Seismic performance of FRP-confined RC columns : stress-strain models and numerical simulation

Abstract

Fiber-reinforced polymer (FRP) jacketing of reinforced concrete (RC) columns has become a very popular and effective means to enhance the seismic performance of existing seismically deficient RC structures. FRP confinement significantly improves the ductility and thus the energy dissipation capacity of an RC column due to the well-known phenomenon that the axial compressive strength and the ultimate axial strain of concrete can be significantly increased through lateral confinement. Despite the popularity of the method, accurate predictive models for the cyclic/seismic performance of FRP-jacketed RC columns have not been established, partly due to the lack of an accurate stress-strain model for FRP-confined concrete subjected to cyclic loading. Such predictive models should take into account various factors that an RC column is subjected to as part of a structure, which include the effects of confinement from transverse steel reinforcement, strain gradient, and fixed-end rotation. Against this background, this thesis presents an in-depth study aimed at the development of rational and accurate numerical models for FRP-confined RC columns subjected to cyclic or seismic loading. Both FRP-confined circular RC columns and FRP-confined square RC columns are examined. The theoretical framework can be readily extended to FRP-confined rectangular RC columns. The first objective of the research was the accurate modeling of stress-strain behavior of FRP-steel-confined concrete under concentric axial compression. This work is particularly important for RC columns designed following modern design standards where the confinement from transverse steel is significant and cannot be ignored. Stress-strain models were developed for FRP-steel-confined concrete in both circular and square RC columns. Under the combined action of axial compression and bending, a column section is subjected to a strain gradient. The second objective of the research was to understand and model the effect of strain gradient on the stress-strain behavior of FRP-confined concrete. With the help of a reliable three-dimensional (3D) finite element (FE) model, the axial stress distribution in FRP-confined concrete column sections under a linear strain distribution was investigated, based on which design-oriented stress-strain models for FRP-confined concrete with due consideration of the strain gradient effect were developed for both circular and square columns. The final objective of the research was to develop reliable numerical models for FRP-confined RC columns subjected to the combined action of axial compression and cyclic lateral/seismic loading. These numerical column models are based on the stress-strain models for FRP-steel-confined concrete developed in the present study as well as loading/unloading rules developed by previous researchers. A hysteretic moment-rotation model was proposed to account for the effect of fixed-end rotations. In addition, an empirical plastic hinge length model was proposed to deal with the issue of deformation localization in FRP-confined square RC columns to achieve objective predictions. The proposed numerical column models provide accurate predictions of the performance of FRP-confined RC columns, especially for the ultimate displacement and energy dissipation capacity, which are very important parameters in seismic design.