Study On Behavior Of A New Concrete-Filled Steel Tube Reinforced Concrete Column Under Axial Compression

Concrete filled steel tube (CFST) has the advantages of high strength and good ductility. However, due to exposure to the air, the steel tube tends to lose the loading capacity under the fire loads. In addition, the fire-retardant properties of reinforced concrete (RC) specimens are better than those of CFST specimens. Therefore, the concrete-filled steel tube reinforced concrete (CFSTRC) column, which consists of CFST column and the wrapping of reinforced concrete (RC), is expected to combine the advantages of both. In this context, the CFSTRC column obtains more and more attention of researchers and is gradually applied to a number of buildings and bridges. However, the research about its behaviour under axial compression is still not deep enough; especially the research in the CFSTRC column with hollow section and several steel tubes is still in the early stages. In this paper, the combination of experimental and theoretical studies was used to investigate the performance of CFSTRC columns under axial compression. The main research works were summarized as follows:(1) In order to demonstrate the necessity of the research in this paper, the relevant research status at home and abroad were systemically summarized firstly.(2) Including 4 CFSTRC columns with a solid square section,4 circular CFSTRC columns with a solid circular section and 4 CFSTRC columns with hollow square section,12 CFSTRC columns tests under axial compression were carried out in this paper. The test parameters contain strength of the concrete in the steel tube and the section type. Meanwhile, the effects of these parameters on the failure mode, ultimate capacity and ductility of CFSTRC columns were studied. The research results indicate that CFSTRC columns with circular section have the higher bearing capacity than those with the square section, and the ductility of CFSTRC columns with solid section is better than that with hollow section.(3) A finite element (FE) model was established. By comparing several concrete models, the proper the concrete constitutive model was selected. The validity of this FE model was fully verified by 38 experimental data.(4) Based on this FE model, the mechanism was deeply analyzed, including the failure modes, the load distribution ratio, the distributions of stress, strain and contact force. Meanwhile, a large number of parameters analyses were performed on the influences of axial bearing capacities of the CFSTRC columns. It shows that the enhance of the strength of concrete, the concrete strength in steel tube, the yield strength of steel tube and the ratio of CFST will greatly improve the bearing capacity of the CFSTRC column. The peak strain of outsourcing by the reinforced concrete structure is approximately equal to 0.0022.(5) Based on the conclusions from the analysis of mechanism and parameters, the practical formulae of strength and stability of the CFSTRC column were proposed. The predicted results of the practical formulae were in good agreement with those of tests. The study may provide reference to practical construction.