Study On Bearing Capacity And Seismic Performance Of Concrete-filled Square Steel Tube Columns With Composite Constraint

The concrete-filled square steel tube(CFSST)column has the advantages of reasonable bearing mechanism,beautiful appearance,convenient construction,especially the simple structure of beam-column joints.It has been widely applied to modern super-high buildings,long-span buildings,and heavyload structures.However,there is an "arch effect" in square section,the concrete inside the square steel tube is not uniformly constrained,and the bearing capacity of CFSST is also limited.The existing studies show that setting inclined ribs in the area near the middle side length of the square steel tube can enhance the bonding between steel and concrete in that weak constraint area,and effectively improve the bearing capacity of the CFSST column,but there are certain requirements for welding quality.By adjusting the placement of the builtin steel,the strong constraint on the corner of built-in steel was used to improve the weak constraint in the middle side length of outer square steel tube,this double advantage way not only makes the constraint in square section more uniform to improve the bearing performance,but also simplify the construction process and improve the construction efficiency.Thereby,two kinds of novel composite square concrete-filled steel tube columns without welding were proposed.The one was CFSST reinforced with rhombic stirrups column,and the other was novel double-skin CFSST(rhombic inside & square outside)column.The latter was replacing the longitudinally discontinuous rhombus stirrups with longitudinally continuous square steel tube,to study the effect of the continuity of longitudinal constraints on the performance of the specimen.In this research,the axial compression mechanism of the two kinds of novel composite columns was investigated by the way combined with experiments,numerical simulation,and theoretical analysis.In addition,the axial compression performance of six kinds of CFSST column with different cross-section was compared.Meanwhile,the seismic performance of the double-skin CFSST(rhombic inside & square outside)column was studied.The main research results are as follows:(1)The axial compression tests of 25 CFSST column specimens were completed.The failure modes of the specimens and the influences of parameters on the axial compression behavior of CFSST columns were clarified,and the mechanism of composite constraints was revealed.The results shown that the failure process and failure modes of six kinds of CFSST columns could be divided into local buckling failure and instability failure during plastic loading according to the aspect ratio.The specimens in SSC group had the highest peak load and the most eximious bearing capacity,there were no obvious decline section in the curves.The longitudinal continuity of the build-in steel determines whether the bearing capacity can be improved by increasing the area of the core concrete.(2)8 double-skin CFSST(rhombic inside & square outside)column(specimens SSCS)and 1 CFSST column(specimen SSCS7)were tested under cyclic loading.The failure process and final failure mode of specimens were observed,the load-displacement hysteresis curves and the variation characteristics of steel’s strain were obtained.And the influences of parameters on the seismic performance indexes of specimens were analyzed.The results shown that the horizontal bearing capacity of SSCS specimens with large the steel content were 30.2% much higher than that of SSCS7 specimen.The interstory-displacement angles of feature points were met the requirements of engineering practical application.The built-in inclined square steel tube could effectively alleviate the weakening effect of P-(35)effect on the horizontal bearing capacity of SSCS specimens.The established mathematic model could predict the hysteretic relationship between load and displacement of SSCS specimens under low cyclic loading,and the calculated hysteretic curve was in good agreement with the measured hysteretic curve.(3)The axial compression performance and seismic performance of double-skin CFSST(rhombic inside & square outside)column specimens were simulated by using ABAQUS finite element software.The influence law of relative parameters on axial compression capacity and seismic performance of composite column were studied.(4)By using the superposition method,the calculation formula considering stability of axial bearing capacity of two novel kinds of composite columns and the bearing capacity calculation formula of SSCS column under bending(bias)condition were derived.The calculation results were in good agreement with the test results.(5)Based on the experimental and theoretical research results,some suggestions for the material selection and section design of the two novel composite columns were put forward.It is an economical way to enhance the bearing capacity of CFSST column with built-in rhombic stirrups.Selfcompacting concrete should be used in the double-skin CFSST(rhombic inside& square outside)column which is more suitable for the design of column structure with large section(multiple cavities)because of its high steel content and good seismic performance.