Comparing the embodied energy of structural systems in buildings

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). sumption of a building (Thormark 2002). Consequently, the structural system should be a primary target for reducing the embodied energy of a building. 1.2 Shortcomings of Existing LCA Tools for Schematic Design There has been much research on the embodied energy of building materials, including structural materials, as evidenced by the Inventory of Carbon and Energy (ICE) produced by the Sustainable Energy Research Team (SERT) at the University of Bath (Hammond & Jones, 2008), This inventory surveys peer-reviewed articles on the embodied energy of construction materials and reports the average values found from these sources. For the purposes of this paper, embodied energy is defined as the total primary energy consumed during resource extraction, transportation, manufacturing and fabrication of construction materials, known as “cradle-to-gate” or “cradle-to-site” as opposed to the “cradle-to-grave” method of calculating embodied energy that would also include primary energy expended on the maintenance and disposal of building materials (Hammond & Jones 2008). As construction and installation methods, building maintenance and demolition can vary greatly, this report focused on the more consistent and quantifiable components of the embodied energy of structural materials. While the environmental impact of manufacturing a pound of concrete versus a pound of steel is well documented, there has been less research into comparing the embodied energy of structural systems. The environmental impact of a 4-story building with a structural steel frame versus a concrete frame is difficult to compare and generalize because buildings are complex entities with structural systems dependent on required spans, fire separations, site conditions, and numerous other criteria. Fully designing alternative structural systems in order to calculate quantities of materials and consequently the embodied energy of each is a tedious task. As there is little time during the early stages of architectural and engineering design for such timeconsuming research, means of quickly evaluating the embodied energy of alternative structural systems are required. While numerous studies have calculated the embodied energy of theoretical office buildings (Cole & Kernan 1996, Scheuer et al, 2003), it is difficult to apply the results of these studies to the design of a new building due to the unique requirements of each building. Furthermore, when the size of the building and material used is held constant, the embodied energy of a structural system, normalized in terms of MJ/m, can still vary by up to 50% depending on the building (Suzuki & Oka 1998). Consequently, comparing case studies of entire buildings is not an accurate means of comparing alternatives for the design of a new building. Commercially available life-cycle analysis (LCA) tools, such as the ATHENA EcoCalculator (AEC), exist to calculate the embodied energy of a proposed structural system during the early stages of the design process. (ATHENATM is a registered trademark of the ATHENA Sustainable Materials Institute, Merrickville, Ontario, Canada.) In a review of fourteen models for the environmental assessment of buildings, Seo (2002) noted a number of shortcomings in existing tools including the need for a more comprehensive assessment model, the ability to readily compare alternatives, the time-consuming effort to input data specifically acquired for the assessments, and the need to be specially educated in the use of the tools due to their complexity. Furthermore, the over simplified nature of these LCA tools can provide misleading conclusions about which structural materials and systems will have the lowest environmental impact. The AEC uses overall building square footage and a selection of predetermined structural assemblies to calculate the embodied energy of the structural system. However, regardless of the size or height of the building, the AEC always concluded that a steel structure had a lower embodied energy than a concrete structure during simulations conducted by the authors. This directly contradicts research to the contrary that shows a steel structure has a higher embodied energy than a comparable reinforced concrete system (Cole & Kernan 1996). Furthermore, the simplified inputs used by the AEC ignore issues of floor-to-floor heights or span lengths that could change the amount of material required. Unfortunately, the proprietary nature of the data and calculations used by commercially available LCA software precludes a better understanding of how the structural systems are being compared. For example, the AEC includes the on-site construction of assemblies, maintenance and replacement cycles over an assumed building service life, and structural system demolition and transportation to landfill. However, as there is no connection between the durability of a structural material or system and the actual service life of a building (O’Connor 2004), the assumptions made about difficult to calculate quantities, such as building service life, could lead to the potentially false conclusion that a steel structural system is always better than a concrete one. In order for 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. 2 EMBODIED ENERGY OF TYPICAL STRUCTURAL BAYS 2.1 Decision Making During Schematic Design 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 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 and quickly compared to one another. 2.2 Typical Structural Bays Because concrete and steel systems are not identical in how they optimize member size and type for a given bay size and assembly (flat plate versus one-way beam system for example), a range of six “model bay” sizes were developed for each structural system based on the schematic drawings of a laboratory building in the schematic design phase (Table 1). As the data calculated will be applied to a laboratory building certain criteria, such as a floor-to-floor height of 4.25 m (14 ft), were used. The area of the bays increases in a linear function as roughly a multiple of the smallest bay size. The model bay contained a single column centered on the tributary area for the given bay size. The width and length of the bay for steel or concrete were adjusted so that the dimensions were appropriate for the materials. For steel, rectangular bays with length equal to 1.25 times the width met the approximate square footage of the “model bay,” while concrete bays were square as is typical in normative practice to maximize the efficiency of each system. Table 1. Typical structural bay sizes used in this study. _________________________________________________________ Model bay Area Steel bay Concrete bay ___________ _____ __________ ______________ m x m m m x m m x m _________________________________________________________ 4.9 x 4.9 24 4.3 x 5.5 4.9 x 4.9 37 6.1 x 6.1 6.7 x 6.7 45 5.5 x 8.2 53 7.3 x 7.3 6.1 x 11.6 71 7.2 x 9.8 8.5 x 8.5 8.5 x 11.0 94 8.5 x 11.0 9.7 x 9.7 11.0 x 11.0 121 9.2 x 13.1 11.0 x 11.0 12.2 x 12.2 149 11.1 x 13.4