Experimental Study On Load Separation Of Axial Compression Short Column Of High Strength Concrete Filled Circular Steel Tube

In recent years,with the booming development of high-rise buildings,concrete-filled steel tube structures are more and more favored by engineers because of their superior mechanical properties.Again due to the civil engineering material course of developing,production requirements of high strength concrete and the difficulty,thus the application of high strength concrete in engineering is becoming more and more widely,based on the above circumstances,fills in high strength concrete of concrete filled steel tube structure also gradually appears in the engineer’s field of vision,so the research of steel tube high strength concrete structure appear more meaningful.At present,there are many achievements in the research of steel tube high-strength concrete,but most of the research is through the overall test and analysis of the results of its mechanical properties,it is not clear to obtain the steel tube and the core concrete in the stress process of the specimens in the stress state.In order to be more clear to the stress of steel tube and core concrete their separation,in this paper,on the basis of predecessors’ designed a will of steel tube and concrete assume their respective load separation of loading device,through the device of concrete filled steel tubular short column specimen axial loading,the core concrete and steel tube can be obtained,respectively,in the process of bearing load bear axial load,and analyzed the test result is obtained by experiment.This paper mainly completed the following work:(1)On the basis of predecessors,this paper designs a loading device that can separate the load of steel tube from that of concrete,and designs 12 short columns of concrete filled steel tube for axial compression test,among which four kinds of concrete are C40,C60,C80 and C100 respectively.Two kinds of specimens which eliminate friction between steel tube and concrete section are compared with those which do not.Axial compression tests were carried out on all CFST specimens and the corresponding test data were obtained.(2)By collating the test data,the ultimate bearing capacity,failure state,load-displacement curve,load separation curve,load-strain curve,ductility performance,concrete bearing capacity improvement coefficient and other performance parameters of the test specimens are compared and analyzed.The similarities and differences of the performances of concrete-filled steel tube specimens with different strength concrete were compared and analyzed.The results showed that the failure modes of C40 and C60 were mainly waist drum type,while the failure modes of C80 and C100 were mainly shear type.The axial load of C40 and C60 specimens increased throughout the test process,while that of C80 and C100 specimens decreased after reaching the peak.The load proportion of steel tube in the specimens with low strength concrete-filled steel tube is large.With the increase of concrete strength,the axial load proportion of concrete in concrete-filled steel tube increases.The lifting rate of concrete bearing capacity decreases obviously with the increase of concrete strength.As the concrete strength increased from C60 to C80,the ductility coefficient of the specimens decreased rapidly.The higher the concrete strength,the worse the ductility of the concrete-filled steel tube specimens.Sticking ptfe film can effectively eliminate cross section friction between steel tube and concrete.(3)Three classical constitutive stress-strain relationships of confined concrete are introduced and compared with the experimental results.The results show that the Saenz constitutive model is the closest to and can reflect the real performance of the constrained concrete materials below C80,followed by the Han Lin-Hai constitutive model,and the Mander model performs the worst due to the excessive prediction of the stress peak.For the constrained concrete material with C100 strength,the three constitutive models are close to the test values in the stress rising section,but the three constitutive models have a large deviation in the prediction of the stress falling section after the stress peak,which is difficult to reflect the real performance of the material.After that,the constitutive model was adjusted based on experimental fitting,and the results of numerical analysis of the constitutive model of constrained concrete fitted well with the experimental results.(3)Sorting the existing is widely used in several countries in the specification for concrete filled steel tubular axial compression bearing capacity calculation formula,and collected 500 of the existing domestic and foreign experimental data of concrete filled steel tubular short column calculation and contrast analysis to the above formula,the formula of the best find specification for Japanese AIJ discipline,the American AISC specification and ACI code times,European Eurocode 4procedures and "technical specification for concrete filled steel tube structure in our country and poor performance.Based on the adjustment of the two formulas for calculating the axial bearing capacity of concrete filled steel tube in China,it is found that the adjusted formula is better than the original formula in predicting the test results.