Case Studies of Failure, Damage Assessment, and Repair of Multispan Bridges in Argentina

The paper is concencerned with the evaluation, strengthening and follow-up of two case studies of multispan prestressed concrete bridges in Argentina. The two case studies, and the ensueing repair works, were prompted by different circumstances: one of them, by the catastrophic collapse of one of twelve identical 60 m span arches -tied by the presetressed concrete deck- after passage of a regular passenger bus, and the other one, consisting of fourteen simply supported spans of cast-in-place prestressed concrete girders, by systematic and excessive deformations and cracking of the longitudinal girders. Although both cases were built more than 30 years ago used for prestressing a postensioning system (Hoyer) no longer applied in Argentina, and were located in the same geographic region with similar mild environmental conditions (temperate weather, with rains concentrated on the summer months), the difference between their respective structural configurations led to very different critical limit states. The tied arch collapsed without warning just after a bus had passed over the bridge; the collapsed span was the last of the twelve spans the bus had passed. At the time of collapse no signs of damage or of abnormal structural behavior had been noticed. The experimental and modelling forensic study revealed that failure of the prestressed concrete tie beams near the arch abutments incorporated to the structure of the deck was the cause of collapse. The failure mechanism involved only partial redistribution of forces from the broken high stregnth steel tendons of the tie beams to the reinforced concrete deck slab before the onset of collapse. The tendon failure was found to have been caused by corrosion associated with the spilling of a water on the initial section of the tie beam by a drainage pipe of the water runoff of the bridge deck. Design of the external prestressing needed to restore the safety requirements of the eleven identical spans that remained in service was immediately undertaken, implemented and controlled by a follow up process of the prestressing force of the new external tendons. On the other hand, the simply supported beams reached a service limit state of excessive vertical deflections and cracking, that led to bridge owner to restrict the circulation over the bridge to a maximum of one truck in one of the two traffic lanes. The experimental and analytical evaluation that followed indicated the the load capacity of the bridge was not at stake, but that new external prestressing to compensate for the excessive losses of prestressing force was needed to improve durability. The paper presents in detail the various aspects of the structural evaluation performed, from dynamic load tests and material properties to the criteria for design of the external prestressing and control of performance after carrying out the repair work.