Fundamental Mechanical Behavior Of Short Concrete-Filled Thin-Walled Steel Tube Columns

Usually, thick steel tubes are used in traditional engineering of concrete-filled steel tubes (CFST). Engineering experiences show that using thin-walled steel tubes in CFST engineering can reduce the consumption of steel, the workload of welding and cost of the project. However, the thin-walled steel tubes are prone to local buckling under axial pressure, especially for the square and rectangular cross-section shape. Referencing steel structure, layouting the longitudinal stiffeners along the thin-walled steel tubes of CFST can provide restrictions for plate panels, consequently postpone the occurrence of local buckling. Behavior and mechanism of Concrete filled thin-walled steel tubular beam-columns were investigated. Two kinds of section tubes were studied in this paper, that is, the stiffened concrete-filled thin-walled steel tubes (SCFTST) and the unstiffened concrete-filled thin-walled steel tubes (UCFTST). The main work in the dissertation includes:(1) Experimental tests on ten concrete-filled thin-walled tubular columns (CFTST) were axially carried out, the main experimental parameters were tube diameter-to-thickness ratio, flexural rigidity and welding ways of stiffener, loading ways. Eleven concrete-filled thin-walled rectangular steel tubular beam-columns were tested, where eccentricity ratio, depth-to-width ratio, welding ways of stiffener were selected as main varied parameters. The behavior of the thin-walled steel tubular columns was analyzed. Parametric analysis was performed on the influences of bearing capacities of the composite members.(2) Load-deformation relationship curves of CFTST subjected to compression and bending were analyzed by finite element method. The predicted load versus deformation relationship curves are in good agreement with those of tests,it demonstrated that the numerical method is reliable in predicting the load versus deformation relationship. A calculation method of flexural rigidity of stiffener was proposed by the finite element method. The study shows that the stiffeners can improve the local buckling performance of CFTST efficiently, enhance the restriction to core concrete by steel tubes, consequently increase the axial compression capacity of members, however the influence on ductility is not obvious.(3) Stress distributions of concrete and failure modes of tubes were analyzed by finite element method to CFTST beam-columns. Parametric analysis was performed on the influences of mechanism of the composite members by the finite element method. The selected parameters were eccentricity ratio, depth-to-width ratio and stiffener, etc.(4) The experimental results are compared with the predicted ones on current design codes, and based on the parametric analysis, simplified calculation methods of bearing capacities of SCFST members subjected to compression and bending were proposed.