There are a number of factors that typically influence the selection of a structural system including code, cost, construction schedule and site constraints. As sustainability increasingly becomes an important goal during the design process, the role of structure in the overall sustainability of a building will need to be considered in terms of embodied energy, building longevity, reuse and deconstruction. The structure of a typical office building contributes roughly one-third to one-quarter of the total embodied energy and double the amount contributed by interior finishes. Consequently, the structure of a building should be a primary target for reducing the embodied energy of a building. While there has been much research on the embodied energy of structural materials, there has been less research into comparing the embodied energy of structural systems. Life-cycle analysis (LCA) tools exist to calculate the embodied energy of a proposed structural system during the early stages of the design process. However, the over simplified nature of these tools can provide misleading conclusions about which structural materials and systems will have the lowest environmental impact. To allow architects and engineers to consider issues of sustainability in the design and selection of a structural system, a transparent and easily understood metric for comparing the embodied energy of structural systems is required. In order to better understand the relationship between structural systems and embodied energy, this paper examines the embodied energy of materials used in typical steel and reinforced concrete structural systems by calculating the amount of material needed for different systems and the embodied energy of selected bay sizes. This method accounts for the varying size and amount of material needed for different spans and columns sizes. By using bay sizes, alternative structural systems are more easily compared to one another. Finally, a 4-story laboratory building, in design at the University of Oregon in Eugene, Oregon, was used as a case study to test the use of bay sizes as a comparative tool. Using schematic plans furnished by the architects to identify the bay sizes used in the building, a one-way concrete slab and beam or one-way joist slab proved to be the structural systems with the lowest embodied energy (both approximately 5,000 GJ).