Design-Oriented Model of Unified Character to Determine Softening–Hardening Stress–Strain Behavior of FRP-Confined Concrete Columns of General Cross Section

Even though many experimental studies have evidenced that fiber-reinforced polymer (FRP)-confined concrete columns under compression might exhibit a postpeak strain-softening response, followed by a hardening behavior (a stress reduction–recovery response), most existing models are only applicable to confined columns developing full hardening behavior. Additionally, their applicability is limited to a certain column cross-sectional shape (circular or noncircular). Therefore, the present study is dedicated to the establishment of a new design-oriented stress–strain model, unified for FRP-confined circular/noncircular concrete columns. For this purpose, first, a new nondimensional confinement stiffness-based index is developed, below which the column response is transformed from a full-hardening behavior (Type A) to a postpeak strain softening–hardening one (Type B). A parabolic-linear stress–strain formulation is proposed for Type A columns by developing a new expression for the calculation of the linear hardening branch’s slope. For the case of Type B, a new methodology is introduced for the simulation of the stress reduction–recovery response beyond the transition zone, whose main elements are calibrated by a large test data set. The dominance degree of the noncircularity effect of the cross section on both Type A and Type B cases was assessed and reflected in the key components of the proposed stress–strain models, based on nonlinear regression analysis. With these considerations, this model could accurately simulate the impact of the noncircularity effect on the stress–strain relationship of Type A and Type B columns, whose predictive performance is validated through a comparative assessment with existing models based on large test stress–strain results.