The ultimate collapse of the midship section of tankers and container ships under combined vertical and horizontal bending moments is determined by an approximate method. That accounts for the load shortening contribution of each plate and stiffener assembly, thus being able to construct the moment-curvature relation for the hull girder. The method is applied to study five tankers and six container ships and the results are used to define an interaction formula proposed to account for the combination of the load effects for design purposes.