The standard hot-rolled sections with various shapes are used to construct the skeletal structural system of the steel civil engineering structures. The conventional design rules direct the engineer to select the smallest section from the manufacturers' catalog that satisfies the considered constraints. However, use of these predesigned sections may result in waste of structural material. This problem can be handled by searching for optimal built-up section designs, which can be manufactured by cutting the plates to the determined dimensions and welding them to each other so as to form the optimized shape. This paper presents the optimal design of prismatic I section beam-columns under stress, non-linear deflection and global buckling constraints with one of the recent metaheuristic algorithms and discusses the influence of variable grouping on the optimization results. Four optimization types are introduced and the contribution of using optimized shapes instead of hot-rolled sections to the structural material economy is demonstrated over numerical examples. It is shown that optimization may lead up to 23% lighter solutions than the hot-rolled sections and it is possible to obtain adequately efficient doubly-symmetric I-section designs (in terms of material amount used) from an engineering point of view.