The Interface Characteristics And Performance Optimization Of Concrete-filled Steel Tube Under Temperature Impact

Concrete-filled steel tube(CFST) is a composite load-carrying member through pouring concrete into steel tube. Its superior performances depend on the composite reinforcement between the steel tube and concrete. The confinement effect of steel tube to core concrete improves the plasticity and ductility of concrete, as well as the global and local buckling strength. However, the thermal properties of steel and concrete differences are significant, which inevitably leads to the interface deboning and void of CFST under service environments, affecting the work performance and structural safety.In this paper, based on the temperature impact tests, finite element numerical analysis, thermal elasticity theory and nonlinear fitting theory, a study on the cross-section temperature field, the interfacial properties and interface optimization theory of CFST under temperature impact were conducted. The research results basically form the interface optimization method of CFST based on the fine design of core concrete expansion properties. The main contents are as follows:1. The temperature impact tests have conducted. The L/D ratio, D/t ratio, confinement index, material strength and other important parameters of the specimens could meet the engineering requirements and the test results could reflect the actual service status of CFST. The temperature distributions and variations of specimens' cross-section were obtained. The contact thermal resistance across the steel-concrete interface should be considered in the finite element analysis because of the existence of interface temperature difference. The experimental temperature fields can be used to compare with numerical data to verify the validity and accuracy of the finite element model.2. The temperature impact analysis and thermal-mechanical coupling analysis of CFST were conducted. The heat transfer model and mechanics model were established. And the thermal and thermal-mechanical coupling FEM analysis theories were also discussed. We obtained the cross-section temperature fields, interface responses and core concrete free expansion rates compensating the interfacial gaps. Comparative analysis of experimental and numerical temperature fields showed that the finite element model could be used for parametric analysis of interface features and interface optimized design. The maximal nonlinear cross-section temperature difference could reach 18.2?. The maximal Mises stress and steel tube and radial stress of concrete could reach 36.6MPa and 3.39 MPa respectively at the cooling stage. The environmental temperature incensement would deteriorate the interfacial performance. A appropriate free expansion rate would alleviate interfacial voiding and improve the bearing capacity. 0.5Nu would begin to make the relative deformation between steel tube and concrete compensate for the interfacial gap caused by temperature impact. 0.7Nu would begin to make the steel tube enter into yield stage and decrease the bearing capacity.3. The interface optimization method was proposed, and core concrete free expansion rate formula,,, compensating the temperature-induced interfacial gaps was built. Firstly, the method of using concrete expansion design was proposed to achieve interface optimization of CFST. Then, the parametric analysis procedures were written by using the Python language. The temperature impact numerical analyses of 270 CSFTs without expansive agent were completed in different geometries and temperature changes. The maximum interfacial gaps of the CFSTs and the corresponding core concrete free expansion rates were obtained. Finally, the core concrete free expansion rate formula was proposed by fitting the 270 free expansion rate discrete data.