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.