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It is difficult to predict welding distortion in large welded structures and even more difficult to achieve both fast and accurate distortion predictions in such structures. Distortion modeling of large welded structures is always associated with enormous computational cost in terms of computer memory requirement and CPU time. This research work was conducted to obtain better understanding and characterization of the plastic deformation that leads to in-plane shrinkage in thin-wall structures. It also aimed to develop a fast and accurate approach for predicting the welding distortion in large thin-wall structures. Some examples of this type of structures are vehicle underbody frame and motor compartment frame. The engineering approach was developed based upon the principle that plastic deformation is the root source of welding distortion. A hypothesis was proposed regarding such development and it is stated as the following: welding distortion can be accurately predicted if the plastic deformation the same as o