Study On Concrete Filled Square Steel Tubular Stub Columns With Stiffeners Under Bending And Shearing Compression

In recent years,mega earthquake occurred frequently,which level has far exceeded the design spectrum level under rare earthquakes in China's codes.The actual destructive forces suffered by buildings during earthquake are often higher than the load-bearing capacity of buildings specified in the codes,which leads to severely damaged and it was difficult to restore its function.However,there is insufficient research on the seismic performance of structures under mega earthquakes and long-period seismic motions in existing codes.How to ensure that the structures under mega earthquakes possess sufficient seismic performance has become an urgent problem we need to solve.Concrete-filled steel tube(CFT)columns are widely used rely on the full use of the mechanical properties of steel and concrete.For the CFT structure,its seismic resistance is mostly dependent on the strength and ductility of the column.When a CFT column encounters a mega earthquake that exceeds the level set by the current specifications,peaks will occur due to local buckling of the steel tube and softening of the concrete stress,etc.The bearing capacity is degraded after loading,and bearing capacity degradation and the negative value of stiffness may cause local deformation to be further concentrated and may cause the whole structure to collapse.In this paper,it is proposed that in the area of plastic hinges at the ends of square CFT short columns,transverse cross-shaped stiffeners are provided on the inner wall of the steel tubes at the ends to improve the local buckling of the steel tubes,thereby improving their seismic performance,especially under mega earthquakes.To verify the effectiveness to improve local flexion of square CFT short columns with cross-shaped stiffeners in the area of plastic hinges,a set of quasi-static tests on plastic hinge area ribbed and ordinary square CFT short column specimens were carried out.The results show that the local buckling of the steel pipe is delayed with the cross-shaped stiffener,and the buckling form changes from a single drum wave with a full cross-sectional width to a double drum wave with a 1/2 cross-sectional width;The hysteretic curve is fuller with the cross-shaped stiffener,the horizontal ultimate bearing capacity is higher,the stiffness degradation is smoother,and it possesses better ductility and energy consumption capacity.In order to further improve the seismic performance of square CFT short columns with cross-shaped stiffeners in the area of plastic hinges,ABAQUS finite element analysis software was used to establish 9 square CFT short column models with plastic stiffeners and 4 squares without stiffeners.CFT short column model was used to simulate the bending and shear compression test to analyze the impact of different width-thickness ratio,concrete strength and axial compression ratio on the seismic performance of square CFT short column.The analysis results show that: as the width-thickness ratio decreases or the strength of the concrete increases,the bearing capacity and the ductility increases;as the axial compression ratio increases,the bearing capacity increases but the ductility decreases.Under the action of bending and shear compression,the horizontal ultimate bearing capacity can be increased by 58.72%-87.98% with the cross-shaped stiffeners,which increases maximum with high concrete strength.