Analytical evaluation of FRP wrapping effectiveness in restraining reinforcement bars buckling

Internal transverse steel reinforcements (e.g. stirrups) are the main internal devices that avoid the longitudinal steel bar buckling, but in many existing RC structures the amount and the spacing between steel stirrups are inadequate. In these structures longitudinal bar buckling can be avoided by applying external reinforcement, in particular, by means of Fiber Reinforced Polymer (FRP) wrapping. A novel analytical approach for the study of longitudinal bar buckling in columns wrapped with FRP is proposed. Longitudinal bars have been considered as axially loaded beams, while the mechanical effect of FRP wrapping on the bars has been modeled by means of springs. The effect of elastic and inelastic behavior has been taken into account by means of the reduced modulus theory. The well-known relations for steel stirrups have been revisited in the case of FRP wrapping to propose an analytical formulation, valid both for circular and noncircular cross sections of columns, for the evaluation of the FRP thickness needed to avoid the longitudinal bar buckling.