The role of spontaneous construction for post-disaster housing

Abstract

The structure of a typical office building contributes roughly one-quarter to onethird 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. the urban population in the United States is estimated to increase by over twenty percent by 2030 (United Nations 2006), parking garages will contribute significantly the environmental impact of the built environment due to the high demand for automotive storage in both urban and suburban areas (Chester et al. 2010). 1.2 Embodied energy and parking garages This paper uses embodied energy as a sustainability metric to compare structural systems as it can also serve as a good indicator of relative raw material depletion, greenhouse gas emissions, and general degradation of the natural environment when comparing alternatives (Ashley, 2009). 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 “cradleto-gate” as opposed to the “cradle-to-grave” method of calculating embodied energy that would also include primary energy expended on the transportation, construction, maintenance and disposal of building materials. As transportation, construction methods, building maintenance, lifecycle, and demolition can vary greatly, this paper focuses on the more consistent and quantifiable components of the embodied energy of structural materials. Specifically, there is no connection to the service life of a building and the material used for its structure (O’Connor 2004). A comparison of North American and European office buildings indicated that the assumed service life of these buildings was not matching the actual service life (Junilla et al. 2006). A life cycle analysis (LCA) study of two theoretical, five-story office buildings, one with a steel frame and concrete slabs and the other with a cast-in-place concrete structural system, showed similar energy use during construction, operation and end-of-life (Guggemos and Horvath 2005). However, the energy used in these steel and concrete structures differed most significantly in the “cradle-to-gate” manufacture of the building materials. Instead of office buildings typical of theoretical and case study based whole building LCA studies, this paper uses parking garages to avoid the variance found in and across office building studies. 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 with uniform grids to the design of a new building due to the unique requirements of each new site and program that making using a similar standardized grid impossible. Due to a wide range of assumptions, it is even difficult to compare one theoretical office building LCA study to another (Robertson, et al. 2012). 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/ft, can still vary by up to 50% depending on the building (Suzuki & Oka 1998). Consequently, using case studies of office buildings to compare alternative structural systems has limited accuracy. As parking garages have predictable loads, consistent floor-to-floor heights and accommodate exactly the same program, there should be fewer variables distorting comparisons between different structural systems. This paper uses real, built parking garages rather than a theoretical design to study the effects of irregularities that develop due to site constraints typical of urban infill projects. One major difference between parking garage structures and those used in office buildings is that garages typically use long one-way spans to create clearances for a driving lane and a row of parking on either side. Office buildings typically use two-way concrete systems or shorter span steel bays.