FAQs about Life Cycle Assessment for the structural engineer

June 2014 » Feature Articles » Sustainable Design
Rebecca Jones

 

How does a building’s embodied impact compare with its operational impact and what proportion of that impact is structural components? How much of the total embodied impact comes from a structure? These are two of the most frequently asked questions about Life Cycle Assessment (LCA) coming from the structural engineering community. By asking these questions, we can begin to better understand how the work of the structural engineer fits into sustainable design in the context of the quantifiable system of LCA. LCA considers the full life cycle of a building, including the embodied environmental impacts (e.g., raw material extraction and construction) and operating environmental impacts of the building during its lifetime (e.g. heating and cooling). 

To determine how structural components contribute to LCA, one should determine the percentage of lifetime impact — from embodied to operational impacts. Structural elements are only one of the many components that contribute to embodied impacts, along with all other materials used, including the architectural finishes. Operational impact is often found to be the largest contributor to LCA results. One of the top 10 frequently asked questions presents test cases where structural detailing can contribute to minimizing the operational impacts of a building during its lifetime.  

Here are a few more of the top 10 frequently asked questions:

Q3a: How do the embodied impacts compare with operational?

Figure 1 shows the typical 20/80 proportion of embodied versus operational impacts of a building for a 60-year lifespan. In the near future, the most effective strategies for reducing building impacts to the environment will be to decrease the operational energy required for a building. However, as the environmental impacts from operational energy decrease, the influence of the structure will increase. As this trend unfolds, savings to the impacts in the structure will have an increasing proportionate contribution to the impacts of the project.

Q4: How much of total embodied impact comes from structure?

Of the 20 percent embodied impacts of the total environmental impact, the structure accounts for approximately 20 to 60 percent of the total embodied impact. 

Q3b: What are some structural innovations that help reduce the operational energy of a building?

For the near future, a more effective way for the structure to participate in decreasing the environmental impact of buildings is to contribute to the reduction of operational impacts. Listed below from a survey of structural engineers are some built examples of structures participating in decreasing the operational impact. See the SEI Sustainability Committee blog (link at the end of this article) for more detailed descriptions of the examples. (The list is currently short and we would love to hear about more. Please comment on the blog to share your examples.) 

  • Use concrete thermal mass to facilitate nighttime low temperatures to cool concrete and harness cooler temperatures during the day.
  • Raise beams into a raised floor system to facilitate air flow at the soffit.
  • Raise the perimeter beam within dimension of a raised floor to increase daylighting.
  • Incorporate geothermal systems into deep foundation elements.

These examples reinforce the idea that synergy is an important component of sustainability — the more you can leverage one element of the design to positively participate in more than one building function and behavior, the more potential there is to minimize building environmental impacts.

These examples also highlight that there are different solutions for different building scales and locations. It is important to be knowledgeable about your specific building type, particularly in regard to location. Some strategies are only applicable to mid-size buildings in moderate climates. But what about a tall building in a climate with large temperature swings throughout the year? 

There is always room for improvement in reducing embodied and operational impacts of the building’s life cycle. Completing a LCA can help one to:

  • Understand which components can be targeted to most effectively reduce environmental impacts.
  • Quantify environmental impacts to ease communication and inspire owners to consider the environment in their decision matrix.

The questions provided herein are part of a series of Top 10 FAQs completed by the LCA Working Group of the SEI Sustainability Committee. Please see the full SEI Sustainability Committee blog post; these numbers can vary depending on height, structural system, location that affects seismicity, and temperature ranges and energy codes that the building’s MEP systems are designed to meet.

For more, see the first article in this series from the April 2014 Civil + Structural Engineer magazine. Subsequent articles will follow to summarize other questions from the Top 10 FAQs. Join the conversation about structures and sustainability on the blog at http://structureandsustainability.blogspot.com/2012/03/top-10-questions-ses-have-about.html

Rebecca Jones is a senior project engineer at Thornton Tomasetti in New York City and currently a member of the ASCE SEI sustainability committee. She can be reached at rjones@thorntontomasetti.com


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