| Along with the increasing of the bridge service life,there are two major diseases in long-span PC girder bridge,namely,the cracking of beams and the continuous mid-span sag.Based on the Hong Yan Xi Bridge project,the new technical measures for tensing the temporary external tendons and the new longitudinal prestress tensioning process on long-span PC girder bridge are put forward to reduce the deflection during the operation of the bridge.Then this paper analyses the impact that the two methods has on force,deformation and prestress of bridge structure with the finite element method.Finally,the influence of the stress of support-section after the completion of the bridge and the shear deformation on structural deformation were analyzed.The main conclusions are as follows:(1)Since creep of concrete is the most important factor of girder deflection,this study has investigated that tensing the temporary external tendons can be used to accelerate the development of the creep in the cantilever construction stage,which can be helpful to reduce the deflection caused by the creep of concrete during the operation of the bridge.Based on the Hong Yan Xi Bridge project,4 kinds of temporary external tendons schemes are taken in account in this research;The scheme 1?3 only lays the temporary external tendons of the baseplate,and the amount of temporary external tendons increased in turn;Scheme a is a comparison of scheme 1,which is provided with equal temporary external tendons of the baseplate and the top flange.The analysis results show that tensing the temporary external tendons of the baseplate in the cantilever construction can significantly reduce the long-term mid-span sag caused by the creep of concrete,and 20 years after the completion of the bridge,the mid-span sag is reduced by 36.96%in the scheme 3;In addition,the removal of the temporary external tendons increases the compressive stress of midspan baseplate,which can improves the anti-cracking performance of midspan section;Furthermore,the reliability of the model is verified by comparing the theoretical and experimental results;(2)Based on longitudinal prestressed system of Hong Yan Xi Bridge,this study puts forward a new prestressed tensioning process that is achieved by postponing 4 pairs of tendons tension in the top plat to the completion of the bridge.On the one hand,the new process increases the compressive stress of the baseplate and reduces the compressive stress of the top flange,which can reduce the long-term mid-span sag caused by the creep of concrete;On the other hand,the tensioning of the delayed tendons cause an arch in the midspan section,and these two points greatly reduce the mid-span sag after the completion of the bridge.The finite element calculation results show that,20 years after the completion of the bridge,the mid-span sag is reduced by 66.06%and the compressive stress of midspan baseplate is increased by 1.1 MPa,which can effectively improves the anti-cracking performance of midspan section;(3)The greater the stress at the upper edge of the fulcrum,the smaller the stress loss during the operation of the bridge,the better the anti-cracking performance of midspan section,and the smaller the mid-span sag after the completion of the bridge.If the shear deformation is not considered,the construction camber of bridge will be smaller.Taking Hong Yan Xi Bridge as an example,under elastic deformation,when considering shear deformation,the maximum cumulative vertical displacement of the main span of the main bridge is 9.05%larger than that without considering the shear deformation.When shear creep is taken into account,the midspan deflection of the span becomes obvious after the completion of the bridge,and if the shear creep coefficient is 3 times of the axial force creep coefficient,the midspan deflection of the 20 year after the completion of the bridge will be increased by 21.97%,which is far greater than 5%,therefore,attention should be paid to shear deformation in the calculation of deflection. |
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