AbstractThe traditional Euler’s buckling theory of slender columns indicates that column capacity depends on flexural rigidity (EI), rather than material strength. As such, the availability of ultrahigh modulus carbon fiber-reinforced polymer (CFRP) plates, which could be much stiffer than steel, can offer a unique alternative for strengthening slender steel columns, in lieu of welding or bolting steel plates. In this study, twelve 2.6 m long S75×8 steel columns of 197 slenderness ratio that represents the upper limit permitted by code were tested under concentric axial loading using pin-ended conditions. The columns were allowed to buckle around their weak axes. CFRP plates were adhesively bonded to the flanges of the steel I-shape sections in nine of the columns. The main parameters studied were the level of initial out-of-straightness [length(L)/8,387 to L/1,020], CFRP modulus (168–430 GPa), CFRP reinforcement ratio (13–34%) and the length of CFRP plate (33–95% of L). The gain in axial strength due to ...