Axial-strength model for FRP-confined concrete based on the improved twin shear strength theory

External confinement by wrapping with FRP sheets is a very effective method for the retrofit of a concrete structure subject to either static or seismic loads. For the reliable and underlying analysis of the confinement, an accurate and theoretical strength model is required for FRP-confined concrete. In this paper, the math-deductive method is introduced based on a review of the existing models. The theoretical method uses the failure criterion as the foundation to establish the axial-strength model. To consider the stress condition of FRP-confined concrete, an improved failure criterion is developed based on twin shear strength theory. Based on a proposed failure criterion, an axial-strength model is proposed for FRP-confined concrete columns. The coefficients in the proposed model are computed with respect to the range of the unconfined concrete strength, and the ranges of the coefficients show obvious trends. The predictions of the proposed model agree well with the comprehensive database.