paper, the available experimental data are considered and, in the light of an equilibrated equivalent truss model, a theoretical explanation of the observed behaviour is presented. A new definition of the design strength of the externally bonded FRP reinforcement is proposed, for both flexural plates and shear ties, taking into account the ultimate anchorage force. In the first part of the paper the model is presented and discussed; in the second part, the proposed model is validated by comparison with more than one hundred experimental results. The application to the available experimental tests shows the robustness of the method, which appears to be fully eligible as a design practice procedure.