Stress–Strain Model for FRP-Confined Circular Concrete Columns Developing Structural Softening Behavior

Even though several stress–strain models have been proposed for fiber-reinforced polymer (FRP)-confined concrete columns subjected to axial compressive loading, very few models can predict an axial response featuring postpeak strain-softening behavior. Furthermore, the reliability of most of these models is limited to only a certain concrete strength class (either normal-, high-, or ultrahigh-strength concrete). This study aimed to develop an analytical model for determining the axial response of FRP-confined concrete applicable to cases with different levels of confinement stiffness and concrete strength. For this purpose, this research proposed a new confinement stiffness threshold dependent on the coupled concrete strength and column dimension size effects to classify quantitatively FRP-confined concrete’s behavior in two distinguished subcategories: strain-hardening behavior and postpeak strain-softening behavior. For FRP-confined concrete with strain-hardening response, a parabolic–linear stress–strain relation was developed, where a new formulation was derived for the slope of the linear second portion, calibrated by 583 test data. To simulate FRP-confined concrete with postpeak strain-softening behavior, a new methodology was proposed whose key components were calibrated by using 121 test data. With these features, the proposed model can objectively account for the integrated influence of concrete strength and confinement stiffness on stress–strain response. The predictive performance of the developed stress–strain model was evaluated by comparing the predictions of a wide range of relevant experimental test data, which confirms the model’s reliability and accuracy. Compared to the other existing stress–strain models, the proposed model performed better.