Experimental And Theoretical Research Of Bearing Capacity Of Steel Tubular Joints Of A Long-span Transmission Tower

The joints are the key parts of steel transmission towers. The joints are intersected by various components, which caused very complex stress states, and the stress concentration here is obvious. Experimental study and theoretical analysis have not yet fully developed at home and abroad.Most of the current research methods are using the finite element method and the stress concentration factor approach to analyze the mechanical properties of this type of steel tube connection. Detailed studies on the destruction mechanism of the tube-gusset plate and concrete-filled double skin steel tubular joints have not yet been carried out. According to the given analytical methods of the existing codes of many countries, the results are too conservative, hence the designers can not accurately grasp the safety factor of the structure, and it will also affect the economy of constructions adversely. At present, there is no specific way to estimate the local buckling of the tube walls and to estimate the difference between stiffened and unstiffened connections. The reference values given by empirical formulas of the codes of foreign countries can not accurately reflect mechanical properties of the joints under different conditions. Therefore, deeper experimental and theoretical research of the ultimate capacity of the tube-gusset plate joints and concrete-filled double skin steel tubular joints has become a key problem to be solved.Based on the project of a long-span tall transmission tower, researches about the mechanical properties of K-shaped tube-gusset plate joints and concrete-filled double skin steel tubular joints were carried out, specifically including the following areas:(1) Through full scale tests of the tube-gusset plate joints, the limit states of damage and failure mechanism were studied. Finite element simulations using Newton-Raphson method with 8-node solid elements considering material nonlinearity and geometric nonlinearity has been undertaken. The results of the test and the finite element method together with the corresponding formulas of the codes of different countries are compared and analyzed.(2) Then established the finite element models to analyze the parameters of the connections, the analysis considered the influence of the axial forces, the widths and the heights of the stiffening rings, the diameters, the thicknesses of the tube walls, the lengths and thicknesses of the plates together with other factors. Mechanical properties of the joints are summarized through analysis.(3) A simplified formula of the ultimate bearing capacity of the K-shaped tube-gusset plate joints is established using the analytical-theoretical-method considering the influence of ring-stiffening plate.(4) Through full scale tests of the tube-gusset plate joints, the limit states of damage and failure mechanism were studied. According to the results of the test, the joints studied show great bearing capacity, which can meet the requirements of the target load and there is a certain security reserve. Then the results of the test and the finite element method are compared and the validity of the finite element model was verified.(5) Widely range of finite element analysis of concrete-filled double skin steel tubular joints was carried out to research the influence of joints parameters on ultimate strength. Results show among all the parameters such as:the ratio of chord diameter to thickness a, that brace-to-chord diameter ratioβ, the ratio of brace thickness to chord thicknessγ, the ratio of brace diameter to chord diameter di/D, and the ratio of the axial stress to yield stress of the steel tube, the brace-to-chord diameter ratio and the ratio of chord diameter to thickness has significant influence on the ultimate strength. However, gap factor B(g/D) changes and concrete strength fcu changes have slight influence. The formula of the ultimate strength of concrete-filled double skin steel tubular joints was obtained by the use of multiple regression methods, which provides a meaningful reference for the design of the engineers.(6) Finally, the problems about the steel tubular joints requiring further researches were discussed.