Contemporary economical and ecological aspects of production and exploitation of the complex thin-walled structures (aircraft, ship, etc.) increasingly stimulate growing emphasis on reduction of the thereby consumed amount of material and energy. Compliance with this requirement demands contrivance of ever more rational and economically feasible design variants, which are consequently and imminently characterized by decreased inherent safety margins. Since evaluation of the structural limit states implies direct consideration of the situations when structure (or some of it's components or joints) loses ability to perform it's intended function, considerably accurate and reliable determination of true structural safety level and it's corresponding safety margin might be employed in the process of structural concept synthesis. In this respect, objective of the presented research is development of the improved progressive collapse analysis method and consequent enhancement of the ultimate limit state evaluation in the concept design of monotonous thin-walled structures subjected to predominantly transverse (vertical) flexural loads during exploitation. By subsumption of various distributed load effects, proposed method enables more sophisticated evaluation of longitudinal ultimate load-capacity (ultimate bending moment) and more accurate and reliable identification of the critical cross section and proprietary collapse sequence due to extreme bending. In that sense, implementation of the proposed method within framework of the optimization based conceptual synthesis can result in a higher quality level of design attributes (cost, mass, safety) for nondominated design variants and more rational and economically feasible preferred design solutions. After detailed description and review of the existing incremental-iterative progressive collapse analysis method (based on Smith's approach), various aspects of the proposed methodological extensions are presented. Detailed discussion regarding geometrically nonlinear kinematics of Euler-Bernoulli beam is presented with special emphasis on the case of symmetric bending devoid of the longitudinal force action, since it could be considered as the appropriate idealization for the most of the aircraft and ship structures. Relevant aspects of the cross sectional warping due to transverse symmetric bending are discussed and existing numerical method for determination of the cross sectional (discrete) distribution of longitudinal warping displacements is described. Furthermore, a method for determination of the corrected normal (longitudinal) strain distribution based on Lockwood-Taylor theory is derived. Various aspects of shear load resistance of the plane plating are reviewed, including elastic shear buckling and various means of it's deferment allowed by the current theoretical description of that phenomena. Local shear buckling imminence criteria for plane stiffened panels is derived, various existing semi-empirical formulations of the ultimate shear strength are presented and accepted elliptical formulation of the interaction between (ultimate) shear strength and longitudinal (ultimate) strength is described. Finally, detailed description of the proposed incremental-iterative progressive collapse analysis method is given. Accuracy level of the proposed method is demonstrated on a few different examples of monotonous thin-walled stiffened box girders submitted to different load cases of (pure and transverse) symmetric bending and results obtained by the initial and proposed method, experimental testing and numerical simulations (NLFEM) are comparatively reviewed.