Comparing the embodied energy of structural systems in parking garages

The structure of a typical office building contributes roughly one-quarter to one-third of the total embodied energy. Although the occupation phase of a building's life cycle currently dominates energy use, as operational energy use is minimized through high-performance design, construction and equipment, embodied energy will play a larger role in the overall energy consumption of a building. Consequently, the structural system should be a primary target for reducing the embodied energy of a building. Parking garages offer an ideal case study for comparing the embodied energy of a variety of structural systems. As above grade parking garages have little operational energy use outside of lighting and have few materials or systems beside the structure, the embodied energy of the structure comprises a majority of the environmental impacts during its life-cycle. By selecting existing parking garages built over the last 10 years of similar height and in the same seismic zone, the design loads, column lengths and structural layouts are similar. This consistency makes more accurate comparisons between structural systems possible. Using material take-offs of three existing parking structures with one-way spans, one pre-cast concrete, one post-tensioned concrete and one cellular steel, this study shows that there is little difference in the normalized embodied energy of structural systems used for parking garages if steel with high-recycled content is used. The most important step architects and engineers can take to reduce the embodied energy of a parking garage structure is to specify steel products with a high recycled content, specifically reinforcing bars and structural sections.