Study On Design And Prevention Of Longitudinal Cracks Of Bottom Plate Of Continuous Rigid Frame Bridges Based On Cracking Laws

The continuous rigid frame bridge has strong spanning ability,beautiful line appearance,and smooth traffic.It also has the advantages of continuous beam and T-shaped rigid frame bridge.It has been built in large quantities in China.China has formed a huge “continuous rigid frame bridge family”! In advocating the strategy of sustainable development and the concept of life cycle,as a large country with continuous rigid bridges,it is also timely to summarize the experience of building continuous rigid frame bridges and summarize the shortcomings,so as to provide a strong scientific basis for follow-up bridge design and construction.Based on the survey of continuous rigid-frame bridges in Shaanxi Province,it is found that the problem of longitudinal cracks in the bottom plate of continuous rigid-frame bridges is prominent.The causes of these problems and how to control them during the design period have become the most concerned issues for all parties in the next step.In response to the above issues,the research and achievements made in this paper are as follows:(1)All continuous rigid-framed bridges in Shaanxi Province were surveyed,and the cracking patterns of the longitudinal cracks in the bottom plate were studied.The cracking rules were summarized.According to the actual situation in Shaanxi Province,the causes of cracks are analyzed,and the direction of further research is obtained: 1)The calculation formula of radial force effect caused by tensioning of prestressed steel should be study furtherly.2)Study on temperature gradient fields in narrow area of Shaanxi Province.3)To investigate the effect of adding cross diaphragms on the stress distribution of the bottom palte of continuous rigid frame bridges.(2)Based on the investigation and statistics of all the continuous rigid-framed bridges in Shaanxi Province,the cracking patterns of longitudinal cracks in the floor and the cracking rules are summarized.According to the actual situation in Shaanxi Province,the causes of cracks are analyzed and the direction of the next research is obtained: 1)Correction calculation of tensile radial force of prestressed steel beam in continuous rigid frame bridge;2)Research of temperature gradient in narrow and long area of Shaanxi Province;3)Research on improving the stress of floor by adding horizontal diaphragm in continuous rigid frame bridge.(3)In this paper,three representative continuous girder bridges in northern Shaanxi,central Shaanxi and southern Shaanxi are selected for continuous observation.The measured data show that there is vertical temperature difference in the bottomplate of box girder bridges.By comparing the regulations of temperature gradient in many countries,the temperature gradient model of three regions was established by using the "three fold line".At the same time,ANSYS finite element simulation model was established,and the measured temperature effect was analyzed.The results show that without considering the vertical negative temperature gradient of the bottom plate,the stress of the bottom plate will have an adverse effect,which is unsafe.(4)Through the finite element simulation analysis,it is found that the addition diaphragm at the mid-span can improve the stress of the bottom plate of the box beam and weaken the peak tensile stress of the bottom plate.The reasonable thickness of the mid-span diaphragm is 55 cm and the reasonable hollowing rate is 30% or 35%.At the same time,the diaphragm can effectively reduce the distortion effect of mid-span section of the box beam.In terms of structural structure or structural stress,the addition of diaphragm is advantageous for sharing the radial force of the steel strands and reducing the longitudinal cracks.