Confinement Model for Axially Loaded Concrete Confined by Circular FiberReinforced Polymer Tubes

An analytical model is introduced in this paper to predict the behavior of axially loaded circular concrete columns confined by fiber-reinforced polymer (FRP) tubes. The model is an extension to the confinement model introduced by Mander et al. for concrete confined by steel reinforcement. The model is based on equilibrium, compatibility conditions, and the biaxial strength failure criteria of FRP tubes. It can be used to predict the behavior of prefabricated FRP tubes totally filled or partially filled with concrete, as well as concrete wrapped with FRP sheets. The model can account for the case of axially loaded concrete core only as well as the composite section of the concrete core and FRP tube. The model is verified by experimental results reported by the authors and other researchers. A parametric study is presented to examine the effect of stiffness of the FRP tube, the effect of loading the FRP tube axially, and the effect of presence of inner hole inside the concrete core. The study showed that increasing the central hole size reduces the confinement effect, increasing the stiffness of the tube improves the confinement, and axial loading of the FRP tube significantly reduces the confinement.