Study On Optimal Tension Technique Of Vertical Prestressing And Standard For Stress Under Anchor Controlled In PC Box Girder

Vertical prestressing system in PC box-girder bridge provides vertical prestressing and reduces the principal tensile stress of webs, thus preventing cracking. However, the cracking could not be completely prevented, which seriously affects the normal function and durability of the bridge structure. At present, researches about vertical prestressing mainly focus on function, principals, setting modes, loss calculation theories, equipments and so on. But researches about construction methods, especially regarding tension timing of vertical prestressing, tension carft and standards for stress under anchor controlled have not been thoroughly conducted. This thesis, based on the technology project by the Transportation Department of Shaanxi Province entitled Researches on Key Techniques of Long-span Continuous Rigid Frame Bridge on the Basis of Life Cycle, takes a bridge to work on simulation analysis of stretch-draw timing of vertical prestressing and data collection of pressure sensor under the embedded anchor. It mainly focuses on:(1) Study the effect of vertical prestressing tension on the crack resistance and the mechanical behavior of the web after cracking with ANSYS. Then it comes to the conclusion that the vertical prestressing tension not only improve the crack resistance of the web, but also can improve the stiffness, slow down the development of cracks and the growth of deflection. Meanwhile, the tension of the vertical prestressing causes the crack of the web to be different from that of non-tension, the direction of the inclined cracks and the direction of the main beam is about 45°.(2) Under the assumption that the longitudinal normal stress of the box girder web is certain assumptions, analyzing the influence of all-tension, batched tension and lagging tension on the box-girder webs through simulating prestressing construction process by using the ANSYS birth-death element in FEM. Then it comes to the conclusion that the webs stress state of lagging two segments draw vertical prestressing almost consistent with all-tension, the maximum vertical compressive stress is only 2.7% larger than the latter, but decreased by 32.3% than batched tension, which can be utilized in the practical project.(3) Embedding sensors in the position where cracks easily occur, collecting effective stress under anchor of life cycle, using the measured data as initial sequence to build GM(1,1) grey forecast model of vertical prstressing loss and do model modification of unequal time. Then it is concluded that the long-term loss of vertical prestressing accounts for 7.92% of the initial tensile force, recommended the effective stress under anchor in vertical prestressing construction should based on 0.6 times reduction coefficient of prestressing concrete compressive stress of vertical prestressing in JTG D62-2004 and vertical prestressing long-time loss, advised that effective stress under anchor in vertical prestressing construction should be more than 0.7 times of the designed tension force.The outcome of this thesis provides theoretical basis and reference for vertical prestressing construction and NDE(nondestructive examination), standardizes construction techniques of vertical prestressing and is of great importance of practical value for the application and development of vertical prestressing system in PC box-girder bridge.