Load-Carrying Capacity And Practical Calculation Method Of Welded Hollow Spherical Joints Connected With Square And Rectangular Steel Tubes

Welded hollow spherical joints are one of the most popular nodal joints in space trusses and lattice shells, with the advantages of simple construction, definite load transferring mechanism and convenient connection. Welded hollow spherical joints connected with square and rectangular steel tubes are employed in the National Swimming Center "Water Cube". However, current studies on welded spherical joints are concerned with those connected with circular steel tubes. For joints connected with square and rectangular steel tubes on previous studies have been reported and no design methods have been established. This thesis investigates the structural behavior and load-carrying capacity of these joints subject to axial forces, bending moments and combined loading of the two, through three approaches including finite element analysis, experimental study and derivation of simplified theoretical solution. The final objective is to establish the practical calculation method.Firstly, based on the many specialty literatures, the development and research advances on the hollow spherical joints are described, as well as the formulas for the calculation of the load-carrying capacity. The background of this thesis is then pointed out and the main contents are put forward.According to the geometry characteristic of the welded hollow spherical joints connected with square steel tubes, a coordinate system is defined and a simplified shell model is given. Theoretical formulas are then derived based on the thin shell theory, and the boundary conditions are also given. Because of the complex of the shell theory and the boundary condition, the analytical solution is very difficult to achieve for the current problem got. So the elastic analysis is carried out by the finite element analysis.Based on the elastic-perfectly plastic model and the Von-Mises yield criterion, a finite element model for the analysis of welded spherical joints connected with square/rectangular tubes is established, in which the effect of geometric nonlinearity is taken into account. A major parametric study is then carried out employing this model. It is shown that, for welded spherical joints subject to combined axial forces and bending moments', the axial force - bending moment correlation is independent of their geometric parameters, which will significantly simplify the calculation method for the bearing capacity of the joint. In addition, the stress distribution of the joints has also been analyzed.Based on the punching shear failure model, simplified theoretical solutions are first derived for the loading-carrying capacity of the welded spherical joints connected with square/rectangular tubes.Consequently, the basic forms for the design equations are obtained.Experiments on ten typical full-scale joints, including six joints connected with square steel tubes and four joints connected with rectangular steel tubes, are conducted to understand directly the structural behavior and the collapse mechanism of the joint, and also to validate the reliability of the finite element model and the theoretical solution.By utilizing the results from the simplified theoretical solution, finite element analysis and experimental study, the practical calculation method is established for the load-carrying capacity of the joints subject to axial forces, bending moments and the combined loading.In addition, penetrated butt welds are adopted to connect the member to the joint in "Water Cube". In order to avoid the brittle weld fracture prior to the steel tube fracture, it is necessary to strengthen the butt welded connections, so that the structure has a good ductility. For moment connections for square steel tubes, based on the unreinforced connection using penetrated butt welds, two types of strengthening strategies are proposed: cover-plated reinforcement and local-thicker-tube reinforcement. Low-cycle reverse loading tests are carried out on these three types of connections. It has been shown that, both the cover-plated and the local-thicker-tube reinforcements can improve the ductility of the connection effectively, and the former one is much better. Furthermore, a modified cover-plated reinforcement is proposed based on the failure mode of the test connections. Low-cycle tests are carried out on moment connections for both circular and square tubes with the modified reinforcement, and the efficiency of new reinforcement method is validated.Finally, the conclusions drawn from this study are summarized, and areas in need of further research are highlighted.