Research On The Hollow And Solid Concrete Filled Stell Tubes With Polygonal And Circle Section Under Axial Compression And Complex Stress

Concrete filled steel tube is a kind of composite structure which is made of steel tube filled with a centrifugal prefab concrete or fill the concrete in site. The interaction force between the steel and concrete made the CFST much more captacitable . that is ,in case of the constraint effect provided by the steel, the strength of the core concrete can be improved,also with the plasticity, toughness can be greatly improved at the same time. On the other hand, it will avoid or delay the occurrence of local buckling of steel tube due to the presence of core concrete which may ensure the performance of steel brought into full play.In the past few years,solid concrete-filled steel tube has been widely used in engineering field.The concrete- filled steel tube may be at the complex stress condition combined by compression,bending, shear, and torsion load in actual engineering structures.The working performance under the combined loads of the CFST member has been began to study,in the existing research ,the investigation on strength and stability capacity correlation equations of CFST member subjected to complex stress condition is obtained by the fitting results based on a large number of finite element model calculations, lack of a derivation as a theoretical support. Therefore the purpose of this thesis is obtained the basic form of the strength and stability capacity correlation equations of the concrete filled steel tubular structures under complex stress through the theoretical derivation. After that we use the finite element model calculations to validate and amendment the parameters to get the final formula. In this thiese, limit equilibrium theory is used to establish static equilibrium equations. Limit equilibrium theory is based on balance equation and plastic theory ,that is ,be regardless of loading history and the deformation process.In this theory, based on the equilibrium condition of structure that depend on limit state, the load value in the limit state is cucaluated .And then establish the static equilibrium equations between structural components and external in which the Internal forces of the structural components and the external loads should meet the condition of static equilibrium.Then use the previous CFST equivalent truss model which obtained when establish static equilibrium equations to do mechanical analysis under complex stress .we can calculate the internal forces of each equivalent truss rod and then determine their yield failed pattern came to the conclusion to yield equation. Made use of the previous balance equation, computing to offset the internal forces items of steel and concrete obtained the bearing and stability capacity correlation equation.We should consider the role of the impact of hoop between the steel and concrete, analyze and summarize the working performace and loading capacity of concrete filled steel tubular structures subject to respective axial compression, bending, shear and torsion. We will introducte a hoop influence coefficient to adjust the various ultimate bearing capacity of items, in the result to get a practical calculation formula for the compression, flexural bearing capacity which can be in good agreenment with experiment and finite element values. This practical formula which considerd the role of the impact of hoop will substitute to the correlationg equationg to adjust the carrying capacity to finalize the expression of the strength and stability capacity correlation equations of the concrete filled steel tubular structures under complex stress..During the derivation,we should consider the influence of different sections and the solid and hollow code concrete.In order to achieve an unity formula with different solid, hollow, circular section, square cross-section and polygonal cross-section. We use a a hoop influence discount coefficient to reflect different section and a hollow ratio to reflect the influence of the hollow CFST capacity.Non-linear finite element software is used to establish CFST modles with different section and hollow ratio. And then use three-dimensional finite element analysis method to investigating the working performace of CFST to validate and amendment the parameters of the theoretical derivation to get the final bearing capacity correlation equations . And then use the collection of previous test results to validate and amendment the parameters in order to determine the accurate bearing capacity formula.Finally use the results of strength experiments of six polygon section concrete filled steel tubes to verify the formula.