Strengthening Concrete Box Girder Bridges

There is, at a theoretical level, a direct relationship between the effects of prestressing, temperature, support settlement, and creep redistribution in concrete box girder bridges. At a more practical level, the net effect of thermal gradients or creep redistribution is to impose a positive secondary moment onto the structure, which can be balanced by internal prestressing (tendons inside the box that are anchored at the abutment diaphragms and pass freely through the pier diaphragms) or external prestressing (jacking the bearings upward). Although intuition tells us to put the prestressing at the bottom if there is a problem with tension at the bottom, the tendons should actually be at the top (just below the upper kern point) to counteract these secondary moments. Intuition also tells us to drape the tendons, but this practice is not particularly efficient. Of the infinite number of possibilities for bearing adjustment (a variation of support settlement), only raising the bearings upward to give the bridge a parabolic profile counteracts these secondary moments.