Bridge Expansion Joint for Bridges with Segmental Box Girders

Precast segmental box girder bridges are constructed by assembling precast concrete segments and post-tensioning them together. The expansion joint design must account for the unique movement characteristics of segmental construction, including the elastic shortening from post-tensioning and the long-term creep and shrinkage. Segmental construction sequence affects the movement at the time of joint installation. The segments are typically assembled using the balanced cantilever method, with segments added alternately on each side of the pier. The post-tensioning is applied in stages as the segments are added. The joint should be installed after the post-tensioning is complete and the elastic shortening has occurred. Elastic shortening from post-tensioning causes the deck to shorten immediately when the post-tensioning force is applied. For a segmental bridge with an average prestress of 10 MPa, the elastic shortening is approximately 33 mm per 100 meters of bridge length. The joint gap must be set after the elastic shortening has occurred to avoid this movement being imposed on the joint. Creep and shrinkage of precast segments are smaller than for cast-in-place concrete because the segments have typically undergone most of their creep and shrinkage before installation. However, the post-tensioning force causes additional creep under the sustained prestress. The joint must be designed to accommodate the remaining creep and shrinkage after installation. Joint geometry for segmental box girder bridges may be complex due to the variable cross-section of the box girder. The expansion joint must fit the cross-section of the box girder at the joint location, which may include webs, flanges, and a variable depth. Custom-fabricated edge beams may be required to match the box girder geometry. Waterproofing of segmental box girder bridge joints must address both the external deck joint and the internal drainage within the box girder. Water that infiltrates the deck joint must be collected and discharged through the internal drainage system without accumulating in the box girder cell.