Study On Restraint Effect And Axial Compression Behavior Of Large Diameter Concrete Filled Steel Tube

Concrete-filled steel tube is a kind of composite material formed by pouring concrete into steel tube.It mainly relies on the interaction between steel tube and core concrete to give full play to the performance advantages of the two materials.It has the advantages of high bearing capacity,good plasticity,good seismic performance,convenient construction and so on.It is widely used in high pier and long-span bridge projects in mountainous areas.In recent years,with the continuous increase of pier height and span,the cross-section size of CFST members is also increasing.For example,the diameter of main pipe of CFST composite structure pier of jinyanghe super large bridge reaches 1900 mm.At present,the research on mechanical properties of CFST and the design and calculation theory of current codes are mainly based on the model test piece with the diameter of about 300 mm.Whether the mechanical properties of CFST are changed after the pipe diameter is increased,whether it is consistent with the mechanical properties of small-diameter CFST,and whether there is difference in the restraining effect of CFST,there are few relevant researches at present.Therefore,in this paper,the restraint effect and axial compression performance of large-diameter concrete-filled steel tube are studied.The details are as follows:(1)Six groups of 12 concrete-filled steel tubular specimens were tested under axial compression.The size of the specimens was 219 mm ~ 600 mm,and the test parameters were diameter and diameter thickness ratio.The results show that: under the parameters of this paper,the diameter of steel tube has no effect on the axial compression failure mode and load displacement curve trend of concrete-filled steel tube specimens.The failure mode of specimens is waist drum type,and the loading failure process is in three stages: elastic stage,elastic-plastic stage and plastic stage.The larger the pipe diameter is,the longer the elastic section of the load displacement curve is,and the shorter the elastic-plastic stage is.With the increase of pipe diameter,the increasing range of ultimate bearing capacity is smaller,while the peak stress and its corresponding peak strain have a decreasing trend.The larger the initial elastic modulus is,the faster the stiffness degradation speed is,and the earlier the lateral deformation coefficient of the specimen begins to change.In addition,the strain difference between the center point of the section and the joint point of steel tube and concrete increases with the increase of the pipe diameter,that is,the constraint of steel tube on the center point of the core concrete is stronger than that of the joint point of steel tube and concrete.(2)At the same time,it is found that with the increase of the ratio of diameter to thickness,the peak stress and its corresponding peak strain decrease,the smaller the combined modulus of elasticity,the faster the degradation rate of stiffness,the earlier the lateral deformation coefficient begins to change,and the faster the change rate,the greater the difference between the strain at the center point and the strain at the boundary point on the same cross section,and the greater the lifting range of the ultimate bearing capacity small.(3)Using the finite element software ABAQUS to carry out the finite element nonlinear analysis,the ultimate bearing capacity,load displacement curve,failure mode and the stress distribution and development of the core concrete at different positions of the section are extracted,which are in good agreement with the model test.Based on the model,the parameters are analyzed deeply.(4)By comparing the test results with the calculation results of the axial bearing capacity of CFST in six codes,The results show that the deviation between the test results and the results of aisc2008 and aisc360-10 is large,and that of EC4,gb50936,JTG / td65-06-2015 and cecs28: In 2012,the results of axial bearing capacity are in good agreement with the test results when the pipe diameter is 219 mm,325mm and 400mm;with the increase of pipe diameter to 600 mm,the deviation between the test results and the calculation results of code bearing capacity is relatively large,which are 84%,90%,81% and 86%,respectively.Then,by using the numerical fitting method,the calculation formula of the bearing capacity of the large-diameter concrete-filled steel tube is put forward.