Torsional strengthening of steel circular hollow sections (CHS) using CFRP composites

Abstract

The use of carbon fibre reinforced polymer (CFRP) for strengthening of steel members has attracted increasing attentions in the structural engineering community. However, there is still a lack of understanding on the mechanisms of CFRP strengthening of steel tubular sections which are subjected to torsion. This paper presents a comprehensive experimental study on the effectiveness of the CFRP material for strengthening of steel circular hollow section (CHS) under pure torsion. Five different types of CHSs with various diameters and wall thicknesses were selected. Furthermore, five different types of CFRP strengthening schemes were used with the fibers aligned in different angles respect to the axial direction of the CHSs. In order to investigate the wrapping sequence, spiral (S) and reverse spiral (R), different mixed S/R schemes, such as SRRR vs. RRRS and etc., were applied to the CHSs. The abbreviations of the wrapping scheme, such as SRRR, refer to four strengthening layers with the spiral (S) and reverse spiral (R) wrapping directions. Various failure modes of the CFRP-strengthened specimens were observed including CFRP rupture, CFRP crushing and adhesive cracking without fiber failure. It was found that the CHS with larger diameter and thickness benefit the most from the CFRP strengthening scheme. In addition, S wrapping was found to be the most effective strengthening scheme. Finally, in order to achieve the best strengthening performance of the system for a mixed S/R CFRP strengthening scheme, it was recommended to apply the S wrapping inside. A theoretical model was proposed for the prediction of the torsional capacities of the CHS specimens. Reasonable agreement was achieved between the predictions and the experimental results.