Assessment and Upgrading of Weakly Shear Reinforced Bridge Decks: A Case Study

During the 60s and 70s many highway viaducts were built in Italy using simply supported prestressed beams, often adopting low shear reinforcement quantities following the prescriptions of the code in force at that time. Nowadays, according to the current assessment procedures at Ultimate Limit State (ULS), based on new criteria and increased traffic loads, shear reinforcements in exiting bridges are often not enough to face shear forces from vertical loads if the conventional truss resisting model is adopted. An alternative assessment procedure may consists in verifying prestressed single span members as elements without shear reinforcement in regions uncracked in bending. In this case, the shear resistance depends on the concrete tensile stress and the concrete compressive stress due to prestressing. With reference to a case study constituted by a prestressed deck of a highway viaduct, this paper shows the inadequacy of the shear capacity of the main girders subjected to traffic loads of the current codes. Subsequently, different upgrading strategies to improve the shear resistance are analyzed, including solutions that increase the bending capacity with external prestressing cables preventing the cracking in bending at ULS