Study On Hot Spot Stress Concentration Factor Of Welded Concrete-Filled Stell Tubular Truss Joints

The intersecting welding joint is the weaker link of the concrete-filled steel tubular(CFST)truss structure,and it is also one of the important topics in the research of steel structure bridge fatigue.The CFST truss welding joints are usually designed with reference to the empty steel tube joints,and the fatigue design method and theory are seriously vacant.The hot spot stress method has become the mainstream method for fatigue evaluation of welded tube joints in the world.The hot spot stress concentration factor is the key to determine the hot spot stress amplitude and then carry out fatigue evaluation.In this thesis,the Y and T type welded joints of CFST are taken as the research object,and the fine model is established by ANSYS finite element analysis software.Then,according to the hot spot stress calculation method proposed by International Institute of Welding(IIW),the hot spot stress concentration at the weld toe of the joint is studied.The finite element modeling process of Y and T joints of CFST was discussed in this thesis.It was pointed out that the influence of steel-concrete friction coefficient on the hot spot stress concentration coefficient of joints was negligible.The influence law of the chord axial load and the joint geometric parameters on the hot spot direct stress concentration factor(SCF)and hot spot shear SCF at the joints welding toes were studied in this thesis.The results showed that:(1)The chord axial load had a significant influence on the hot spot direct SCF and hot spot shear SCF at the joints welding toes.As the chord axial tension increased,the hot spot direct SCF of chord crown toe and brace saddle toe increased remarkably.As the chord axial pressure increased,the hot spot direct SCF of chord crown toe and brace saddle toe was significantly reduced,the hot spot shear SCF of chord and brace weld toe was significantly increased.While,under the chord axial tension or pressure load,the hot spot direct SCF of chord saddle toe and brace crown toe was basically unchanged.(2)When the two chord ends were consolidated,the hot spot shear SCF was small and joints geometric parameters had a significant influence on the hot spot direct SCF.As the angle ? increased,the hot spot direct SCF of chord weld toe and brace saddle toe increased continuously,which of the brace crown toe was basically unchanged.As the diameter ratio ? increased,the hot spot direct SCF of chord crown toe and brace weld toe,out of crown toe,was decreasing continuously,while the chord crown toe and the brace crown toe was basically unchanged.As the wall thickness ratio ? and the diameter-thickness ratio ? increased,the hot spot direct SCF of chord weld toe and brace saddle toe was increasing,while the brace crown toe was basically unchanged.Based on the analysis of the geometric parameters influence on the welded toe hot spot direct SCF,the calculation results of the 34 finite element models are taken as samples,the hot spot direct SCF calculation formula of Y type CFST joints was proposed,by improving the equivalent wall thickness calculation method proposed by Det Norske Veritas(DNV),and by referring to the hot spot direct SCF calculation formula of empty steel tubular joints.Comparing the hot spot direct SCF maximum value of the entire weld toe,saddle toe and crown toe,it was proposed that the saddle toe and crown toe SCF values obtained by the hot spot direct SCF calculation formula should be enlarged by 1.2 times,and then used as the hot spot direct SCF of the joint.