Issues in design of long-span prestressed concrete box girder bridges

Prestressed concrete box girder bridges are widely used because of advantageous structural behavior and perfect shapes.However,in the past few years,excessive deflection at the midspan and cracks in the box girders have commonly appeared in some long-span prestressed concrete box girder bridges after a period of operation,reducing the serviceability and durability of the bridges.The causes of cracks and excessive deflection are still not clear.Long-span prestressed concrete box girder bridges are studied from several aspects,such as longitudinal prestress design,vertical prestress design,vertical prestress loss,composition design,configuration of stirrups,characteristics of concrete.Two methods of longitudinal prestress design are compared,and the study indicates that the layout of bent-down tendons is an effective means for avoiding diagonal cracks in the webs.Moreover,the influences of shear deformation and development of diagonal cracks in thin-walled box girders on long-term deflection could not be ignored.