Effect of Internal Stirrups on the Eccentric Compression Behavior of FRP-Confined RC Columns Based on Finite-Element Analysis

Fiber-reinforced polymers (FRPs) have been widely used in the retrofitting and rehabilitation of reinforced concrete (RC) columns because they can provide substantial lateral confinement to concrete. Conventionally, the presence of internal stirrups is neglected in the retrofitting design due to a lack of understanding on the effect of internal hoops when both external FRP and internal steel confinement exist, particularly under eccentric loading. In this study, a finite-element (FE) method is developed to investigate the eccentric compression behavior of FRP-confined RC columns. A reliable concrete plastic-damage model that accurately considers the confining stiffness ratio between the FRP and steel is proposed for the first time, which facilitates the study of a difficult problem: the behavior of RC columns with the dual confinement system under eccentric loading. The accuracy of the model is verified by test results. An extensive parametric study of various affecting factors is carried out to investigate the confinement mechanisms of the concrete columns in terms of the axial stress and confining pressure distributions, lateral principal stress ratio, local stress–strain response, and interaction between the FRP/steel hoops and concrete. It is found that the nonuniform confining pressures under eccentric compression are significantly affected by the internal stirrups. With the existence of discrete transverse steel reinforcement, the axial resistance of the concrete core is increased, thereby improving the ultimate load capacity of the columns. An ultimate axial load model for RC columns that considered combined FRP–steel confinement is subsequently proposed. More accurate results are obtained using the proposed model compared with those calculated from existing design codes.