Research On Strength Of External Stiffener Stiffened Circular Tubular X-and T-joints Subjected To Brace Axial Tension

Steel circular tubular structures are widely used in building structure because of their good looking appearance and good mechanical performance.Joints usualy are the weak points of circular tubular structures,and the key to decide the whole structural performance.Improving the performance of joints is the most direct and the most effective way to improve the performance of the whole structure.Taking external structural measures to strengthen the joint is the simplest way to improve the joint performance.In the various types of tubular joints,intersecting joints has become the most common joint in engineering,because of their simple appearance,excellent performance,easy construction and other advantagesThis thesis studies the influence of external stiffeners strengthening on the mechanic performance of intersecting circular hollow section X-and T-joints under brace tensile load.This research is based on full-scale test of 12 joint specimens,including finite element simulation method,studies the failure modes,ultimate bearing capacity and bearing capacity calculating formula of X-and T-joints strengthened by external stiffeners.The tests in the research consist of 3 control groups of tubular X-joints and 3 control groups of tubular T-joints,totally 6 control groups.Each group includes one joint not strengthened,and one joint strengthened by external stiffeners.Through comparison,the strengthening effect of the external stiffeners is obtained.Accurate finite element model is established based on test results.Parameteric analysis is carried out on joint geometric parameters which affect the joint bearing performance,based on the batch analysis method of finite element model.Specifically,??ratio of brace to chord diameter d₁/d₀????ratio of chord diameter to wall thickness d₀/2t₀??hs?external stiffener height?and t_s?external stiffener thickness?are chosen,304 X-joints models and 256 T-joints models are established,and the main factors to affect the ultimate bearing capacity of X-and T-joints are obtained.Finally,the formulation of the ultimate bearing capacity is derived based on the ring model.The conclusions are as follows:1.Strengthening with external stiffeners is an effective joint strengthening method.It can significantly improve tubular X-and T-joints'ultimate bearing capacity and initial stiffness.For the X-joints with?of 0.25,?of 0.50,and?of 0.73,the bearing capacity is improved by 3.7%?16.6%and 19.3%respectively,and the initial stiffness is improved by12.5%?9.6%and 15.1%respectively.For the T-joints with?of 0.25,?of 0.50,and?of 0.73,the bearing capacity is improved by 14.1%?27.9%and 32.4%respectively,and the initial stiffness is improved by 15.5%?9.6%and 30.1%respectively.2.In the test,T-joints specimens show two kinds of failure modes,that is weld failure and chord plastification failure.In finite element analysis,three kinds of failure are found in T-joints.They are chord plastification failure,brace tensile yield failure,and chord bending failure.However,only two kinds of failure are found in X-joints.They are chord plastification failure and brace tensile yield failure.3.The thickness of external stiffeners has little impact on the ultimate bearing performance of X-and T-joints,and the differences are all within 1%.As for X-joints,chord wall thickness and the size of external stiffeners are positively associated with the joint bearing performance.The bearing performance of joint will be increased with the increase of the two factors.And the influence of external diameter of brace on improvement coefficients shows no uniform rule.As for T-joints,the external diameter of brace,chord wall thickness and external stiffeners'size are positively correlated with joint bearing performance.The bearing performance of joint is increased with the increase of these three factors.4.The strengthening effect of external stiffeners derives from the increase of the effective length B_e.The calculation formula of bearing capacity of external stiffeners strengthened X-joints under brace tensile load is obtained with the help of statistical analysis software SPSS and the bearing capacity of X-joints brace tensile static force formula in Chinese standard?standard for design of steel structures?.