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Seismic retrofit of concrete columns with FRP composites is a well-known method for enhancing their strength and ductility. Behavior of rectangular concrete columns confined with FRP composites depends on several parameters, including unconfined concrete strength, confinement level, aspect ratio of cross-section (defined as the depth /width of the cross-section), and the sharpness of the section corners. For proper design of rectangular concrete columns confined with FRP composites, a good understanding of the stress–strain behavior of FRP-confined concrete prism under axial monotonic compression is necessary. In recent years many design oriented stress-strain models with simple closed-form expressions have been developed for FRP-confined concrete columns. Also some analysis oriented models are proposed in which the stress-strain behavior of circular columns is generated with an incremental process. But to the best knowledge of authors, there is not an analysis oriented stress-strain model for FRP-confined rectangular columns in the literature. Thus in this paper a base model for actively confined concrete is used to develop a new analysis stress-strain model for rectangular concrete columns confined with FRP. This model considers all parameters that affect behavior of rectangular columns. The procedure for generation of analysis oriented stress–strain curves for FRP–confined concrete based on active confinement model is as follows:
1) For a given axial strain, find the corresponding lateral strain according to the lateral-to-axial strain relationship;
(2) based on force equilibrium and radial displacement compatibility between the concrete core and the FRP jacket, calculate the corresponding lateral confining pressure provided by the FRP jacket;
(3) use the axial strain and the confining pressure obtained from steps (1) and (2) in conjunction with an active-confinement base model to evaluate the corresponding axial stress, leading to the identification of one point on the stress–strain curve of FRP–confined concrete;
(4) Repeat the above steps to generate the entire stress–strain curve.
     It is obvious from above procedure that the main relations in analytical modeling are the lateral-to-axial strain relationship, lateral confining pressure provided by the FRP jacket, peak axial stress on the stress–strain curve of actively confined concrete, axial strain at peak axial stress, and stress–strain equation. Thus in this paper these relations for rectangular sections are presented and when these relations be defined, the stress-strain curve can be generated using above mentioned procedure. In this paper an experimental database containing 167 axial compression test results of externally confined rectangular columns is assembled and used for stress-strain modeling. The proposed model considers different parameters that can affect the behavior of rectangular columns, including aspect ratio, corner radius, confinement ratio, and unconfined concrete strength. Also both the strain hardening and strain softening behavior of rectangular columns can be modelled by the proposed formulation. Comparison between experimental results and those of model predictions indicates that the proposed model provides good predictions for different parts of stress-strain curve such as compressive stress and strain also ultimate stress and strain. Also the shape of predicted stress-strain curve is in a good agreement with the test results.

Keywords: FRP, Rectangular Concrete column, Confinement, Stress-strain model, Analysis oriented model

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