In order to aid the design process of complex structures the use of numerical tools is state of the art in engineering. Especially increased demands in light weight construction enforce the improvement of numerical method in accuracy and complexity. The aim of this master thesis is to enhance a method for numerical fatigue assessment of seam welded steel structures, which can be integrated into a postprocessor. The advantages and disadvantages of certain fatigue approaches are identified within a comprehensive literature and application study. The ability to implement the method into to existing fatigue assessment postprocessor FEMFAT WELD is of special interest for the application engineer. The research work is conducted for steel materials and includes the effect of plate thickness, beginnings and endings of weld seams as well as multiaxial loading. The proposed assessment method, based on the critical distance method, uses coarse meshed finite element models to compute structural stresses with the finite element method. These structural stresses are multiplied with the fatigue notch factor in order to obtain the effective notch stress. The determination of the structural stress can be done in several ways, whereat the linear extrapolation of stresses at two reference points is to favor, as the investigated application study shows. The damage is calculated with the given SN-curve and the hypothesis of linear damage accumulation. The fatigue notch factors are stored in a database for different types of weld joints and load cases. Therefore they are calculated with finite element models with a notch radius of r = 0,05 mm at the weld root and weld toe. Regarding the modeling of the weld joint in a preprocessor, some possibilities are discussed and a suggestion to receive appropriate results is made. In order to afford an experimental verification of the numerical determined fatigue life, several fatigue tests are recalculated in this work. These calculations are also done with FEMFAT WELD and further local approaches, based on fine meshed finite element models. Hence a comparison of the estimated fatigue life among the different methods is possible and it can be illustrated, that the introduced fatigue assessment method based on the critical distance method improves the fatigue strength calculation, especially for complex structures and loading cases.