The fatigue strength of structural steel components subjected to cyclic loading is often characterized in terms of S-N curves derived under constant amplitude (CA) loading. However, these are often subjected to load histories of variable amplitude and mean stress, i.e. variable amplitude (VA) loading. To take into account this loading condition for the design or the verification of structural components, damage accumulation models and modification of S-N curves have been proposed in the literature. However, these models require experimental characterization to verify or eventually calibrate some model parameters, e.g. the critical value of the Palmgren-Miner damage, among others. This paper investigates the CA fatigue behavior of transverse welded steel attachments by reporting on an experimental characterization. Fatigue cracks have been monitored and fracture surfaces analyzed to obtain information about nucleation sites and growth rates. Moreover, the paper reports on the design of VA fatigue tests using a load history obtained from traffic measurements from a Swiss road bridge. As a design tool, the preliminary study using advanced probabilistic fatigue prediction models calibrated on similar welded details is presented. This model can also be used to define other load spectra to be applied in future VA experimental studies.