AbstractThis paper presents an experimental study on the seismic behavior of polyethylene naphthalate (PEN) fiber–reinforced polymer (FRP)-jacketed circular reinforced-concrete (RC) columns. A total of seven specimens were tested under constant axial load and cyclic lateral load. The key parameters studied were the thickness (one, two, and three layers) and type (PEN FRP and CFRP) of FRP jacket. Test results indicated that the control column failed by buckling of the longitudinal reinforcement and shortage of ductility, while concrete spalling and bar buckling were effectively inhibited by the application of external FRP jacket. It is also observed that PEN FRP-jacketed specimens had more additional strain capacity compared with CFRP-jacketed specimens at the final condition. The additional strain capacity can serve as a safety reserve for structures and make PEN FRP more suitable for strengthening large or noncircular columns. Furthermore, it is found that 1-ply PEN FRP and 1-ply CFRP had a similar strengthening effect and thus the design of PEN FRP-strengthened column can simply refer to the design method of CFRP-strengthened columns in current codes. Different FRP thickness and types had marginal impact on the hysteretic curves of the specimens, which was attributed to the low axial load ratio (0.15) and high slenderness ratio (5.25) adopted in this paper. Finally, based on a cyclic stress–strain model for longitudinal reinforcement, including buckling effect, developed by the authors in a previous study, the test columns were simulated by the Open System for Earthquake Engineering Simulation (OpenSee)s to achieve an in-depth understanding of the experimental findings and associated strengthening mechanisms. Further parametric analyses showed that considering bar buckling played a significant role in accurately predicting the hysteretic response of FRP-jacketed columns.