Study On Fatigue Performance Of Steel Cable Tower Anchor In A Highway-railway Cable-stayed Bridge

In recent years,with the rapid development of long-span highway-railway cable-stayed bridges,steel pylons with integrated anchor boxes have been increasingly applied.Steel cable tower anchor has various forms,complex structure,multi-directional plates are welded orthogonally or obliquely,and there are many welds.As the force transmission structure of steel tower and cable,it bears large cyclic loading,and the local stress concentration phenomenon is obvious,the fatigue resistance control of structural details is one of the key problems in the design.In this paper,a highway-railway cable-stayed bridge is taken as an engineering example,and the fatigue performance of the anchorage structure of the anchor box steel pylon and its influencing factors are studied by using the hot spot stress method through finite element analysis.The main research contents and results are as follows:(1)The finite element model of the whole bridge cable-beam element is established,the typical cable tower anchorage structure is determined to be located at the tail cable of the main span side tower through the analysis of live load cable force amplitude.Based on the rain flow counting method and Miner linear damage accumulation criterion,the equivalent fatigue cable force amplitude of 2 million times of typical cable tower anchor caused by highway and railway fatigue load is calculated,and the combination mode of the two is discussed.It is found that the combination value of cable force amplitude obtained by the linear superposition method of British code BS5400 is the largest.(2)The multi-scale spatial finite element model of typical cable tower anchor is established,by analyzing the stress amplitude characteristics of the main plates of the anchorage structure and the stress distribution state of the base metal at the weld toe of the main weld under the action of fatigue loading cable force amplitude,three kinds of main fatigue details of the typical cable-tower anchorage structure in the engineering example are summarized and found: the shear detail of the connection side between the shear plate and the web of anchor chamber,the bending and tensile detail of the connection side between the bearing plate and the web of anchor chamber,and the bending and tensile detail of the connection side between the shear plate and the bearing plate.(3)Using the hot spot stress method,on the basis of determining the hot spot stress of each fatigue detail,the influence of element type,grid size and extrapolation method on the finite element calculation value of hot spot stress of each fatigue detail is analyzed,the fatigue life of each fatigue detail is calculated based on the S-N curve of the hot spot stress method.The calculation results of the engineering example show that under the action of fatigue loading cable force amplitude,the bending and tensile detail of the connection side between the shear plate and the bearing plate is the control position of the fatigue life of the anchorage structure,the fatigue life of each detail is more than 10 million times,which is much larger than 2 million times,the structure is safe within 100 years of design life.(4)The single parameter analysis method is used to analyze and summarize the influence laws of the main bearing plate geometric dimensions and structural geometric structure changes on the hot spot stress and fatigue life of each detail in the cable tower anchor.The results show that: in the geometric dimensions,the changes of the thickness of the shear plate,the thickness of the anchor chamber web and the height of the shear plate have a great influence on the hot spot stress and fatigue life of the three kinds of details.In the geometric structure,cancellation of partially stiffened plates and the change of the distance between the shear plates have a great influence on the hot spot stress and fatigue life of three kinds of details.