Collapse of prestressed reinforced concrete jetties: durabilityand faults analysis

Catastrophic collapses often are caused by minor details and lack of adequate precautionary measures. Such a case is presented in this paper, where the authors investigate the failure and collapse of jetties in a marina (built in 1974). The inquiry begins with collecting the original design documents and with the examination of the structural elements recovered from water. The original calculation report and drawings have been analysed in the light of the former and the current structural codes. Particular attention is drawn to the conceptual approach of the original designer. Back in the seventies, it was customary that a fully prestressed structural element did not have any ordinary reinforcement for bending. Therefore, the resistance depended only on the prestressing forces, and so the ratio between resistant bending moment and design bending moment MR/ME varied along with the positions of the strands. The calculations performed by the authors using up to date theories, have shown that the service life was almost adequate. The main cause of the failure was hidden behind a small detail: the external section of the strands, where bond between steel and concrete was prevented by a PVC duct, did not have an adequate protection. Hence, the sea water could penetrate the core of the element, causing severe corrosion.