Besides an efficient component dimensioning, the utilization of modern high-strength materials is essential in order to fulfill the increasing demand of weight reduction and sustainability. Due to the use of ultra high-strength fine-grained steels and the knowledge of in-service loads, the design of lightweight structures is enhanced even further. By applying suitable welding processes, complex structures can be developed, which are often used as structural components in crane and automotive engineering. Therefore, this master thesis investigates the service strength of welded ultra high-strength steel butt joints under operating-like variable amplitude loading. In order to research the effect of in-service loads during operation properly, two different load spectra are investigated. The first one is a straight line spectrum that reflects the loads caused by rough terrain and small vibrations. The second one is a Gaussian spectrum load and denotes typical operating conditions of mobile cranes. The fatigue tests are carried out at pulsating and alternating normal stress on untreated welded butt joints in order to determine the effect of fluctuating mean stress in addition to the influence of the different load spectra. After a statistical evaluation of the test data, the results under variable amplitude loading reveal a significantly higher service resistance in both low-cycle and high-cycle fatigue life region compared to the results of the constant amplitude tests. The experimental results under variable amplitude loading are compared with the results for constant amplitudes by means of a linear damage accumulation, from which a real damage sum is determined. The obtained values are in a range from 0.19 to 2.21, which correspond to the recommended values in literature if the scatter band is taken into account. Furthermore, the test results of the variable loading are transferred into a damage-equivalent stress range for constant amplitude loading. In this evaluation case, a damage sum of D=0.5 is obtained, which is independent of the spectrum shape and which corresponds to the values of common fatigue design recommendations. In summary, it can be concluded that the evaluated damage sums are in accordance with the values from literature and standard guidelines and are well applicable for a service strength assessment of ultra high-strength welded joints.