Wave loads and slamming loads acting on the ship’s hull lead to the superposition of low-frequency wave-induced stresses and high-frequency stresses from whipping as well as springing effects. Whipping, mainly caused by slamming loads, may impact the ultimate strength of the ship’s hull. Both whipping and springing increase the fatigue damage of structural details of the ship’s hull. Fatigue damage assessment typically relies on rainflow counting and the Palmgren-Miner rule for linear damage accumulation, but it is not yet proven whether this approach works well for superimposed high- and low-frequency stresses. Loads recorded in full-scale measurements onboard a containership are the basis of this investigation. Fatigue assessment of measured stress shows a significant damage increase due to high-frequency contribution. However, it has to be emphasized that this is not reflected by observed damages of the fleet in service. To get more insight into the fatigue damaging mechanism of combined low- and high-frequency loads, fatigue tests have been performed in cooperation between DNV GL and TUHH. Transverse stiffeners on a continuous plate have been selected for tests as a representative structural detail. The Palmgren-Miner rule will be verified by test series with measured unfiltered load sequences as well as with low-pass filtered load sequences.