Rigidity Of Unstiffened Circular Tubular X-Joints And Its Effects On Globalperformance Steel Structures

Because of the unique advantage, tubular structure is widely used in space structures. With the development of multidimensional numerical control incision technology and design software for tubular structures, unstiffened tubular joints are more and more adopted at connections between hollow section members in tubular structures. Loads always cause local deformation at the connection of chord and branch, which brings the relative deformation and rotation at the ends of the branches, the behavior of the joints is neither as same as the hinge nor the rigid connections whether in elastic or in elastic-plastic phrase, the semi-rigidity characteristic of the joints has obvious influence on the deformation, mechanical behavior and global stability of the tubular structures.Based on the reviewing of the research achievements relating to this problem, systemic research on the rigidity of tubular X-joint - including axial rigidity, out-of-plane bending rigidity, and in-plane bending rigidity - was carried out by theoretical analysis and experimental study, from which the formula is obtained to calculate the rigidity of tubular X-joint. The influence of joint rigidity on global performance of tubular structures is analyzed, a useful conclusion is presented that the rigidity of tubular joints decreases with the increasing internal force, which should not be taken as constant in analyzing the structural internal force or global stability.Taken the practical single-layer spherical shell as engineering background, aiming at the typical tubular X-joints, the tests on six joint specimens were performed to research the rigidity under axial force, out-of-plane moment, and in-plane moment respectively, the mechanical behavior and failure mode of the joints were studied, and the joint specimens were analyzed by nonlinear finite element method using shell element. The results show that the joint rigidity is big when the ratio of branch diameter to chord diameter approaches to 1, and the tubular joint can be taken as rigid connection in structural analysis, while with decreasing of the ratio value, the joint rigidity deceases gradually, it is improper to take the tubular joint as rigid connection anymore.The factors influencing rigidity of tubular joints were analyzed systematical one by one, using the finite element model which was established by orthogonal method. By analyzing 144 tubular X-joints with axial force on branch using nonlinear finite element method, curves of axial force against joint deformation, failure mode of the joints, and stress distribution regulation under different factors were obtained, the mechanical behavior of tubular joints with axial force on branch was analyzed, and the axial rigidity was calculated. The regulation of axial rigidity changing with the influencing factors was summarized, the dimensionless parameters of axial rigidity factorηN and bearing capability factorωN were presented. Based on the relationship of the two factors, the curve ofηN-ωN was put forward, and the function between the two factors was developed by multi-element regression technology, then the formula calculating the axial rigidity of tubular X-joints was obtained.Nonlinear finite element analysis on 142 tubular X-joints was carried out with branch suffering from out-of-plane moment and in-plane moment respectively, the curves of joint moment against rotation, failure mode, and stress distribution regulation under different influencing factors were obtained. The mechanical behavior of tubular joints was analyzed, the out-of-plane and in-plane bending rigidity of the joints were calculated respectively. The variety regulation of out-of-plane and in-plane bending rigidity changes with the factors was summarized, the dimensionless parameters of bending rigidity factorsηW,ηIand bearing capability factorsωW,ωIwere presented, the curve of them was put forward, and the function between bending rigidity factors and bearing capability factors was developed by multi-element regression technology, then the formula calculating the out-of-plane and in-plane bending rigidity of tubular X-joints was obtained.Following the experimental and numerical studies on joint rigidity, a numerical model was established to calculate the tubular X-joint rigidity in global structural analysis. Importing the computational formulas of tubular X-joint rigidity proposed by this paper into the static analysis and global stability analysis model block of the tubular structure design software STCAD, the static analysis and global stability analysis were carried out aiming at the typical structures with tubular X-joints-a single-layer ribbed reticulated spherical shell project and a single-layer tubular structure of a gymnasium, the influence of tubular X-joint rigidity on mechanical behavior, deformation, and stability of the whole structure was studied. The results show that the joint rigidity has great effect on the global behavior of single-layer tubular structure, for the gymnasium structure, the critical load factor considering the joint rigidity effect decreases to about 50% comparing with that of rigid connection without considering the joint rigidity effect. Setting the joint rigidity as a constant in global analysis, that is the initial joint rigidity just related to geometrical parameters, the critical load factor increases to about 30% comparing with the value calculated by the formulas presented in this paper. All of the results verified that the joint rigidity had better not be taken as constant in structural analysis considering the influence of the tubular joint rigidity, the fact that joint rigidity decreases with the increasing structural load should be taken into account.