Three Dimensional Analysis For Prestressed Concrete Thin-Walled Box Girder Bridge And Research On The Cracking Reasons

A great deal of investigation has indicated that representative cracks such as skew cracks in web with a significant distribution area exist in prestressed concrete box girder bridges which hampers the further application and popularization of this type of bridge in highway construction. Therefore, it is very emergent and significant to take deep research on and to avoid the cracking of prestressed concrete box girder bridges to prolong the service life, to improve the performance and to provide with some useful referenced information for the design.The reason for skew cracks in web is studied combined with three dimentional finite element analysis. After established the local three dimentional model based on a sample bridge on Xiangjiang river, the response of the structure under most disadvantageous loads is analyzed. By comparing the three dimentional calculation results with those of plane in the same working condition, it is concluded that the additional shear stress caused by box girder local deformation under deflecting loads is less than 1 MPa and the additional vertical normal stress and the additional longitudinal normal stress are not remarkable but the difference in principal tensile stress is notable relatively. The three dimentional calculation shows that σ₁ of the sample bridge has little compressive stress storage under combination I while the plane calculation indicates a minimum compressive stress storage of 1.2 MPa. Furthermore, the three dimentional analysis model for web is established. The mechanic responses under the action of vertical prestressed reinforcement are analyzed and the simplified calculation expression for elastic compressive loss of vertical prestressed reinforcement is obtained. It is also found that the vertical prestressed reinforcement performs more effectively when slurry casted than not. Stress concentration and loss of effective prestress are significant relatively if no slurry casted. With the vertical prestress sensitivity analysis, it is known that the principal tensile stress is susceptive to vertical normal stress much. So estimating the effective vertical prestress correctly is the key to control principal tensile stress. Finally, a possible reason for web cracking is obtained with three dimentional analysis results as that the stress distribution characteristic of box girder is not confirmed well and thus the safety storage of normal tensile stress is not sufficient in conventional design. And the loss of vertical prestress estimated insufficiently is another principlereason for web cracking.