Bearing Capacity Of Lightweight Aggregate Concrete Filled Thin-Walled Steel Tube Under Axial Load

With the development of the modern civil engineering structure, the construction material and structure are higher required. As a new type of concrete filled steel tube, lightweight aggregate concrete filled thin-walled steel tube (LACFTST) has many advantages, such as high bearing capacity, fine plasticity and toughness, fire-resistant, seismic, excellent economic benefits and convenient construction and so on. The lightweight aggregate concrete filled thin-walled steel tube is widely used in the engineering construction, especially multi-story and low-rise buildings. However, little study has been done about its mechanical behavior. Related code or specification have not been complete, therefore, it is of great importance to investigate the mechanical performance of the short columns under axial pressure.In this paper, Mechanical analysis and calculation method of bearing capacity of the lightweight aggregate concrete filled thin-walled steel tube under axial load is carried out based on the unified strength theory, finite element simulation, the main contents are listed as follows:1. Based on the unified strength theory, considering the effects of the intermediate principal stress and the multi-axial strength criterion for the lightweight aggregate concrete, by introducing parameterαu andβu, loop tensile stressσθ, longitudinal compressive stressσz and radial compressive stressσr of ultimate bearing condition of the thin-walled steel is obtained. Thus the calculation formula of ultimate bearing capacity of short columns of the lightweight aggregate concrete filled thin-walled circular steel tube is deduced. The ultimate bearing capacities of short columns of the lightweight aggregate concrete filled thin-walled square or octagon steel tube are deduced by equivalent section method taking account of the effects of the cold curved effect and the effective width in thin-walled steel tube. Compared with the obtained solution and the test results in reference, good agreement can be found. The results indicate that the unified strength theory has the good applicability in the calculation of ultimate bearing capacity of short columns of the lightweight aggregate concrete filled thin-walled circular steel tube. Meanwhile, the various yield criteria can be obtained by varying the value ofα, b and k, so the formula can be applied to derive the ultimate bearing capacity of short columns of the lightweight aggregate concrete filled steel tube and the self-stress lightweight aggregate concrete filled steel tube. These solution can provide theoretical foundation for the design of the lightweight aggregate concrete filled steel tube.2. Based on the mechanism of bond between the steel tube and the concrete in the lightweight aggregate concrete filled thin-walled steel tube, the equation of stress of steel tube, bond stress, and relative slip btween steel tube and concrete are deduced through theoretical analyses. And the relationship of bond stress and slip is established. According to the test results and through statistics linear regression, the formula of ultimate bond strength of the lightweight aggregate concrete filled thin-walled steel tube is deduced by considering these factors (slenderness ratio, diameter-thickness ratio, confinement index, etc). The calculation formula of ultimate bearing capacity for short columns of the lightweight aggregate concrete filled thin-walled steel tube is deduced based on the unified strength theory. The effects of bond-slip behaviors of interface between the steel tube and the concrete is considered in the solution. The results indicate that the bond-slip behaviors has little influence on the ultimate bearing capacity of short columns of the lightweight aggregate concrete filled thin-walled steel tube.3. In this paper, the stress-strain curve and stress contour of the lightweight aggregate concrete filled thin-walled steel tube are obtained using the commercial finite element software ANSYS. In addition, the influences of the confinement index and the strength grade of core lightweight aggregate concrete on the mechanical performance of the short columns under axial pressure are discussed according to numerical results.