Modeling the Effect of FRP Sheets on the Behavior of Short Coupling Beams Exhibiting Diagonal Tension Failure

Reinforced concrete (RC) coupling beams in pre-1970s coupled wall structures often feature small amounts of stirrups, and thus are susceptible to shear failure along inclined cracks. In short beams with span-to-depth ratio smaller than about 2.5, the failure occurs along diagonal cracks and can limit the ductility of the member, which in turn limits the seismic performance of the entire coupled wall system. To suppress diagonal tension failure, the coupling beams can be strengthened with externally-bonded fiber-reinforced polymer (FRP) sheets. However, as the FRP sheets exhibit debonding and rupture, their contribution to the shear resistance cannot be evaluated without an explicit consideration of the compatibility of deformations with the existing beam. Therefore, this paper proposes a mechanical model based on deformations which predicts the complete behavior of FRP-strengthened short coupling beams exhibiting diagonal tension failure, including the effects of debonding and rupture of the FRP. The model is an extension of a two-parameter kinematic theory (2PKT) for RC coupling beams and uses two degrees of freedom to evaluate the deformations of the beam. The extended 2PKT is validated with tests from the literature and is used to study the effect of FRP sheets. The model predicts the influence of existing diagonal cracks as well as the effect of the preparation of the beam edges prior to the application of the FRP sheets. According to the model, the effectiveness of FRP sheets to increase the shear resistance increases with the span-to-depth ratio of the beam. Furthermore, for the relatively very short beams studied in the paper, it is predicted that U-sheets and side sheets are nearly as effective as fully-wrapped FRP sheets.