Experimental Research On Mechanical Property Of Circular Concrete-filled Thin-walled Steel Tubular Column Under Axial Load

Concrete-filled thin-walled circular steel tube has its unique advantages, such as convenient and efficient welding, quick installation, steel savings, and cost savings. But the existing concrete-filled steel tube code limit the radius-thickness ratio of concrete-filled thin-walled circular steel tube within 135(235 /)y?f, which is slightly larger than the limit value of h ollow steel tube in order to guarantee the local stability, but not considering that the stable bearing capacity of concrete-infilled circular steel tube increases a lot. Meanwhile the existing literature has relatively less research on the radius-thickness ratio value of concrete-filled thin-walled circular steel tube, and the only research is only involved with the theory. The strength specification stipulates the confining factor range is 0.5 ~ 2.0, the concrete-filled thin-walled circular steel tube with coupling coefficient less than 0.5 is not applicable. As you can see that both stability and strength restrict the development of concrete-filled thin-walled circular steel tube, so this paper attempted to make a breakthrough in the limitation restricting the stability and strength of the concrete-filled thin-walled circular steel tube combined with the structure characteristics and the existing unified formula theory of concrete-filled circular steel tube. Considering that the diameter-thick ratio of practical engineering will not be too large, and local instability is influenced by the cohesive force and other factors, the research on the stability in this paper is to calculate the safety value when local instability will not occur, and critical value of diameter-thick ratio isn’t verified when local instability occurs.The main work of this paper can be summarized as the following aspects :The classic formula is used to deduce the diameter-thick ratio limit of the hollow circular steel tube under the ideal state of zero defect, but the diameter-thick ratio is far below the theoretical value when the local instability occurs for the hollow steel tube with the actual defects. Theory and finite element prove the concrete-filled circular steel tube makes the outer steel tube not sensitive to defects due to the existence of the in-filled concrete, that is, the outer steel pipe can be regarded as zero-defect steel pipe, so the stub column of concrete-filled circular steel tube under axial load in the theoretical diameter-thick ratio limits will not have local buckling.6 groups of experiments of 18 concrete-filled thin-walled circular steel tubes are carried out. According to the comparison of experiment bearing capacity and superposition bearing capacity and concrete-filled steel tube appearance in the experiment and other phenomena, local instability situation of concrete-filled steel tube is distinguished, and the thickness ratio which will not cause the local instability in the experimental range is got.The finite element method is used to judge whether local instability of concrete-filled steel tube happens from two aspects of bearing capacity and the steel tube and concrete separation. Considering the diameter-thickness ratio isn’t too big in the practical engineering due to the limitation of processing, transportation and construction conditions, achieving the minimum from the theoretical value of diameter-thickness ratio, finite element value and experimental value is regarded as the diameter-thickness ratio value meeting the actual demand of the engineering when the concrete-filled circular steel tube doesn’t have local instability.The parameter analysis is carried out on the concrete-filled thin-walled circular steel tube and the analysis suggests that b earing capacity can be viewed as the linear function of the confining facto r. The FEM analysis is carried out on the concrete-filled steel tube and receive the formula of concrete-filled thin-walled circular steel tube. Based on the limit capacity of test, the paper verified the limit capacity formula of concrete-filled thin-walled circular steel tube under axial load. Finally, the calculation formula of axial compression bearing capacity of concrete-filled thin-walled circular steel tube is obtained.