I have done fatigue check of a bride twice. Once with trainload generated with Ella and once with load generated with stepping method. However, I got very different results! Utilization level of members under load generated with Ella is very higher (4.5 times) than those generated with load stepping method. First picture shows utilization level of fatigue for load generated with Ella and second one shows utilization level for trainload generated with stepping method! I have checked stress utilization level in ULS and SLS as well as maximum internal forces of members. Those results are so close to each other. Anyone knows why this large difference occurs? Trai command for both cases generated in the same way. Does anyone know what is going wrong? If results of stepping method is true, I can done a very large optimization in cross-section of the Longitudinal stifnenings (Längssteifung).
stepping method can miss load position which causes extremal effects, ella however places load on most unfavourable postition of influence line/area. For example for single moving point load stepping method should give the same results as ELLA if step is lesser than node spacing. Fatigue is important in short elements and for them missing load postion with stepping is highly possible. Try finer steps.