Prestressed Composite Box Girders for Highway Bridges

In a PC (prestressed concrete) box girder, the need to contain and deviate the longitudinal tendons of internal prestressing increases the web thickness and the self-weight of the deck. As a span increases, the reduction of dead load is a primary need. It is the most important load, resisted first by the construction equipment, and then by the structure itself. Reductions to dead load permit savings in equipment costs while creating a reserve for service loads. External prestressing allows the combination of conventional materials to create innovative prestressed composite structures (PCS). On the longest spans, variable depth PCSs built by balanced cantilevering are less affected by construction stresses. On shorter spans, the same goals can be met with both incremental launching and segmental construction with launching gantries.