Life cycle assessment and structural engineering practice

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    "Don’t drive your sport utility vehicle to the farmers’ market, buy one food item, and drive home again … even if you are using reusable bags." This admonition in a New York Times article (Dec. 10, 2007) on the local food movement typifies the big picture of sustainability.

    What does this quote have to do with the practice of structural engineering? It’s all about putting the details in context and seeing the big picture—about seeing the proverbial "forest for the trees."

    The myriad details of sustainable design can be dizzying at times—materials certification, recycled content, local materials, rapidly renewable materials, and construction waste management, to name a few. Life cycle assessment (LCA) provides a framework for all of those details and helps building design professionals and their clients see the big picture of sustainability.

    LCA is a method of measuring a structure’s various environmental impacts from its creation with the extraction of raw resources used to create building materials to its demolition or end-of-life. LCA was originally developed to measure the impacts of processes and products, but it is now being applied in the more complex realm of building design, construction, and operation.

    Structural engineers are increasingly expected to consider the environmental impacts of their designs alongside the traditional performance measures of strength, serviceability, and cost. The key to a successful sustainable building design is a highly integrated design process in which all of the professional disciplines interact closely throughout design and construction. Therefore, structural engineers should expect to participate in the LCA process, or even to perform limited-scope LCAs of the building structure, its materials, and assemblies.

    LCA considers four phases in the building life cycle: initial construction, building operation, recurring maintenance and renovation, and end-of-life. Energy use during operation is one of the most prominent environmental impacts and one that dominates many green building decisions. As structural engineers, we often view our work as primarily confined to initial construction, not operations, and therefore we view our role in reducing environmental impacts as minimal.

    Sustainable design in general, and LCA in particular, do not let us get away with that kind of compartmentalized reasoning. Buildings have other significant environmental impacts, such as solid waste generation, water pollution, and natural resource use. Previous LCAs have demonstrated that these impacts are strongly influenced by the construction and demolition of structures, and thus are well within the realm of structural engineering practice.

    Structural engineers can make design decisions during initial construction that will reap substantial sustainability benefits later in the building’s life, creating value for the building owner. Designing the structure to consider durability, maintenance requirements, and future deconstruction and adaptability are some examples of such sustainable design techniques.

    The methodology for LCA has been set forth in the International Organization for Standardization (ISO) Standards 14040 and 14044. An LCA has four phases: goal and scope definition, inventory analysis, impact assessment, and interpretation. As with any complex engineering analysis, LCA is an iterative process and each phase will need to be revisited and revised.

    • Goal and scope definition specifies the fundamental parameters of the LCA, such as the intended use and audience of the study, the boundaries of the system, the quality and uncertainty in the data, and any other necessary assumptions.
    • Inventory analysis describes and quantifies all of the energy and material flows associated with the building throughout its life cycle.
    • Impact assessment defines the environmental indicators such as energy use, solid waste, global warming potential, air and water pollution, or natural resource use. It associates each item of the inventory with one or more impact categories.
    • Interpretation analyzes the results and translates the results into practical terms that can be used to improve the system in question.

    The application of LCA to buildings is still evolving, and the use of whole-building, comprehensive LCA is not yet a routine design tool. ISO Technical Specification 21931 and ASTM Standards E2432 and E2114 are the first steps in defining terminology, principles, and assessment methods for the sustainability of buildings. Present efforts in building-specific LCA are aimed toward standardization and verification of LCA data for building products and assemblies. ISO Standard 21930 and ASTM Standards E2129 and E1991 provide the basis for data collection and LCA of building products.

    The National Renewable Energy Laboratory maintains the Life Cycle Inventory Database (www.nrel.gov/lci/), which includes building and construction products. Both the LEED and Green Globes rating systems will soon incorporate a limited form of LCA at the building assembly level, based on customized versions of the ATHENA EcoCalculator for Assemblies (www.athenasmi.org/tools/ecoCalculator/).

    As building-specific LCA methodology becomes standardized, additional data becomes available, and design tools are developed, structural engineers will be expected to participate fully in LCA as a part of sustainable design teams. Structural engineers who become knowledgeable in this area and can actively participate in the LCA process will add value to sustainable building design teams.

    Stephen Buonopane, Ph.D., P.E., is an assistant professor in the Department of Civil and Environmental Engineering at Bucknell University in Lewisburg, Pa. He is a member of SEI’s Sustainability Committee. He can be reached at stephen.buonopane@bucknell.edu. The SEI Sustainability Committee website is www.seinstitute.org/committees/sustainable.cfm.