The designer of an aerial guideway structure for new transit systems has a choice of two major construction materials-concrete and steel. His selection will depend largely on four considerations: esthetic, structural, economic, and acoustical.Esthetic considerations cover such matters as configuration, compatibility of the structure with the environment, color, and surface texture. Structural considerations cover foundation requirements and size and shape of members to accommodate the design loads. Economic considerations include comparative costs of girders of various lengths, ease of installation in available right-of-way, comparison techniques possible through the use of one material or another, capability of local materials producers in terms of both technological expertise and plant capacity, and comparative cost of maintenance. Acoustical considerations include the ability of the material to minimize noise or to permit treatment which will absorb noise.Neither steel nor concrete offers all of the characteristics sought. By a process of both objective analysis and good taste, the designer selects one or the other, or produces a composite design to incorporate the better features of both.