Study On Fatigue Life Analysis Method Of Prestressed And Reinforced Concrete Bridge

Prestressed concrete box girder bridge is widely used in bridge engineering due to its advantage of long span, high stiffness, good torsional performance, easy to construct and so on. But during the period of its serivce, many durability problems gradually exposes, such as cracking in webs, long-term continuous deflection of main structure. Further, after the concrete web cracking, in order to fully reflect use performance of the bridge, the assessment of the fatigue life of the webs is worth taking seriously. These problems are not only related to the normal use of the bridge, but also mean a threat to the safety under some serious conditions. Therefore, study on the evaluation of fatigue cracking and fatigue life of prestressed concrete box girder bridge webs is very urgent. Besides, fatigue issues of reinforced concrete bridges which are widely used in short span bridge engineering are getting more concern, how to compute their fatigue life fast and accurately becomes hot subject in bridge engineering field.The fatigue issues of prestressed concrete box girder bridge webs and reinforced concrete bridges is systematically researched in this thesis, and life prediction methods of prestressed concrete box girder bridge webs and reinforced concrete bridges is purposed to apply in the practice. The main research contents and achievements are concluded as below:1. Deficiency of the existing research is clarify by concluding the research status of the development of prestressed concrete box girder bridge, structure analysis theory of box girder bridge, the causes of web inclined cracks and fatigue issues of webs.2. By refering a large amount of literature, the S-N curve of concrete and reinforcement, the fatigue characteristics of webs by only considering vertical shear effect and the fatigue characteristics of webs caused by transverse effect of box girder is introduced to describe fatigue behavior of prestressed concrete box girder bridge. the specific meaning of the fatigue life of the box girder webs is defined finally.3. Combining the strength characteristic of box girder, a simplify method of box girder bridge is purposed based on beam finite element model, variable angle truss model and the plane frame model. In order to reflect the real state of stirrup stress after web concrete cracking, a algorithm considering crack influence coefficient is put forward, and parameter analysis method is also purposed by considering vehicle loads, crack depth and girder heights. Based on linear fatigue damage accumulation theory and the simplify method of box girder bridge, a fatigue life prediction method of prestressed concrete box girder bridge webs is purposed, and a real case is studied, the influence of the crack depth, the transverse effect of box girder and coupling effect of prestress and vehicle loads to web fatigue life are discussed4. Some basic concepts of linear fracture mechanics is introduced briefly. Because of deficiency of the existing fatigue life prediction methods, fatigue life prediction method of reinforced concrete bridges based on damage and fracture mechanics combined theory which can be used to consider both the crack initiation and crack propagation is purposed. Three different results of fatigue life computed by using damage mechanics method, fracture mechanics method and damage and fracture mechanics combined method is discussed though studying an existing bridge. The rationality of damage and fracture mechanics combined method is illustrated.