Alongside corrosion effects the service life of existing bridges mainly depends on the previous, present and predicted traffic loads. In view of increasing traffic loads, many existing bridges are not designed for the resulting fatigue stresses. For that reason, monitoring of existing, aged bridge structures becomes an economic as well as safety relevant task in civil engineering. This paper describes a monitoring program on a box girder bridge. The measurements are carried out by a structural monitoring program consisting of strain measurements at the superstructure and displacement measurements at the expansion joint. The measured data are used to explore the appearance of passing heavy goods vehicles and to determine their total vehicle weights. These results can be used to determine the fatigue stresses caused by the current traffic loads and to determine the remaining service life of the structure.