A large number of existing bridges need to be rehabilitated due to increasing traffic and/or loading requirements and also corrosion action. In this paper, a procedure is presented for estimating the ultimate capacity of a steel bridge over the Danube in Bratislava – Old Bridge (built in 1945). The development of a simplified Finite Element Model (FEM ) and basic modal parameter calculations preceded the experimental investigations of the bridge via static and dynamic in-situ loading tests, so that the main assumptions adopted in the FEM were assessed through comparison between measured and predicted dynamic and modal parameters of the bridge structure. The bridge structure computational model was then optimized by structure variables (primarily, steel structure joints mass and corrosion grade) to achieve the minimum differences between the experimental and theoretical results. The calibrated FEM with the optimal combinations of the mentioned variable values were defined and finally used for structure calculations and for strengthening the design of the real bridge structure.