| The connection between the beams and cables in long-span cable-stayed Bridges is one of the most important structures to transfer forecs.It is characterized by large force,significant stress concentration,many complex welds,etc.So its fatigue performance has been widely concerned in bridge engineering.In this paper,the steel anchor box in the anchorage zone of a certain Yangtze river bridge was taken as the research object,and the work were carried out which included the structural stress monitoring under the random traffic flow and the local finite element analysis under the action of the cable force,as well as cable force amplitude.In addition,the fatigue performance of the steel anchor box was evaluated based on the nominal stress method and the hot spot stress method,respectively.The main works are as follow:(1)The work was carried based on the model of the Yangtze river bridge.In addition,on the basis of cable force value and cable force amplitude,the steel anchor box corresponding to JS26 cable in the middle span of the south tower was determined as the research object.According to the model of the whole bridge,the influence line of the lane was extracted,and the fatigue cable force amplitude used in this study was determined by combining the domestic and foreign specifications and relevant literature.(2)This paper carried out the local finite element analysis of the steel anchor box structure under the action of fatigue cable force amplitude,and we obtained the stress amplitude distribution characteristics of the main plate of the anchor box and the stress characteristics of the main weld,and the stress amplitude of the main plate and the stress concentration position were also summarized.The results showed that the three most concentrated stress areas of the anchor box structure were the top end of the the upper and lower plate,and the outer weld of the bearing plate which connecting with the web of Steel girders.The former two were characterized by pressure and shear stress,while the latter was characterized by bending and tensile stress.(3)Based on the results of finite element analysis,this paper carried out the stress monitoring under the random traffic flow passing the bridge deck,with obtained stress records and stress spectrum at the main details.In addition,the work has been done based on analyzed the stress time-history characteristics of the structural details at the main joint welds.The stress time-history showed the stress characteristics of the main structure,and an individual truck only caused an effective stress cycle.The fatigue life calculation was carried out in combination with the China and foreign specifications.The results showed that the evaluation results obtained based on the American specifications were the most conservative.The fatigue details at other positions satisfied the design life except that the top end of the lower plate which was not satisfied with the design life requirements under the E-grade of the American specifications.In the evaluation results obtained based on the British specification and the European specification,all test locations m et the design life requirements.(4)Based on the hot spot stress,three stress concentrated areas of the steel anchor box were analyzed,and three types of stress details including compression,shear and tension were summarized according to the stress characteristics.The influence of four factors on the hot spot stress,including grid size,element type,the number of layers in main component and extrapolation scheme,was studied,and the fatigue life was analyzed according to the obtained hot spot stress.According to the results,four parameters indicated kind of effects on the three kinds of structure details.And the element type and the number of element layers have little influence on the calculation value of the structural hot-spot stress,while the mesh size and th e extrapolation mode have notable effects on the hot-spot stress,and under different extrapolation method three kinds of main structure details satisfied the design fatigue life. |
