Research On Experimental Study And Mechanical Behavior Of Bolted End-plate Connections For Composite Steel And Concrete Structures

Steel-reinforced concrete filled steel tubular columns formed by inserting structural steel into concrete-filled circular (square) steel tubes, is a new type of modern composite column. This type of composite column has the advantages of both steel-reinforced concrete structures and concrete filled steel tube structures. There has been a lot of research about the mechanical behavior of the composite column, but the research on its connection is relatively backward. In view of the above-mentioned situations, the bolted end-plate connections, commonly used in steel structures, is introduced in composite structure this dissertation. This new type of connection is formed by end-plate and steel-reinforced concrete filled square steel tubular columns joined together using high strength bolt with equal-length in both ends, and the steel beam is welded on the end-plate. In this dissertation, the mechanical properties and design method of this new type connection have been extensively investigated both experimentally and theoretically based on previous research achievements of steel-concrete composite connections. The main contents are as follows:1.7specimens of steel beam-to-steel-reinforced concrete filled steel tubular column bolted end-plate connections are designed and tested under cyclic loading. Macroscopical deformation characteristics, failure mechanism, failure modes, ductility, strength degradation, rigidity strength degradation, energy dissipation of this new type connection under low reversed cyclic loading are observed and studied. Influences of the axial compressive ratio, thickness of end-plate, bolt diameter, concrete strength grade on the bearing capacity of the composite connections are investigated. Hysteretic model for these end-plate connections to square steel tube columns filled with steel-reinforced concrete is also constructed according to experimental results, which can be used in the elasto-plastic time-history analysis of structural system.2. The nonlinear finite element analysis (FEA) of7specimens is carried out using ANSYS program, and some complicated nonlinear problems are considered in the analysis, such as geometric nonlinear, material nonlinear and state nonlinear. With comparison to the test results, it is verified that finite element method can efficiently simulate and analyze the bolted end-plate composite connections. Some characteristics of end-plate connections which are difficult to be measured by tests, such as the stress characteristics of concrete in panel zone, the magnitude and distribution of primary stress and the distribution characteristics of contact compressive stress between the end-plate and coloumn flange can be gained in FEA analysis. Therefore, FEA method can reflect the mechanical behavior of connections in a more complete and accurate way.3. According to theoretical analysis and test results, the mechanism of force transfer in panel zone is analyzed, and a mechanical model is established in order to derive theoretical equations for calculating the shear capacity of the panel zone. And skeleton curves of shearing force and shear deformation for the panel zone in end-plate connections are obtained in the condition of coincident deformation, which accommodates a brief calculation method for engineering design.4. Considering the effect of plate to the deformation of the bolt under pre-stress, the high strength bolt and compressive plate are idealized as a "bolt-plate element" the calculation formula of the rigidity for the "bolt-plate element" is deduced. The bearing capacity and deformation are calculated according to mechanics character of the bolts in each stage, and the moment-rotation curve of the high strength bolt is established.5. Based on the theory of yield line and thin plates, bearing capacity and initial stiffness of the end-plate under different supporting conditions are calculated, and theory analysis method of complete loading process of the end-plate is proposed.6. A theretical analysis model is presented for analyzing the complete loading process of the bolted end-plate connections on the basis of the superposition principle. The moment-rotation curves can be derived directly from construction dimension and material properties, and avoids the complicated finite element analysis and experimental research. The calculated moment-rotaion curves agree well with the experimental results. In elasticity-plasticity analysis of actual framework structure, if the moment-rotation model presented in this dissertation can be applied to structure analysis model, the influence of connection behavior on the whole structure can be well understood so that it can provide a reference for the design and research in the future.7. A complete set of static and seismic design method for semi-rigid end-plate connections are proposed on the basis of achievements of aforementioned cyclic loading tests research, the finite element analysis and relative research achievements. These are all beneficial supplements to the popularization and application of the steel-concrete composite structure. Mechanical model for calculating initial rotational stiffness of the end-plate connections is established by using component methord. According to the mechanical model, the calculation formula for initial rotational stiffness of the end-plate connections is deduced which can be used in elastic design of composite framework structure.