| At present, in our country, main problems of large span prestressed concrete girder bridges are excessive midspan lag and cracks in box girder. So, it is quite necessary to study how to control long-term deflection of large span prestressed concrete girder bridges.In this paper, prestress, box girder rigidity, first time loading age of segment, model and duration for shringkage and creep are studied to compare their influences on short-term and long-term deflection. Based on some continous girder and rigid frame bridges we have performed construction control, in situ measured displacement, long-term displacement, internal force and prestress tendon amount are investigated and analyzed to find their relationship.First, using top slab prestress to control long-term deflection of midspan is studied. Top slab tendons are redesigned on the basis of their bending moment is approximately equal to structure selfweight bending moment at the longest cantilever state, internal force and displacement are compared with original design.Second, in order to control long-term midspan deflection, bottom slab tendon amount is increased, efficiency of bottom slab tendons at different location to control long-term midspan deflection is compared, proper tension time of bottom slab tendons is studied.Third, using temporary cables together with designing prestress according to integral cast construction to control long-term midspan deflection is studied. Longitudinal prestress tendons are designed according to internal force of integral cast construction, sum of prestress bending moment, selfweight bending moment and bridge deck pavement bending moment is approximately zero. During construction stage, with the help of temporary cables, box girder will not deflect, so that displacement during construction is very little. After the construction, long-term shrinkage and creep displacement will be very little as well, as a result, long-term midspan deflection is controlled.
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