The paper concerns the analyses carried out on an old, severely damaged 34.70 m long and 9.20 m wide R.C. grillage deck, composed of four longitudinal beams and five cross-beams. The deck is part of a 28-spans viaduct, built in 1968, which exhibits a severe corrosion state, due mainly to salted water coming from the roadway platform. The study is aimed at assessing how the bearing capacity of the structure varies over time, depending on the progression of corrosion phenomena, and to searching for the corresponding collapse mechanisms. The work starts by presenting the hypotheses and algorithms at the base of the Limit Analysis procedure developed for the specific purpose of this study, followed by a brief recall of the damage model assumed to represent the deterioration in time of the mechanical properties. The most important conclusions highlight how the damage, represented according to different scenarios, determines not only a reduction of the local bearing capacity, but also an internal force redistribution towards the undamaged structural elements. The conclusions of the study were taken into account in the actual design of the deck repairing interventions. In fact, the design focussed not only on substituting/repairing the elements which had undergone the most damage, but also assessed the robustness of the deck in its entirety, verifying that the undamaged elements had sufficient bearing capacity to carry the increase of internal forces derived from the redistribution mechanism.