Fiber-Element Model for Slender HSS Columns Retrofitted with Bonded High-Modulus Composites

This paper presents an analytical model developed to predict the behavior of concentrically loaded cold-formed square hollow structural section (HSS) slender columns, strengthened with high-modulus carbon fiber reinforced polymer (CFRP) sheets. The model predicts the load versus axial and lateral displacements, and accounts for plasticity of steel, the built-in through-thickness residual stresses, geometric nonlinearity, initial out-of-straightness imperfection, and the contribution of CFRP sheets. The model was verified using test results. Since the gains in strength due to CFRP retrofitting of the tested columns were found to be sensitive to the inherent out-of-straightness, the model was used to uncouple the effects of out-of-straightness and CFRP reinforcement ratio. It was shown that axial strength and stiffness of HSS columns could be increased substantially when retrofitted with longitudinal CFRP sheets, due to reduction of the lateral displacement induced by buckling. It was also shown that CFRP effectiveness increases for columns with larger out-of-straightness deficiencies and columns of higher slenderness ratios.