In the structural optimization of a ring-stiffened cylindrical shell the unknown variables are the shell thickness as well as the thickness and the number of flat rings. The shell diameter enables to realize a belt-conveyor structure inside of the shell. The uniformly distributed vertical load consists of dead and live load. The design constraints relate to the local shell buckling strength, to the panel ring buckling and to the deflection of the simply supported bridge. The cost function includes the material and fabrication costs. The fabrication cost function is formulated according to the fabrication sequence and includes also the cost of forming of shell elements into the cylindrical shape as well as the cost of cutting of the flat plate ring-stiffeners. As an alternative to safety factors one may try to describe the uncertain data via a non-probabilistic description of uncertainty, in particular the fuzzy-set based analysis. Several procedures are described and the optimum design level can be obtained either based on failure possibility or of membership value satisfaction. The fuzzy-based optimization becomes a sequential minimization of unconstrained convex scalar functions, from which a Pareto solution is obtained. A branch and bound procedure is associated with this algorithm to provide a discrete solution
