A novel design equation for reinforced concrete columns confined by FRP and steel based on advanced finite element analysis

Abstract

Strengthening of reinforced concrete (RC) columns using externally-bonded fiber-reinforced polymers (FRP) is a widely accepted and used retrofit technique. The FRP encasing increases the vertical load capacity and ductility through a confinement effect, which acts in addition to the confining mechanism of the internal reinforcing steel. This contribution produced by the transverse steel confinement is commonly ignored in the design of FRP retrofit, thus often leading to an over-conservative design. This over-conservativeness is particularly evident for RC columns designed using current design codes, which have higher ductility requirements and higher transverse steel reinforcement amounts than older columns. In addition, a reliable and accurate prediction of the nonlinear structural behavior is crucial for reliability-based and performance-based design applications. To address this issue, a new FRP-and-steel confined concrete model was recently developed to rigorously account for the effects produced by the simultaneous confinement of concrete by FRP and steel on the nonlinear behavior of axially loaded RC columns retrofitted with FRP. This study proposes a modification to the ACI 440-17 design equation of the pure axial compression capacity of FRP-confined RC columns, which is based on advanced finite element (FE) modeling and structural reliability principles. Statistical information available in the literature is used to derive the probability distributions of all involved modeling and design parameters. In particular, the probability distribution for the cross-sectional capacity of the columns is obtained via Monte Carlo simulation based on nonlinear inelastic FE response analyses using a zero-length FE with fiber section in OpenSees. The iterative Hasofer-Lind RackwitzﾖFiessler algorithm is employed to assess the first-order reliability indices corresponding to the use of the proposed equation for multiple realistic combinations of design parameters. It is found that the current ACI 440-17 equation is increasingly over-conservative for increasing amounts of transverse steel, whereas the proposed design equation provides an approximately uniform reliability index under the different design conditions considered.