Flexural and axial behaviour of carbon fibre reinforced polymer (CFRP) strengthened steel circular hollow sections.

2017-03-22T01:44:55Z (GMT) by Haedir, Jimmy
External bonding of carbon fibre reinforced polymer (CFRP) sheets with epoxy resin has been effectively used to strengthen and repair concrete structures. In recent years, there has been resurgence in the use of CFRP to strengthen steel bridges and structures as a result of increased service loads, corrosion and inadequate maintenance. This strengthening system may be extended to steel tubular structures, which may be prone to local buckling. Only a few studies tested steel tubular sections of wider section slenderness range, such as compact, non-compact and slender sections in bending, and addressed the behaviour of such sections reinforced using bidirectional (longitudinal and hoop directions) CFRP wraps. In addition, few studies have investigated the response of fully effective and not-fully effective composite steel hollow sections in compression. The research aims to understand the structural behaviour of steel circular hollow section (CHS) beams and short columns externally reinforced with CFRP sheets by means of static tests and theoretical analysis, and to develop rules for strength design of the composite sections. The effects of much wider section slenderness ratios, amount of CFRP and fibre orientation were examined. Initially, the flexural behaviour was investigated by conducting experiments on eighteen CHS beams (five control specimens and thirteen reinforced with CFRP sheets) under pure bending. The diameter-to-thickness ratios of the tubes used in the tests ranged from 12 to 96. Rotation and strain measurements at mid-length were taken. The reinforced beams had better performance compared with their bare steel counterparts, especially for tubes with larger diameter-to-thickness ratios. The experimental results have demonstrated that it improved the bending strength and rotation capacity of the bcams. The improvement in the behaviour of the CFRP-reinforced tubes appears to be related to the delay or elimination of elastic local buckling in slender section tubes as a result of the restraint provided by the hoop CFRP layers. The longitudinal fibre layers were shown to be effective in providing strength in the CHS beams. The increase in bending capacity obtained in the tests was between 3 and 90%. The main failure mode of the strengthened beams is associated with crushing of the CFRP sheets. The second test programme was performed on short CHS columns subjected to axial compression. A total of ten columns were tested, of which six were reinforced with CFRP sheets and four were control specimens. The tubes have diameter-to-thickness ratios of ranging between 37 and 78. The parameters investigated included the amount of CFRP, the diameter-to-thickness ratio of the steel tube and the fibre configuration. The axial load-shortening and strains at mid-length of the tube were recorded throughout the period of loading. A theoretical model was developed to calculate the ultimate moment capacity of CFRP-reinforced steel CHS beams subjected to bending. The ultimate capacity was calculated based on equilibrium of forces and strain compatibility between CFRP and steel. The model assumed that the steel is either elastic, elastic-plastic or plastic at ultimate. Two cases were included in the analysis, one of which considers the CFRP in compression and the other does not account for the CFRP in compression. It was found that the CFRP in compression has significant influence on the ultimate capacity of CHS beams. Further analysis was carried out to predict the moment-curvature response of CFRP-reinforced steel CHS beams. Material nonlinearity was implemented using a tri-linear stress-strain relationship for the steel and a linear elastic relationship for the CFRP material, involving subdivision of the section into elemental areas. The inclusion of volume fractions of the fibre and adhesive is necessary to determine the elastic properties of the CFRP. The nonlinear equilibrium equations were solved using an incremental-iterative method. The analytical predictions were verified with the experimental results. It was shown that the volume fraction of the fibre has an effect on the initial stiffness of CHS beams. The effects of local imperfections in slender section tubes and strain hardening in the post-yield stage would need to be taken into account in the analysis to improve the accuracy of the results. A design method based on the modular ratio concept was proposed for the CFRP¬-reinforced steel circular tubular beams and short columns. The applicability of current section slenderness limits for steel hollow sections for the design of CFRP-reinforced steel CHS has been discussed. Design expressions for CFRP-reinforced CHS were derived as a function of the parameters related to the amount of CFRP, the elastic modulus of hoop CFRP, the geometrical slenderness, and the yield stress of steel. Design curves illustrating the significance of strengthening parameters were developed, and they form the basis of a simple and effective design procedure for CFRP-reinforced steel CHS tubular beams and columns.