Axial Load Behavior And Strength Of Tube Confined Steel Reinforced Short Columns With Ultra-high Strength Concrete

The development of high-rise buildings and long-span bridges has led to a stringent requirement of pillars and piers'load-bearing capacity and ductility. Thus, composite structures and high performance materials, especially concrete filled steel tubular (CFST) structures and high strength concrete, have been extensively studied in recent years. However, CFST structures have several limitations, such as the complexity of beam-column joints, local buckling of the tube due to vertical load and poor fireproof performance."Tubed column", first came up by Tomii M, Sakino K et al., is a new type of composite structure, which can overcome these limitations mentioned above. The outside steel tube is not applied to axial load directly so the essential work of the tube is to confine the concrete core. The failure of tube confined high strength concrete column is caused by a critical diagonal shear crack crossing the midrange of the column. It is exceedingly effective to impose a steel core in the concrete to enhance the shear resistance ability to prevent the spreading of the critical diagonal shear crack, thus, load bearing capacity and inherent ductility of the columns would be significantly improved.This paper elaborates on experimental and analytical studies on the behavior of tube confined steel-reinforced ultra-high strength concrete short columns subjected to axial compressive load. A total of twenty specimens were fabricated and tested to investigate the failure mode and axial load behavior of tube confined steel reinforced concrete (SRC) columns. Parameters for the tests includes:tube shape, he shape of steel, diameter (width)-to-thickness ratio, tube tensile strength. Test result indicates that the confinement effect of circular tube is greatly superior to that of square tube, because circular tube can efficaciously prevent the ultra-high strength concrete from brittle failure, but the square tube cannot. O-shape and double H-shape of the steel exhibited better mechanical performance than H-shape of the steel. The larger area of the concrete that could be constrained by the steel shape, the better performance it could provide. Elastic-plastic analysis of the tube was utilized to investigate the mechanism of tube confined SRC columns under axial compression. A bearing capacity formula for tube confined SRC columns was proposed based on test results, which can be as reference for further study and guidelines engineering application. The main conclusions of this paper are as follows:1. For SRC columns, the confinement effect of circular tube is greatly superior to that of square tube, for the reason that circular tube can efficaciously prevent the ultra-high strength concrete from brittle failure, however, the square tube cannot.2. O-shape and double H-shape of the steel exhibited better mechanical performance than H-shape of the steel. The larger area of the concrete that could be constrained by the steel shape, the better performance it could provide.3. The decrease of diameter/width-to-thickness ratio can effectively improve the strength of the SRC columns.4. The enhancement of confinement effect due to the increase of the tube strength is limited and has minimal influence on the strength of the columns.