The assessment of the total service life of a steel structure should gain importance in the near future due to the increasing significance of building preservation and building modernization. The main cause of the failure of existing structural steelwork under cyclic loading effects is material fatigue. Most steel structures are, however, failure-tolerant. For economic reasons, this behaviour can be considered by including the crack propagation phase in the assessment. This contribution presents newly developed software tools and the results of some simulations of the total lifetimes of existing structures.