Axial Compressive Behaviors Of Composite Special-shaped Concrete-filled Steel Tubular Columns

The indoor edges can be avoided and the building usable area can be improved by using traditional special-shaped concrete-filled steel tube, but the steel tube of traditional special-shaped concrete-filled steel tube is butt welded together after bending steel plate, the thicker steel plate is, the greater bending difficulty is, so that it is difficult to make in the project site, meanwhile the deformation of negative angle part is serious under loading. In view of the above disadvantages, a composite special-shaped concrete-filled steel tube is proposed by our group, the rectangular steel tube and the U-shaped steel tube are directly welded together to form special-shaped steel tube, the above disadvantages can be effectively overcome by using composite special-shaped concrete-filled steel tube, there should be a broader application prospect in engineering. Therefore, with the support of 2012 Hubei Province Natural Science Fund Project(project number: 2012FFB05112) and 2014 Central Universities Fundamental Research Funds Project(project number: WUT 2014-IV-125), using the composite special-shaped concrete-filled steel tubular column as the research object, the axial compressive behaviors of composite special-shaped concrete-filled steel tubular columns are studied by experimental study,theoretical analysis and numerical calculation. The main contents and conclusions are shown as follows:(1) The axial compressive behaviors of 18 composite T-shaped concrete-filled steel tubular specimens and 18 composite L-shaped concrete-filled steel tubular specimens are carried out through static test, where the influence of steel tube thickness, concrete strength and slenderness ratio on mechanical behaviors of the specimens are analyzed. The axial compressive bearing capacity is calculated by using various code design methods of concrete-filled steel tube, which are compared with the test results. Based on the code of AIJ-CFT:1997, the formulas are proposed to calculate the axial compressive bearing capacity of composite T-shaped concrete-filled steel tubular specimens and composite L-shaped concrete-filled steel tubular specimens, in which the influence of the steel tube constraint effect and slenderness ratio are considered. The results show that the failure modes could be classified into three categories: waist drum failure mode, local buckling(or cracking) failure mode and bending failure mode; steel tube thickness shows a significant influence on axial compressive bearing capacity; when slenderness ratio is relatively large, steel tube thickness shows a more significant influence on axial compressive bearing capacity; with the concrete strength increasing from C30 to C50, the axial compressive bearing capacity increases and the increase range is about 15%; when steel tube thickness is relatively small, slenderness shows a more significant influence on axial compressive bearing capacity; design methods in current codes are conservative for the specimens; the results calculated by the proposed formulas agree well with the test results.(2) The constraint characteristic of steel tube to core concrete is analyzed based on the experimental results of composite T-shaped concrete-filled steel tubular specimens and composite L-shaped concrete-filled steel tubular specimens. According to the existing strain-stress relation of confined concrete, the axial compression strain-stress relation of core concrete in composite T-shaped concrete-filled steel tubular specimens and composite L-shaped concrete-filled steel tubular specimens are suggested. The finite element nonlinear analysis of composite T-shaped concrete-filled steel tubular specimens and composite L-shaped concrete-filled steel tubular specimens are carried out by using the proposed strain-stress relation, the calculated results are compared with the test results in two aspects mainly from the failure modes and load-strain curves to verify the rationality of the proposed strain-stress relation. On the basis of the finite element nonlinear analysis, the longitudinal stress distribution of core concrete and the constraint stress of steel tube to core concrete are analyzed emphatically. The results show that the core concrete longitudinal stress of the specimen central section are closed to the measured value of uniaxial compressive, when axial load reaches ultimate bearing capacity; the constraint stress of steel tube to core concrete is mainly distributed in steel tube corner and the scope distance steel corner 25 mm, the constraint stress of steel corner is max, the constraint stress is smallest in the central of steel tube edges, which close to 0.Finally, on the basis of summarizing the work, the prospects of the research on this topic are put forward.