UC Davis

Application of fiber-reinforced polymer (FRP) composites in strengthening of reinforced concrete (RC) structures has become an increasingly accepted engineering practice. In particular, the use of externally-bonded FRP wraps as a confining material for concrete can enhance both the compressive strength and the ultimate strain of concrete, making it suitable for strengthening and/or seismic retrofit of existing reinforced concrete columns. The confinement effect produced by the externally-bonded FRP acts in addition to the confining mechanism of the existing internal reinforcing steel, thus increasing the load-carrying 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 excessively conservative retrofit design.This dissertation presents a confined concrete material constitutive model for use in finite element analysis, which is able to accurately model the combined confinement effects of FRP and internal steel reinforcement on the structural monotonic, cyclic, and/or dynamic response of reinforced concrete RC columns confined with externally-wrapped FRP. The proposed material constitutive model for FRP-and-steel confined concrete explicitly models the simultaneous confinement produced by FRP and steel on the core concrete to predict the combined effect on the structural response of circular RC columns. This proposed material model is combined with a force-based frame element to numerically predict the load-carrying capacity of FRP-confined RC columns subjected to different loading conditions. Numerical simulations are compared to experimental test data available in the literature and published by different authors. The numerically simulated responses agree very well with the corresponding experimental results. The proposed model is found to predict the ultimate load for FRP-confined RC circular columns with better accuracy than models that do not consider the simultaneous confinement effects of FRP and steel. The proposed FRP-and-steel confined concrete model is employed in a comprehensive parametric study to numerically investigate the steel confinement effects and the relative importance of key modeling and design parameters on the axial strength of FRP-confined RC columns. The results show that the steel confinement effect can significantly increase the axial strength of FRP-confined RC columns, particularly for large cross-sections, low concrete compressive strengths, and low amounts of confining FRP. The steel confinement effects induce two distinct behaviors depending on the ratio between the FRP lateral confinement and the unconfined concrete peak strength. These two behaviors can be described as functions of two different relative confinement coefficients. The numerical investigation of the effects of the transverse steel confinement on FRP-confined circular RC columns from the case of pure axial loading is also extended to the case of combined axial load and bending moment. A thorough parametric study is conducted to investigate the effects of different design parameters when varied within their range for practical applications. The two synthetic coefficients previously proposed to quantify the effect of transverse steel confinement on concentrically-loaded FRP-confined circular RC columns are modified to incorporate the effects of load eccentricity. The outcome of this research could lead to significant benefits in terms of safer and/or more economic design of FRP-confinement retrofits of RC columns. Based on the results from the parametric study, a modification is proposed to the ACI 440.2R-17 design equation of FRP-confined RC circular columns subjected to axial loading. 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 ACI 440.2R-17 design equation, which produces increasingly excessively conservative retrofit designs for increasing amounts of transverse steel reinforcement. A practical design procedure based on the proposed design equation is also presented. In order to reduce the associated computational cost of the proposed FRP-and-steel confined concrete model, and to increase the corresponding numerical robustness, this dissertation also proposes a new optimization procedure to obtain an analytical expression for the iteratively-generated monotonic envelope of the original stress-strain model. Several analytical functions are tested and their capability to fit the iteratively-generated uniaxial stress-strain model for the monotonic envelope curve of FRP-and-steel confined concrete is evaluated. The newly-proposed analytical formulation of the uniaxial stress-strain model for FRP-and-steel confined concrete is compared with the original iterative formulation in terms of computational cost for an application example consisting in a nonlinear seismic time-history analysis of a five-span bridge structure with FRP-retrofitted RC piers. It is found that the use of the newly proposed optimization-based analytical monotonic envelope can reduce by more than 30% the computational time associated to the original iteration-based monotonic envelope with negligible changes in the structural response prediction at both global and local levels.