| Owners and much of the design and construction units largely favor the prestressed concrete bridge, especially the P.C. continuous beam bridge, which makes a rapid development and extensive application, because it has a series of advantages, such as big structural stiffness, good smooth-going function when driving a vehicle, less expansion joint and simple maintenance. However, when entering 21 century, engineers find general problems about P.C. Bridge cracking. Most of the early bridges began to crack after 3 to 5 years. Even more seriously, during the construction period, various extent of cracking phenomenon appeared. Later, the bearing capacity of the bridge had been largely affected after many years. Therefore, we should try our best to solve the problems of prestressed concrete cracking, which is inevitable.In order to seek the mechanism and causes of P.C. Bridge cracking, to find preventive measure, and also to offer technical premise and decision basis for similar bridge building in the future, this paper, starts with P.C. Bridge mechanics behavior, makes use of the numerical analysis method, analyses the Stress distribution rule and mechanics characteristics of Box Girder section about box girder in each construction stage and under completion state, and researchs how to affects the characteristics of cross-section on porthole of Post-tensioning prestress about P.C. Bridge. Therefore, it offers theoretic basis to this box girder cracking problems.First of all, this paper summarizes the development situation and research of dynamic state about methods and theories of the P.C. Bridge. Meanwhile, it points out the deep meaning of researching the stress properties by means of numerical analysis. Also this paper demonstrates the background of this work and difficulties that we may meet, and outlines the mechanics behavior about box girder of P.C. Bridge, calculation method and basic theory about finite element analysis, to offer theoretical basis to numerical analysis.Then, based on Chongqing Yudong Yangtze River Bridge (260m prestressed continuous rigid frame) , taking advantages of large-scale structure finite element analysis software ANSYS to establish three-dimensional analytical plane model, linking with plane frame structural software Dr.Bridge full-bridge model, this paper makes a numerical research for every construction phase about the 0-2 # part of box girder(singe box twin room).At last, making conclusions, and having a outlook about the trend of development in the field of P.C. Bridge on mechanics properties and cracking reasons.The content on numerical analysis is consisting of four parts:1. Using APDL programming in ANSYS to achieve plane numerical imitation under the circumstance of porthole effect (before tensioning prestress and grouting) and no porthole effect (after tensioned prestress and grouted), it compares the stress distribution of cross-section and stress concentration extent around the prestressed porthole. All of the above offers premise to three-dimensional numerical analysis and confirms the necessity and feasibility about the three-dimensional numerical to the P.C. Bridge.2. Making use of plane frame structure software DR.Bridge 3.0, we set up full bridge model of the real bridge, which calculates the bearing situation during construction stages and completion state about Chongqing Yudong Yangtze River Bridge. It provides reference basis for three-dimension numerical analysis.3. Using ANSYS secondary development tools and ANSYS Parametric Design Language, we build the three-dimension solid modeling about the 0-2 # block part of Box Girder of Chongqing Yudong Yangtze River Bridge, and have a three-dimension numerical analysis in construction phases and under Completion State. By simulating the construction process such as prestressed tension and grouting, thereby, it analyzes and masters the stress distribution of the channels with prestressed box girder bridge section in the entire construction process. Based on the results of numerical analysis, combining with calculating results of plane frame and constructions test data, we can make a comparison with the results of three-dimension analysis, have an overall evaluation about the numerical analysis results and verify its reliability.4. It compares and analyzes the Stress distribution conditions on the typical cross-section of Box Girder at the typical construction stages according to data results obtained from Ansys three-dimensional numerical analysis , under different circumstances of whether having the porthole effect. Thereby, it studies how the later-tensioned prestressed porthole affect mechanics characteristics of the P.C. box girder section.
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