Analysis Of Ultimate Bearing Capacity And Fatigue Of Tubular Uniplanar Gap K-joint

With the wide application of steel structure in civil engineering, tubular uniplanar K-joint is frequently applied to truss structure, rack structure, etc. At present, previous researches about tubular uniplanar K-joint mechanical performance have obtained a lot of achievements, but there are still many problems solved imperfectly. Hence, further attention is still needed. Besides, recently, studies on the properties of K-joint stress mostly focus on the ultimate bearing capacity of node, with little attention on fatigue life. In addition, according to the steel structure specification of state, the formulas about computation ultimate bearing capacity of nodes have not taken the influence of boundary condition into consideration, leading to conservative results. Therefore, the present study focuses on tubular uniplanar gap K-joint, and mainly investigates its ultimate bearing capacity, failure mode, and failure life.Firstly, the finite element method is adopted to study the ultimate bearing capacity and failure mode of tubular uniplanar gap K-joint, and meanwhile, the standard formula is used to calculate the bearing capacity of tubular uniplanar gap K-joint. In the present study, the influence of axis angle θ between the main pipe and the branch pipe, the diameter ratio β (d/D) of branch and main pipe, and the diameter-thickness ratio γ (D/T) of main pipe, as well as four kinds of boundary conditions and loading modes on nodes’ultimate bearing capacity and failure mode are analyzed. Besides, the ultimate bearing capacity of tubular uniplanar gap K-joint is calculated by using the standard formula, and then, the result is compared with that of finite element analysis. In addition, the causes of calculation deviation are also investigated in the present paper. Results show that the node geometric parameters θ、β and γ influence the ultimate bearing capacity and failure modes for the tubular uniplanar gap K-joint differently. Among the three parameters, y is the main parameter influencing node failure modes, and when β and γ are small, the influence of boundary conditions on the ultimate bearing capacity of nodes can be ignored, but when they are larger, the influence will show up gradually.Secondly, the fatigue life of tubular uniplanar gap K-joint is also analyzed and calculated by adopting the Paris formula. During the analysis, on one hand, the finite element software is employed to establish a two-dimensional standard finite element model of three-point bend specimens with a through crack on the surface, as well as to calculate the stress intensity factor of the specimen; on the other hand, the theory formula is also applied to calculate the stress intensity factor. And then, the finite element analysis results and the theoretical formula calculation results are compared, which shows that the crack model is reasonable. Finally, the finite element model of tubular uniplanar gap K-joint which contains an initial semi elliptical surface crack is established with the above mentioned method. The fatigue life of node is calculated by firstly obtaining the effective stress intensity factor of nodes from the finite element analysis and then plugging them into the Paris formula. In terms of the analysis of the fatigue life of nodes, the influences of the branch and main pipe diameter ratio β,and the main pipe diameter-thick ratio y, as well as the crack depth and wall thickness ratio a/T on fatigue life of nodes are focused and considered. Results show that fatigue life of nodes decreases with the increasing of y and a/T, while it increases as β increases; what’s more, as to thinner main pipes or branch pipe nodes of smaller diameter, the effects of β and y are more obvious;and moreover, as for thinner main pipes or nodes of larger initial crack a, the fatigue life of node will decrease sharply as a increases.