The Research On The Weakening Of The Web's Section Caused By The Pre-stressed Concrete Box Girder's Vertical Pre-stress Passageway

The structural system, of continuous (rigid structure) box girder bridges of prestressed concrete, is applied broadly and developing rapidly. In order to reduce and control the principal tensile stress of the web and to prevent cracking, setting up prestress tenders becomes an important content in design. However, the cracking is still commonly found in the webs of box girders, even have been vertical prestressed. One of the main causes of cracking is too much loss or invalidy of prestress, which results in difficulty to design. This article mainly studies the variation of the webs' stress level and the stress concentration around the passageway, caused by the weakening of the section's shear resistance stiffness, flexural rigidity and pressure resistance stiffness, in the condition that the vertical tendon's passageway is not grouted or not enough. The detailed research methods are expounded as follow.Firstly, based on the assumption that the stresses are equal and other assumptions, the analysis on stress of the box girder's web of the Chishan Bridge in Yiyang City, Hunan Province and of Yuanshui Bridge in the freeway from Changde to Zhangjiajie is carried out. Thus, influence on vertical prestress in the web because of the weaken of vertical prestress passageway can be found. Furthermore, equivalent principal tensile stress incremental method is applied to analyzes the vertical prestress of the web. Secondly, this article sets up the finite element simplified model of box girder's web and carries on the centralized analysis of the shearing stress around the passageway to get the concentration factor tentatively. Finally, cooperating with finite element analysis, experimental study on a test web model has been done to comparatively study the influence of the vertical prestress passageway, which is grouted or not, on the specimen's stress level and shear resistance, as well as the centralized analysis of the vertical prestress passageway on the specimen.Stress analysis of the box girder's web, with variable section finite element analysis and experimental study on stress concentration show that if vertical prestress passageway is not grouted, weakening the section and stress concentration around the passageway will increase the principal tensile stress of the box girder's web in the bridges' operation, and weaken the effect of the vertical prestress. So in the course of design, it is necessary to discount the vertical prestress to make up the disadvantageous effects owing to the weakening of the passageway on the structure.