Precast prestressed concrete girders have a tendency to creep upwards over time due to the prestressing force and other effects. If a girder''s ends are restrained by a pier or diaphragm, forces will build up in the girder due to the creep.
Typically, the ultimate restraint moment in a girder is a positive moment. This moment combines with other moments in the girder, creating an increase in the mid-span positive moment and a decrease in the negative moment over the piers.
In essence, the continuity of the girder is reduced, and the continuous forces move towards simple-span values.
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