How to optimize the environmental performance of structural materials

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    Q: In your last article you discussed how recycled content is not necessarily the best measure of the environmental performance of structural materials. How can I, as a structural engineer, make design choices that better optimize overall environmental performance of my materials selection?

    A: Despite the popularity of specifying high recycled content in our concrete and steel as a means of achieving credits in LEED, this strategy only addresses a sliver of the wide array of environmental burdens. Other notable impacts are resource depletion, water pollution, greenhouse gas emissions, ozone depletion, and contribution to smog and other air pollutants. Recycled content only directly relates to resource depletion and indirectly affects other categories with variability depending on the processes required to extract, transport, and manufacture the material. The most complete method available for evaluating the multitude of environmental burdens is Life-Cycle Assessment (LCA).

    The U.S. Green Building Council (USGBC) also recognizes the need for a more holistic evaluation of the environmental impacts of materials and has introduced a pilot credit — Life-Cycle Assessment of Building Assemblies and Materials — into its LEED rating system. The credit requires use of the Athena EcoCalculator, a free, easy-to-use spreadsheet tool created by Athena Sustainable Materials Institute. For complete details on the pilot credit, visit http://www.usgbc.org/ShowFile.aspx?DocumentID=6350

    During the pilot period, one credit in the “Innovation in Design” category will be awarded for evaluation of structural and envelope assemblies using the EcoCalculator. The credit structure is set up to eventually allow for obtaining up to five points plus up to two more innovation points for exemplary performance. Given the large proportion that structure represents in final EcoCalculator results, pursuing these credits will potentially entice clients and design teams to think earlier about structural material design decisions. Thus, it is important that structural engineers become familiar with the LCA credit and the Athena EcoCalculator tool so that they are not forced to make awkward material choices based on how the materials impact a LEED score. For instance, currently the cement content in concrete assemblies cannot be adjusted; this significantly affects the associated environmental impact. If, on the other hand, large amounts of cement are replaced with carbon neutral alternatives, a concrete solution might make more sense than a timber structure in many situations. But without the flexibility to designate cement content, concrete is severely penalized in the LCA score and consequently timber consistently looks much better.

    The EcoCalculator allows users to input the square footage of the built-in assembly types used for the structure and envelope of a building: framing, floors, walls, roof, and windows. In version 3.3, the user chooses one of 15 geographical regions and two building height categories that best match the building to be evaluated. Results are reported in a summary of eight impact categories, which the user then needs to input to a LEED template that applies weightings to the categories and comes up with a final LEED LCA score. In the future compliance path, this score would equate to a number of LEED points, but for now USGBC will not make any judgments on the score; one only needs to participate in the pilot program to receive the one Innovation in Design credit.

    The elegance of the EcoCalculator is in how simple it has made a calculation that is traditionally lengthy and complex. There are essentially two steps in LCA-based embodied environmental impact calculations: deriving material quantities, and assigning environmental impact data. The latter effort, gathering the environmental data for the assemblies represented, is the vastly more intricate part of such LCA tools, and Athena has already built the results of that work into its programs.

    Athena developers are not structural engineers, and are open to our advice on refining the material quantity derivations embedded in the tool. The more structural engineers who try the LCA credit while in pilot period and engage with Athena and the USGBC regarding the EcoCalculator, the more quickly it can be revised and improved.

    The LCA credit is an excellent step towards capturing a more complete picture of the environmental impacts of our material choices, and it will potentially integrate structural engineers into the design process at the beginning. However, it needs some improvement to ensure structural material decisions are based on information that accurately reflects environmental impact and structural feasibility. The time to react is now.

    Frances Yang, P.E., LEED AP, specializes in materials sustainability for the Structures and the Energy and Resources disciplines of Arup in San Francisco. She is a member the SEI Sustainability Committee and active on the LCA working group. Any comments can be sent to frances.yang@arup.com. The committee’s website is www.seisustainability.org