UC Davis

Reinforced concrete (RC) columns can be effectively rehabilitated or strengthened by externally bonded fiber-reinforced polymer (FRP) wraps, which can improve the axial load capacity and ductility of these columns through a confinement effect. Modern design codes require that seismically designed RC columns have significant minimum amounts of both longitudinal and transverse steel. This transverse steel can apply a considerable confining pressure into the concrete core in addition to that produced by the external FRP sheets. This simultaneous confinement can be accurately modeled using a recently developed FRP-and-steel confined concrete model for finite-element analysis. This paper presents a comprehensive parametric study to 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 a relative confinement coefficient. The results of this study can be used to achieve more efficient and economical retrofit of RC columns through FRP confinement.