Mechanical Behavior Of Circular-Steel-tube-confined Concrete-filled Square Steel Tube Columns Under Axial Compression

Concrete-filled steel tube can better exert the performance of steel and concrete through combination.Among them,concrete-filled square steel tube has been widely used in engineering due to its simple joint structure and high bending rigidity.However,due to the weaker constraint of square steel tube on concrete,the bearing capacity is limited and the core concrete is brittle.In order to overcome the shortcomings of concrete-filled square steel tube,this paper first proposed circularsteel-tube-confined concrete-filled square steel tube,which is a new type of composite members.The member consists of intermediate concrete-filled square steel tube,outer circular steel tube and sandwich material between the two.The outer circular steel tube and the sandwich material are not connected to the end of the member and the beam-column joint,and the main function of them is to provide lateral restraint to the concrete-filled square steel tube.In this paper,the combination of experimental research and theoretical analysis is used to study the axial compression performance of circular-steel-tube-confined concrete-filled square steel tube short columns,which lays a foundation for further research on the mechanical properties of bending,shearing and torsion of the new composite members.In this paper,12 circular-steel-tube-confined concrete-filled square steel tube short column specimens and 12 ordinary concrete-filled square steel tube short column specimens were designed to study the macroscopic deformation characteristics and overall failure mode of each specimen under axial load;The measured longitudinal strain and transverse strain are used to analyze the constraint of the outer circular steel tube on the core concrete,and the improvement effect of the outer circular steel tube on the compressive strength and ductility of the core concrete and the improvement of the member failure mode are proved.Stress state changes and interactions between the inner and outer steel tubes and the core concrete are further studied.The test results show that the outer circular steel pipe provides effective restraint and improves the performance of the core concrete,so that the circular-steel-tube-confined concrete-filled square steel tube composite column significantly improves the bearing capacity and ductility than the concrete-filled square steel tube with the same steel content.Based on the analysis of the stress on the corners of the outer circular steel tube,the calculation formula of the axial bearing capacity of the circular-steel-tube-confined concrete-filled square steel tube short column for engineering design is proposed.The finite element model of circular-steel-tube-confined concrete-filled square steel tube axially compressed short column was established by ABAQUS.Considering the contact and material nonlinearity,the core concrete constitutive model suitable for the new composite member was selected and adjusted.By comparing the experimental results,the correctness of the finite element modeling method and the reliability of the analysis results are verified.Based on the finite element model,the force mechanism,deformation development process,stress variation of the middle section and the interaction between the inner and outer steel tubes and the core concrete of the circular-steel-tube-confined concrete-filled square steel tube axially compressed short column are analyzed.The analysis results and the test verify each other,further revealing the working mechanism of the component.The outer circular steel tube generates lateral restraint force at the initial stage of loading.When the peak load is reached,the longitudinal stress of the square steel tube is higher than the yield strength,and the bearing capacity of core concrete is much higher than that of plain concrete.Finally,the parameter analysis of circular-steeltube-confined concrete-filled square steel tube columns was carried out,and the effects of several parameters such as steel content change of inner and outer steel tubes,strength grade of core concrete and strength grade of steel were studied.The research shows that the circular-steel-tube-confined concrete-filled square steel tube firstly proposed in this paper can effectively overcome the shortage of traditional concrete-filled square steel tube,significantly improve the bearing capacity and ductility,and avoid the shear brittle failure of core concrete.The main working mechanism is that the outer circular steel tube provides more effective lateral restraint,avoids the overall buckling of the square steel tube,and improves the plasticity and compressive strength of the core concrete.Compared with directly increasing the wall thickness of the square steel pipe,adding circular steel pipe with the same steel content to the concrete-filled square steel tube to form the circularsteel-tube-confined concrete-filled square steel tube has significant advantages.