Analysis-Oriented Modeling of Partially FRP-Confined RC Columns Incorporating Non-Uniform Deformation Effects

This study proposes a generalized mechanics-based analysis-oriented model (AOM) for predicting the complete axial stress–strain response of circular reinforced concrete (RC) columns under partial (discontinuous) FRP–steel confinement. The proposed formulation couples an enhanced dilation model with a unified axial stress–strain relationship to explicitly account for non-uniform lateral expansion between wrapped and unwrapped regions, additional axial deformation developed in unwrapped zones, and FRP–steel confinement interaction through a stiffness-based framework. A spacing-dependent confinement reduction coefficient and a calibrated failure-surface function are further introduced to account for the effects of discontinuous FRP wrapping. To support model development and validation in a concrete-strength range for which experimental data are scarce, axial compression tests were conducted on five circular RC columns, comprising two unconfined reference specimens and three partially FRP-confined specimens with a concrete strength of approximately 50 MPa. The tests showed that FRP confinement increased the axial capacity by up to 365% relative to the reference capacity and increased the post-peak strain ratio to as much as 12.8 times the reference value. At four FRP layers, increasing the strip spacing from 36 to 71 mm reduced the axial capacity by approximately 49%, demonstrating the pronounced influence of confinement discontinuity. Validation against the present tests and independent experimental datasets demonstrated close agreement between the predicted and measured full-range axial stress–strain responses, including the initial response, nonlinear transition, strain-hardening branch, and ultimate response. Across the investigated datasets, the proposed formulation consistently reproduced these response characteristics more accurately than the generalized Teng et al. analysis-oriented model and the ACI 440.2 design-oriented model.