Research On Axial Load Behavior Of CFT Columns With Inner GFRP Tube

CFT columns with inner GFRP tube is a novel composite structure, which consists of an ordinary concrete filled steel tubular column and a concentric inner GFRP tube. The bearing capacity and ductility of CFT columns increase due to the increasing confinement of the concrete core that was provided by the inner GFRP tube. What’s more, as the GFRP tube set inside the CFT columns, the brittle failure of the concrete core confined by the GFRP tube could be avoided.In this paper, experimental study and theoretical analysis have been carried on the axial compression performance of CFT columns with inner GFRP tube. The research work and results are as follows:(1) Based on the diameter and wall thickness of GFRP tube, nine CFT columns with inner GFRP tube were tested under axial compression. By experimental research, the failure pattern, bearing mechanism, load-displacement curve, ultimate bearing capacity and ductility of CFT columns with inner GFRP tube subjected to axial compression have been studied on. The test results show that the bearing capacity of CFT columns with inner GFRP tube are larger than that of concrete filled steel tubular columns, and the bearing capacity of the post failure stage is still higher than that of concrete filled steel tubular columns. What’s more, the ductility of CFT columns with inner GFRP tube is better than that of concrete filled steel tubular columns. The failure mode of the CFT columns with inner GFRP tube is different from that of concrete filled steel tubular columns. Due to the existing of the outer layer of concrete, the failure mode of the concrete confined by the GFRP tube was ductility, which was from local failure experience for a long time development to comprehensive destruction, eventually led to the CFT columns with inner GFRP tube. In addition, different from concrete filled steel tubular columns, the failure mode CFT columns with inner GFRP tube behaves "rise-a small wave down-secondary up".(2) Based on the test results, the effects of the parameters including GFRP tube diameter and GFRP tube thickness are studied on ultimate bearing capacity, the failure characteristics, load-displacement curve and ductility of CFT columns with inner GFRP tube. Experimental results show that the axial bearing capacity and ductility of the CFT columns with inner GFRP tube increases with the increase of GFRP tube diameter when the GFRP tube thickness is the same. The axial bearing capacity and ductility of the CFT columns with inner GFRP tube increases with the increase of GFRP tube thickness when the GFRP tube diameter is the same. And the effect of GFRP tube diameter of CFT columns with inner GFRP tube is more significantly than that of GFRP tube thickness.(3) Through the analysis of the failure stage of the load-displacement curve of CFT columns with inner GFRP tube, the study found that the bearing capacity of CFT columns with inner GFRP tube after the damage is related to diameter ratio. And the relationship between Np/Nue (The ratio of the bearing capacity of the ultimate bearing capacity and the bearing capacity after destruction) and D/d (the ratio of GFRP tube diameter and the diameter of steel tube) is got through the method of statistical regression.(4) Based on experimental research, a fiber model suitable for CFT columns with inner GFRP tube is proposed. Using the proposed fiber model, full nonlinear analysis of the axial behavior of CFT columns with inner GFRP tube is taken. The calculate curves are found to exhibit good agreement with test results, so the proposed fiber model is suitable for CFT columns with inner GFRP tube.(5) A parameter study using the proposed model is carried out to investigate the behavior of CFT columns with inner GFRP tube. The study found that the axial bearing capacity and ductility of CFT columns with inner GFRP tube increases significantly with the decrease of GFRP tube diameter-thickness ratio d/t2.