Models for determining the compressive strength of concrete columns confined by FRP have been presented in previous studies. In this study, a large set of experimental data regarding circular columns confined with different types of FRP has been collected. In order to increase the accuracy in the existing models, three modified models for predicting the compressive strength of circular columns confined with FRP has been proposed by using the collected data. The FRP strain efficiency factor in the proposed models is considered as: (i) a function of the strain ratio, (ii) a function of the confinement stiffness ratio, and (iii) a function of the combination of these ratios. Studying the analytical results using the proposed factors revealed that models wherein the FRP strain efficiency factor is a function of the strain ratio or the combination of the confinement stiffness ratio and strain ratio give results closer to the experimental results.
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