The design resistance in prEN 1993-1-9 for welded connections subject to fatigue loading refers to ISO 5817 quality level B. However, the background document does not explicitly support the link between weld quality level and fatigue resistance. To address this gap, an analysis was conducted on the experimental database used for prEN 1993-1-9, in conjunction with published results from literature encompassing over 1000 experimental fatigue tests. In addition, experimental tests were conducted on butt weld connections welded from one side to evaluate the impact of undercut on fatigue strength. Non-destructive testing methods, including 3D scanning, and radiographic testing were employed to characterize weld imperfections. Fatigue strength was evaluated in accordance with prEN 1990 requirements. The findings revealed that mean undercuts height between 0.54 mm and 1.48 mm in a 15 mm plate thickness have decreased the fatigue strength to 62 MPa (13% reduction). Moreover, the analysis revealed that some experimental tests used in the background document did not conform to quality level B tolerance. Finally, the use of 3D scanning provided detailed measurements of geometric imperfections that cannot be captured by visual inspection, which may downgrade the quality levels to D or below. This highlights the need to align weld quality classifications with the non-destructive test method employed.