Research On Axial Compression Performance Of Concrete-filled Circular Steel Tube Short Columns Based On Load Separation Test

Concrete-filled steel tube refers to a component formed by filling concrete in a steel tube.The restraining effect of the steel tube makes th e internal concrete in a three-way stress state.The internal concrete can avoid premature buckling of the steel tube.The combined structure of the steel tube concrete makes properties of these two materials can be fully utilized,so that concrete-filled steel tube has a series of characteristics such as high bearing capacity,good plasticity and tough ness,and good fire resistance.These characteristics of concrete-filled steel tube make it use more frequent.In order to meet the requirements of high-rise building and large-span development,under the environment of the rapid development of civil engin eering materials and the improvement of construction technology,concrete-filled steel tube structures filled with high-strength concrete are also constantly appearing,so the research on high-strength steel tube concrete appears more meaningful.Scholars at home and abroad have conducted a large number of experiments,theoretical studies and numerical simulations on the composite structure of concrete-filled steel tube since 1967,but most of them are related mechanical performance studies obtained from overall analysis.The stress of the two materials of steel tube and core concrete is not clear.In order to simulate the stress of concrete-filled steel tube in actual projects,the load shared by steel tube and concrete is determined.Based on the Hong-Song Hu square steel tube concrete axial compression separation test,this paper designs a circular steel tube concrete loading device that separates the load,and designs plain concrete columns and empty steel pipes with the same parameters to study the restraint effect of this composite structure and the stress-strain relationship of the core concrete.This article mainly completed the following work:(1)Based on the innovative device proposed by related scholars to separate the steel tube and the core concrete,four groups(one for each group)of load-separated short columns of concrete filled steel tube axial compression tests,and four groups with the same parameters(two for each group)A plain concrete column and an empty steel pipe,the variable is the concrete strength(C40,C60,C80,C100),all specimens are subjected to axial compression test and the corresponding test data are obtained.(2)The test data was sorted and analyzed,and the failure state,peak load,load-displacement curve,load-strain curve,stress-transverse deformation coefficient relationship curve,ductility performance and other performance parameters of each group of test specimens were compared.The results show that the load-bearing capacity and ductility of the filled concrete are improved under the restraint of the steel tube.As the strength of the concrete increases,the failure state of the concrete-filled steel tube will change,from a drum-shaped failure with obvious failure characteristics to a diagonal split fa ilure with no obvious failure characteristics.For the load-displacement curve,the higher the strength,the longer the elastic rising section,and the boundary point between elasticity and elastoplasticity becomes blurred.C40 and C60 specimens continue t o rise at the end of the elastoplastic stage without a descending section.C80 and C100 specimens are in the elastoplastic stage.At the end of the peak,the bearing capacity drops instantly.The proportion of the load bearing the steel tube in the overall load gradually decreases as the strength of the concrete increases.The development stage of steel tube and concrete is more consistent with the overall development stage of steel tube concrete.As the strength of concrete increases,the ductility changes significantly,reflecting the small deformation and extremely brittle characteristics of high-strength concrete.(3)Four types of confined concrete stress-strain relationships are introduced,and their respective advantages and disadvantages are analyzed by comparing the calculated peak stress and peak strain with the experimental values.It is found that the Mander model is better in predicting the ascending section.For C40 and C60,the Mander model and Han Linhai model are adjusted.For the C80 and C10 0 specimens,the residual bearing capacity is adjusted based on the test results,and a more reasonable core is proposed.Concrete constitutive model.(4)Taking the hoop restraint stress as the research object,the existing calculation models of hoop restraint force by relevant scholars and the calculation codes of bearing capacity widely used in several countries are sorted out,and compared with 430 existing domestic and foreign test data,Showing the accuracy and rationality of the calculation formulas and models.Based on the test results,the relationship between the hoop restraint stress and the restraint coefficient was summarized,and the calculation formula was put forward and the calculated value was compared with the test value to verify the acc uracy of the formula.