The confinement effect of fiber-reinforced polymer (FRP) jacket on the plastic hinge length of square RC columns is studied through experimental testing and analytical study. Seven half-scale RC square columns with different confinement ratios were tested. The strain of longitudinal reinforcement bars and the extent of yielding were measured by strain gauges mounted inside the reinforcement bars. The variation of the strain field of the external column face was recorded continuously by digital image correlation (DIC) during a test. More rational approaches of data analysis are used for identification of plastic hinge length from measured strain fields. The obtained results show that compared with unconfined RC columns, FRP jacketing increases the plastic hinge length when the confinement level is low, but reduces it when the confinement level is high. The analytical study shows that all existing models of plastic hinge length are inadequate in one way or another because no model includes all important factors. The proposed model can correctly capture the trends but it needs to be further improved for accuracy when more data are available.
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