An experimental investigation was conducted in this study on axially loaded square hollow section (SHS) slender columns retrofitted by carbon fiber reinforced polymer (CFRP). A total of seven specimens with identical cross section dimensions and raw material properties were compressed between pinned ends to identify the influence of the CFRP, in which six specimens were retrofitted by the CFRP. The effects of many influential factors including different layers of the CFRP, retrofitting directions of the CFRP and retrofitting sequences of the CFRP on the behaviour of the CFRP strengthened SHS slender columns were carefully evaluated. The column strengths obtained from the experimental investigation are compared with the design strengths calculated using the design equations given in the British Standard (CIRIA) and the equations modified based on the section conversion method proposed by Canadian Standard (CAN/CSA-S16-01). It is shown from the comparison that the ultimate strengths of the SHS slender columns were enhanced by retrofitting with the CFRP. The retrofitting directions of the CFRP in the longitudinal directions were found to have a great influence on the ultimate strengths of the CFRP strengthened SHS slender columns. Furthermore, a correction factor (βc) is proposed in this paper for the stability of the CFRP retrofitted SHS slender columns under axial compression.
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