Axial Compressive Test Of A New Composite Cfst Pier Column

In order to meet the needs of modern structures for long-span,super-high-rise and heavy-load development,researchers continue to study the mechanical properties of new piers.In this paper,a new type of cross-section of compound concrete filled steel tubular pier column with eccentric double circular steel tubes in rectangular steel tubes is proposed.Compared with ordinary concrete-filled steel tubular members,this new type of member has better confinement effect on concrete in the tube,higher bearing capacity and better ductility.Firstly,this paper summarizes the test results and theoretical analysis results of various cross-sections of concrete-filled steel tubes at home and abroad,and then analyzes the mechanical mechanism and bearing capacity characteristics of this new type of pier by combining axial compression test with finite element analysis.The main research contents include:（1）Seven specimens of concrete-filled steel tubular composite pier columns are designed,and whether or not circular steel tubes are built in,the pouring section of concrete and the height of the specimens are taken as investigation variables.Through axial compression tests,the axial compression performance of the new pier columns under the above variables is studied,and the deformation characteristics of each specimen in elastic stage,elastic-plastic stage and descending stage are analyzed.（2）There are three kinds of axial compression failure modes of concrete-filled steel tube composite pier columns:drum failure,shear failure and empty steel tube failure.The reason for these different failure modes is that the stress state of the concrete in the tube changes after it is restrained by the inner and outer steel pipes.Concrete is in a three-dimensional stress state,and its failure will be determined by the compressive strength of the"principal stress surface"and the compressive and shear strength of the"dangerous slope".（3）The steel consumption and ultimate bearing capacity of rectangular steel tubes with eccentric double circular steel tubes are increased by about 4.69%and 22.42%respectively,and the axial ability coefficient is increased by 19%at the late stage of loading.It can be seen that the layout of inner steel tubes has a great influence on the overall bearing mode of piers and columns.The concrete section has little influence on the initial stiffness,but has some influence on the ultimate bearing capacity.The hoop effect of double round steel tubes on concrete improves the compressive strength,thus ensuring that the pier column can maintain a stable bearing capacity at the later stage of loading;Decreasing the pier height will be affected by the end effect and increase the initial stiffness of the specimen,but the ultimate bearing capacity and ductility are not obviously improved.（4）At the initial stage of loading,steel pipe and concrete are in a cooperative state,and the hoop effect of steel pipe on concrete is weak.When the members yield,the vertical compressive strain of the inner and outer steel tubes increases sharply,and the lateral strain also develops rapidly.At this time,the hoop constraint of steel tubes on the lateral deformation of concrete is gradually obvious.Therefore,the strength factor coefficient SI and concrete contribution rate coefficient CCR are proposed to further reflect the working mechanism of axial compression between steel tube and concrete.（5）Based on the test,the finite element model is established,and the model results are consistent with the test failure mode.Based on the pier column RC88-L,the working mechanism of concrete and inner and outer steel pipes in three stages is analyzed through stress nephogram,and the parametric analysis of axial compression performance is carried out.Based on the double shear unified strength theory,the calculation method of the axial compression bearing capacity of the new pier column is proposed.The average value of N_um/N_ujof all pier columns is 0.992 and the variance is 0.003,which shows that the research on the axial compression bearing capacity of the new pier column in this paper can put forward reliable suggestions for engineering application.