Modelling the complete Behaviour of shear-critical coupling beams with FRP sheets

Fibre-reinforced polymer sheets (FRP sheets) have been used widely for the strengthening of existing shear-critical concrete members. One such application is the strengthening of short coupling beams in coupled-wall structures that work in double curvature with high shear stresses. Such members do not obey the classical plane-sections-remain-plane hypothesis and are typically designed and assessed with strut-and-tie models. However, because FRP sheets are linear-elastic and exhibit debonding from the concrete surface, they are not well suited for strut-and-tie modelling based on the theory of plasticity. Instead, there is a need for mechanical models that account for the compatibility of deformations between the coupling beam and the FRP layers. This paper discusses such a model based on a two-degree-of-freedom kinematic description of the deformation patterns of short coupling beams. The model was originally developed for reinforced concrete members and accounts for four mechanisms of shear resistance (i.e. diagonal compression in the critical loading zones, aggregate interlock, tension in the transverse reinforcement, and dowel action of the flexural reinforcement), while in this paper it is extended by adding a fifth mechanism due to the FRP sheets. The debonding and rupture of the sheets is modelled explicitly based on an existing constitutive model. The presentation will discuss the main assumptions of the model and will include comparisons with tests. As the model accounts for the subtle superposition of the five shear mechanisms, it will be used to study the effectiveness of FRP sheets in suppressing diagonal shear failures in short coupling beams.