The problem of effective breadth of a stiffened panel with two different edge or support conditions, namely (i) stress-free edges and (ii) displacement restrained edges, has been solved for the first time, in this paper, by the eigenfunction approach to the end problem of semi-infinite strips. It is shown that these two edge conditions lead to complex eigenvalue problems and thus the plane-stress problem of the plating cannot be solved by the classical methods which have been hitherto successfully used to obtain the estimates of effective breadths for the two other possible mixed edge conditions, namely, (i) vanishing direct stress and tangential displacement and (ii) vanishing normal displacement and shear stress. It is brought out that this end eigenfunction formulation, using a generalized orthogonality of Papkovich, provides a general analytical framework for the stress field problems of stiffened panels in bending.