Study On Mechnical Behavior Of Unstiffened Tubular K-joints With Circular Chord And Square Braces

Steel tubes have been widely used in space truss structures increasingly because of their beautiful figure and excellent mechanical performance. The braces in the multiplanar tubular truss mainly bear the effects of pressure-bending or tension-bending and the chords mainly axial force. The circular chords and square braces can perform better than Circular-Hollow-Section or Square-Hollow-Section members used in the multiplanar tubular truss according to the respective mechanical behavior of the circular and the square (rectangle) sections. Joints in the space tubular structure are complex and their mechanical performances are one of the engineering problems needed to be resolved. It is thus necessary to investigate the mechanical performance of the joints.On the basis of the unstiffened uniplanar tubular K-joints mode tests, nonlinear FE analysis on joints is made. The mechanical behavior, joint failure mechanism, joint ultimate capacities and concrete in chord on the ultimate capacity and stiffness of joint are presented in the paper.At first, experimental research on static behavior of unstiffened tubular K-joints with circular chord and square braces are conducted. Five specimens of hollow section K-joints and one K-joint with concrete filled steel tubular chord were tested under monotonic loading. The test procedure, mechanical behavior, joint failure mechanism, joint ultimate capacities are studied and the effects of thickness of braces and concrete in chord on the ultimate capacity, stiffness and ductility of joint are also discussed. Results of tests show that: firstly, compared the calculated results of ultimate capacity of tubular joints according to the current codes with ultimate capacities of joints obtained from the tests, it is proved that the performance of the available design guidance about tubular K-joints such as the circular and square hollow section K-joints for predicting such joint ultimate capacities is poor and obviously low. The main reason may be that the thickness of brace changes the joint failure mode and improves the ultimate capacities and joint ductility. Secondly, although the ultimate capacities and initial stiffness of joints with concrete filled steel tubular chord increase by comparing with hollow joints, the ductility keep the same. Thirdly, the displacement of this kind of joints is mainly caused by local displacement of the tensile brace.Then, Finite Element(FE) validation studies of the unstiffened tubular K-joints are performed on the ultimate capacity. A effective and exact FE model including all kinds of factors are set for parameter analysis. On the basis of the FE model, comparison between the test result and FE proves the validation of FE method available.Following the experimental and numerical studies on the kind of unstiffened tubular K-joints, finite element analyses are conducted on 64 the type K-joints. The distribution of stress and failure mechanism are analyzed. The effects of the geometry parameters on the ultimate capacity of joint are also investigated. Results indicate that the parameters which make a different influence on the ultimate capacity of the K-joints are the ratio of chord diameter to thickness and brace-to-chord thickness ratio. The effect of the brace-to-chord diameter ratio upon the joint ultimate capacity is relatively weak.Whereafter, with reference to the ultimate capacity of the circular hollow section K-joints in the IIW(International Institute of Welding) formula, a new strength equation is developed for unstiffened tubular K-joints with circular chord and square braces through multi-variable nonlinear regression analyses. By statistical analysis for the joints in the paper and the full-size test about Vietnam National Stadium conducted by Chen yiyi, it is proved that the given formulae is accurate and reliable.At last, the problems those require further studies are discussed.