In this paper, a notional load method is presented for providing analytical treatment of complex distortion effects on fatigue behaviors of lightweight shipboard structures through a distortion decomposition technique. Its applications for analyzing secondary bending stresses caused by nonlinear interactions between four common distortion types induced by welding and remotely applied load are discussed in detail. In addition, two sets of lab-scale specimens and nine full-scale stiffened panel fatigue tests involving complex distortion shapes are also analyzed using the closed form analytical solutions developed. The analytically calculated stress concentration factor results are validated by direct finite element computations in all cases. Furthermore, fatigue test data obtained from both butt-welded thin plate lab specimens and full-scale stiffened panels are shown not only in a good agreement with one another, but also fully contained by the master S-N curve scatter band adopted by ASME Div. 2 since 2007.