| The steel box girder cable-stayed bridge has many advantages,such as large span capacity,light weight,high torsion stiffness,good wind resistance stability,high construction precision and fast construction speed.It has a very wide range of applications all over the world.Generally speaking,in order to consider the factors of shear lag,the designers often add two longitudinal diaphragms in the steel box girder,the form of the longitudinal diaphragm can be used in the form of plain plate,and the trusses can be used too.But after a period of using,it is frequently found that there is many cracks in the connection between the trusses and the joint plate,which has produced a huge potential danger to the regular use of the bridge.The finite element method is used to analyze the behavior of the longitudinal diaphragms in a steel box girder cable-stayed bridge,and the reasons for the emergence of cracks are explored.The research ideas of this paper are as follows: first,the whole bridge model is established by Midas Civil,and then the local segment model of the longitudinal diaphragms is established by ANSYS,and the boundary conditions of the corresponding segments in the whole calculation are loaded into the local model for the analysis of the force behavior.The main conclusions are as follows:(1)The stress of the pipe web member is less than 200 Mpa and less than the yield strength of the Q235 steel during the whole process of the longitudinal diaphragm,which is full of vehicle load on the side of the bridge deck and the loading of the fatigue car.At the same time,it was observed that there was a very obvious stress concentration at the intersection of the pipe and the joint plate.In the process of fatigue vehicles passing through the bridge deck,the tension-pression stress cycle appeared at No.1-4 concern point.The pressure-tension stress cycle appeared at No.5-8 point of concern and the maximum stress amplitude reached 96.13 Mpa.At the point of concern,there are two factors of stress concentration and alternating stress.Therefore,it is concluded that the cause of the crack is due to the continuous reciprocating of the vehicle on both sides of the longitudinal diaphragms to cause large alternating stress to cause fatigue cracking.(2)For the longitudinal diaphragms using the channel steel bar,the various situation stresses of the longitudinal diaphragms are close to the pipe longitudinal diaphragms,not beyond the yield strength.At the same time,it was observed that there was obvious stress concentration at the intersection of channel steel corners with longitudinal diaphragms joints plate.In the process of fatigue vehicles passing through the bridge deck,the pression-tension stress cycle appeared at No.1-4 concern point,and the tension-pression cycle appeared at the 5-8 point of concern,and the maximum stress amplitude reached 86.94 Mpa.In the same way,there are two factors of stress concentration and alternating stress at the point of concern.It is concluded that the cause of the crack in the longitudinal diaphragm of the channel steel is also due to the heavy alternating stress caused by the continuous reciprocating of the vehicle on the both sides of the longitudinal diaphragm to cause the fatigue cracking.(3)The fatigue life of two different abdominal bar forms is evaluated according to the SN curve of the specifictions for design of highway steel bridge(JTG D64-2015).The conclusion is that the fatigue performance of the longitudinal diaphragm of the channel steel bar is better.The maximum decrease was 89.6%.(4)The stress concentration of the two kinds of longitudinal diaphragms has appeared,so it can be improved from the idea of changing the contact form between the web member and the joint plate.Through the circular circle processing of channel corners,it is found that the stress concentration phenomenon of contact points disappear under the same working conditions.The conclusion has some reference significance for the design of longitudinal diaphragms in similar bridges. |
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