Reliability-Based Calibration of New Design Procedure for Reinforced Concrete Columns under Simultaneous Confinement by Fiber-Reinforced Polymers and Steel

External confinement of reinforced concrete (RC) columns with fiber-reinforced polymer (FRP) jackets and/or wraps is a technique extensively used for strengthening and retrofit of structurally deficient columns. The confinement effect produced by the externally bonded FRP acts simultaneously with the confining mechanism of the existing internal reinforcing steel, thus increasing the vertical load capacity and ductility of the member. The transverse steel confinement contribution can be significant, although it is generally ignored in existing design guidelines for FRP wrapping, potentially leading to an overconservative retrofit design. This paper proposes a modification to the design equation of FRP-confined RC circular columns subjected to axial loading that is given in the current US standards. The proposed design equation is calibrated through a structural reliability analysis approach, in which the capacity model (corresponding to the probability distribution for the axial load capacity of the columns) is generated via Monte Carlo simulation based on advanced nonlinear finite-element response analyses for multiple realistic combinations of design parameters. Under different design conditions, the newly proposed design equation provides a significantly less variable reliability index than that obtained using the current accepted design equation, which produces increasingly overconservative retrofit designs for increasing amounts of transverse steel reinforcement. A practical design procedure based on the proposed design equation is also presented.