Experimental Investigation of Axial Compression Behaviour of Stub Circular Concrete-Filled Steel Tubes with Q690 High-Strength Steel

Circular concrete-filled steel tubes (CFSTs) can fully utilize the composite action of the steel tube and infilled-concrete, and thus its load-bearing capacity and ductility can be significantly enhanced. Therefore, they are widely used in engineering structures. The application of high-strength steel (HSS) in CFST columns can reduce the size and self-weight of the members and allows structures to achieve greater usable space while saves material, which is more economical and environmentally friendly. However, the current design codes for CFSTs are primarily based on research achievements using ordinary steel strength grades, and whether the codes are still applicable to CFSTs with high-strength steel remains unknown. In addition, existing research on the axial compression behaviour of CFST stub columns with HSS mainly focuses on the structural behaviour at the ultimate load, while little attention is given to the axial force contributions of the steel tube and the infilled concrete at different deformation stages. Therefore, this paper experimentally investigated the axial compression behaviour of CFST stub columns with HSS. Firstly, axial compression tests were conducted on six stub columns, including three CFST columns and three pure steel tubes. The main test parameter was the steel grade, including Q355, Q460, and Q690. Subsequently, based on the test results, the failure states, load-deflections, load-bearing capacity, and axial force contributions of the steel tube and infilled concrete were analysed. Finally, the applicability of the current design codes in predicting the load-bearing capacity of circular stub CFST columns under compression was discussed. The test results indicate that all the pure steel tube and CFST specimens exhibited good ductility. Due to the presence of concrete, the load-bearing capacities of CFST columns were increased by more than 30% compared to these of pure steel tubes. Based on the strain gauges attached on the surface of the steel tube, the axial resistance contributions of the steel tube and infilled concrete of CFST columns were separated. The results show that the confinement provided by the steel tube significantly increased the axial force carried by the concrete, and the increase in concrete axial force was more pronounced with higher steel grades. Although the steel tube provided lateral confinement, its axial force contribution was not significantly reduced compared to its yield load-carrying capacity. Comparative analysis between the experimental results and the predicted results by design codes reveals that the current Chinese design code (GB 50936—2014) can effectively predict the compression resistance of CFST columns using ordinary grade steel, but when high-strength steel is used for the steel tube, the code tends to provide overly conservative or potentially unsafe predictions. The current European design code (EN 1994-1-1) provides reasonable and conservative predictions of the compression resistance for CFST columns using both ordinary steel and high-strength steel. It can still be applied to CFST columns with Q690 HSS. However, EN 1994-1-1 underestimates the axial resistance contribution of the steel tube and overestimates the axial resistance contribution of the infilled concrete.