This study aims to develop and validate a wide-ranging simulation tool to predict welding distortion in stiffened plates and shells, with particular emphasis on out-of-plane deformation. The approach adopted in this study uncouples the thermal, elasto-plastic and structural effects leading to distortion. The computational models and results are supported by realistic welding tests and appropriate measurements to validate the simulated thermal fields and out-of-plane distortions. The simplest and most computationally efficient model makes use of algorithms, instead of numerical computation, to link the thermal welding strains to the elasto-plastic and structural responses of the welded assembly, via a static, single-load-step analysis. Alternative, more computationally intensive models are explored which simulate the full transient thermal and elasto-plastic structural responses in an uncoupled fashion. These provide a cross-reference for the more rudimentary but computationally efficient models. The experiments and computational strategies are applied to welded assemblies incorporating double-fillet-welded stiffeners.