This study investigates an analytical beam model to improve the analyses of welded thin plates with welding-induced curved distortions. The model addresses the rigidity of a butt-welded joint and its effect on plate bending and structural stress by including a rotational spring at the welded end. The spring rotational stiffness, k(a), is replaced by the fixity factor, rho(a) . The validity of the model is based on the assumption of small displacement and moderate rotation of the mid-plane of the welded plate. Using the Finite Element Analysis of a two-dimensional model, a semi-analytical method for the fixity factor computation is developed. Compared with the numerical analysis, the beam model showed a maximum error of 3% in deflection and hot-spot structural stress. Results suggest that the fixity factor is mainly dependent on the width of the weld bead and the far-end constraint. The introduction of rho(a) can improve the analytical solution by 9% in the evaluation of the hot-spot structural stress. Neglecting the non-ideal joint rigidity may lead up to 54% underestimation in terms of fatigue life, when the S-N curve slope, m, equals 5. However, the relevance of rho(a) decreases for increasing geometric slenderness of the welded plates.