Study On Mechanical Properties And Size Effect Of Concrete-Filled Double-Skin Steel Tube Columns Under Axial Load

Concrete filled steel tubes are widely used in projects such as long-span bridges,subways,high-rise buildings,and factory buildings.In addition to the advantages of the classic concrete-filled steel tube structure,such as high bearing capacity,high plastic toughness,convenient construction,and good economic effect,the concrete-filled double-skin steel tube structure also has the characteristics of lighter weight and greater flexural rigidity.In addition,it is difficult to test full-size large components in engineering,so it is of practical engineering significance to study the bearing capacity and size effect of double-layer concrete-filled steel tubelar columns.In this thesis,the concrete-filled double-skin steel tube column is taken as the research object,and the experimental research,theoretical analysis and finite element simulation are carried out on it.The specific content is as follows.Axial compression tests were carried out on 14 concrete-filled double-skin steel tube columns with different sizes and different inner and outer steel tube thicknesses.To explore the influence of important parameters of concrete-filled double-skin steel tube columns such as steel content,thickness of inner and outer steel tubes,hollow ratio,hoop coefficient and size on the failure mode,bearing capacity and ductility of the specimen.The results show that the restraint of the steel pipe is correlated with the ductility of the specimen,the development law of the load-strain curve and the peak bearing capacity.The greater the hoop coefficient of the specimen at the same height,the better the ductility performance,and the greater the slope and ultimate bearing capacity of the load-displacement curve in the elastic stage.The ultimate bearing capacity and ductility of the specimen decrease with the decrease of the hoop coefficient of the specimen.Even if the thickness of the inner and outer steel pipes is different,as long as the specimens with the same hoop coefficient have the same load-strain curve development law and peak strain value.The load-strain curve of large-sized specimens increases after reaching the peak bearing capacity,and the ductility becomes worse,and the corresponding longitudinal strain decreases when the damage coefficient of the specimen reaches 1.0.It shows that the failure development speed of the specimen is faster,and the size has an influence on the mechanical properties of the concrete-filled double-skin steel tube columns.In addition,for the specimens with different thicknesses of inner and outer steel pipes but similar steel content,the development trend of the load-strain curve and the corresponding strain value of the peak load are relatively similar.The load-strain curve of large-sized specimens increases after reaching the peak bearing capacity,and the ductility becomes worse,and the corresponding longitudinal strain decreases when the damage coefficient of the specimen reaches 1.0.It shows that the failure development speed of large-scale specimens is faster,and the size has an impact on the mechanical properties of double-layer CFST columns.Based on the bearing capacity test,the ratio of the theoretical calculation bearing capacity of the existing code to the test bearing capacity is obtained,and it is found that this value increases with the increase of the specimen size.It shows that this value increases with the increase of the specimen size and the development of bearing capacity of concrete-filled double-skin steel tubular columns affected by dimensional changes.The estimated bearing capacity of large size concrete-filled double-skin steel tubular columns is too high.According to the test and calculation results,the four size effect laws are introduced into the existing code bearing capacity calculation formula,and the revised bearing capacity calculation formula is obtained.The effectiveness of the revised formula is evaluated by statistically analyzing the test bearing capacity data of the existing concrete-filled double-skin steel tube columns.The finite element software ABAQUS is used to establish the model of the concrete-filled double-skin steel tube columns,and the validity of the model is verified based on the test data.The stress analysis of the whole process of compression of the concrete-filled double-skin steel tube columns shows that the outer steel tube,core concrete,and inner steel tube do not produce obvious interaction at the initial stage of stress,the stress state of the concrete is clear,and the stress distribution of the steel tube is uniform.With the loading of force,the hoop effect between steel pipe and concrete affects the development of stress,and the stress of each part shows obvious correlation until failure.The finite element parameter expansion analysis of concrete-filled double-skin steel tube columns was carried out to obtain the development law and failure mode of the specimen bearing capacity under different hollow ratios,diameter-to-thickness ratios,and concrete strengths.