Research On Setting Of Large Span Prestressed Concrete Continuous Box-girder Back-up Prestressed Strand

Prestressed concrete continuous box-girder birdge has developed quickly, because it has characteristics of large span, good structural rigidity, perfect dynamic performance, simple structure , low project cost and extensive use of cantilever construction. But in recent 20years, with the increase of built-up bridges and service time, some defects have been discovered in large span prestressed concrete continuous box-girder bridges of service phase. Mid span deflection and structure crack are typical defects.In view of defects of large span prestressed concrete continuous box-girder bridge, this paper makes cause analysis in all these areas of computation theory, materials, outside surroundings, construction errors, operation and maintenace. And the paper takes (60+2x110+75+55)m continuous box-girder of Zengjiang Bridge as example to discuss how to prevent mid span deflection and structure crack in bridge's design phase or control them in later service phase. The paper mainly analyzes concrete shrinkage and creep, prestressing loss, then comes up with new ideas of the layout of back-up prestressed strand. Comparing internal strand with external strand, considering different forms of the layout of strand have different effects on improving structural behavior and mid span deflection, it puts forwards the rational layout of strand. Combining regular monitoring results, back-up strand should be applied before structure crack.With the constant improvement of concrete shrinkage and creep computation theory, the mature of external prestress computation theory and the improvement of aseptic technique , the layout of prestressed strands of large span prestressed concrete continuous box-girder will renovate continually. Internal prestressed strand and external prestressed combined strand will be developing direction of large span bridges in future, which will bring distinct economic and social interests for adjusting and improving structural behavior in service phase.