Abstract
This article presents guidelines developed by the Research Institute of Roads and Bridges for estimating the loss of the load capacity and determining the time between failures of steel road bridges resulting from corrosion. Mathematical relationships which enable the determination of load capacity and durability of bridge structures were discussed. Reduction coefficients required to calculate the percentage decrease of the load bearing capacity of the corroded structure and reduction coefficients that are significant in determining the degree to which lifespan of the structure is shortened were defined. Mathematical algorithms of presented method made it possible to obtain information, that are necessary for further maintenance and possible modernization of the bridge in Wisla.