Axial Compressive And Seismic Behavior Of Spiral-confined Ultra-high-strength Concrete-filled Square Steel Tube Columns

Square concrete-filled steel tube(CFST)columns have been widely used in skyscrapers.Since these columns are subjected to very high axial loads,the use of ultra-high-strength concrete(UHSC)is preferred in order to reduce their cross-sectional size.For ductility requirement,additional confinement to the UHSC may be necessary.Among various approaches,spiral confinement is an effective and practical way for square CFST columns.Those columns with spiral called as square spiral-confined concrete-filled steel tube(SCCFST)columns.A systematic and in-depth study has been carried out on the SCCFST columns,and following achievements are attained:(1)Twelve SCCFST specimens and two conventional concrete-filled steel tube(CFST)specimens with the concrete compressive strength of 112 MPa were tested.The test variables included the amount and yield strength of the spiral and the ratio of the spiral-confined area to the total concrete area.Based on the test results,the failure mechanism,axial load-carrying capacities,deformation capacities of the SCCFST columns were investigated.(2)The load-deformation response of the steel tube was assumed based on an available stress-strain model for the steel tube in CFST columns,and the load carried by the concrete infill can be determined by subtracting the steel tube contribution from the measured total load.The improvement of the postpeak performance of the concrete infill generally increased as the spiral confinement index increased,which indicates the effectiveness of the spiral confinement index for quantifying the ductility of the concrete in SCCFST columns.The indices for ductility measure of the concret infill based on the energy concept were used and the experimental results show that the spiral confinement index can reasonably capture the trend of the ductility of the concrete in SCCFST columns.Regression analyses were conducted to derive simplified expressions for the relationships between the spiral confinement index and the ductility index of the concrete.The design expression derived based on the test results can be used to determine the required quantity of spirals for square spiral-confined concrete-filled steel tube columns using 110 MPa concrete.(3)SCCFST specimens for seismic design were subjected to cyclic loading.The specimens were varied in axial compression ratio and with-or without-spirals.Based on the test results,the failure mechanism,hysteretic characteristics,load-carrying capacities,deformation and energy dissipation capacities of the square spiral-confined ultra-high-strength concrete-filled steel tube columns were investigated.In conclusion,the results of this research indicate that the square spiral-confined ultra-high-strength concrete-filled steel tube columns broader prospects of applications in skyscrapers.