Research On Strength Of External-ring-stiffened Tubular X-joints Subjected To Brace Axial Compressive Loading

Owing to excellent mechanical properties,simple and beautiful model,steel tubular structures are widely used in marine engineering,constructional engineering and many other fields.One of common connection forms adopted by steel tubular structures is the tubular joint,and the connection of various steel tubular members,namely the joint design is an important part of affecting the whole structure safety.In the actual engineering application,simple tubular joints sometimes cannot meet the bearing capacity or stiffness requirement,so local reinforcement must be performed for them.Based on the experiment and finite element analysis method,ultimate strength,failure mode and the effect of various geometric parameters of tubular X-joints reinforced with external stiffening rings on the ultimate strength were studied under the axial load.The analysis model used for such tubular joint and formula of the ultimate strength enhancement coefficient were proposed in this thesis.Three groups of comparative experiment were firstly designed,where each group of experiment included one unreinforced X-joint and one X-joint reinforced with the external stiffening ring.The experiment device and loading scheme were introduced and the failure mode and structural performance of X-joint were discussed.Subsequently,nonlinear finite element analysis was made for X-joint reinforced with external stiffening ring,and the element type,material properties,weld modelling and mesh generation were considered comprehensively to build an accurate and effective finite element model for parametric analysis.Moreover,the finite element analysis results were compared with test results to verify the validity of the finite element models.At last,finite element parameter analysis was made for160 finite element models,including 144 tubular X-joints reinforced with the external stiffening ring and 16 unreinforced tubular X-joints to explore??ratio of brace to chord diameter d₁/d₀?,??ratio of chord diameter to twice chord wall thickness d₀/2t₀?,the external stiffening ring width factor _r??2b_r/d₀,where br means the width of external stiffening ring?and the external stiffening ring thickness factor?_r?t_r/t₀,where t_r means the thickness of external stiffener ring?on the ultimate strength enhancement coefficientR_e of X-joint reinforced with external stiffening ring.Moreover,the formula of the ultimate strength enhancement coefficient was summarized according to the yield body analysis model and nonlinear regression analysis.According to the yield body model,it was assumed that only the intersecting volume between brace and chord yielded;whereas in a ring-stiffened joint,both the intersecting volume between brace and chord and the volume of entire external stiffening ring yielded.The strength enhancement of ring-stiffened CHS X-joints therefore was attributed to the increase in volume of yielding materials in comparison with the unreinforced joints.The following conclusions are drawn after analysing and summarizing the above work:1.The external stiffening ring can significantly increase the ultimate strength and initial stiffness of tubular X-joints.The ultimate strength of reinforced joints was enhanced by 86%,75%and 58%respectively than that of unreinforced joints,and the initial stiffness was enhanced by 121%,145%and 128%respectively.Moreover,the error between the results of finite element analysis and the experiment results was less than 11%,indicating the finite element method can predict the structural performance of the reinforced and unreinforced X-joints accurately.2.Two failure modes were observed in the experiments,which were chord plastification and local buckling of brace member.Where the chord plastification existed in all the experiment members,but the local buckling of brace member could only be observed in the reinforced joints,and the local buckling of brace member occurred earlier than the plastification,indicating the ultimate strength of X-joint was enhanced because the existence of the external stiffening ring postponed the occurrence of the chord plastification.3.The ultimate strength enhancement coefficientR_e decreases with the increasing of?.However,after?exceeds 0.4,its impact on the ultimate strength enhancement coefficientR_e declines dramatically.In addition,??ratio of chord diameter to twice chord wall thickness d₀/2t₀?,the external stiffening ring width factor _r??2b_r/d₀?and the external stiffening ring thickness factor?_r?t_r/t₀?are all positively correlated to the ultimate strength enhancement coefficientR_e.The relationship between the ultimate strength enhancement coefficientR_e and the four parameters obtained from the yield body model is consistent with the conclusion of parameter analysis,which proves the reliability of the yield body model.4.When ring size?width and thickness?is small,optimization effect of two schemes is better for the case of reducing the size of external stiffening ring and for the case of increasing the size of external stiffening ring,better optimization effect will be obtained when the width of ring is near 50mm and the thickness is relatively large.In conclusion,the optimization effect of scheme one,which is achieved by reducing width of external stiffening ring along transverse direction or increasing dimension of external stiffening ring along longitudinal direction,is better than that of the scheme two,which is achieved by reducing size of external stiffening ring along longitudinal direction or increases width of external stiffening ring along transverse direction.It is suggested that the optimization scheme one should be selected in actual projects.