The introduction of LEEDv4 in November 2013 offers new opportunities for structural engineers to be recognized for their contributions toward sustainable buildings. Scot Horst, senior vice president of the U.S. Green Building Council (USGBC) has said, “One of the things we want to try to do is to initiate a discussion between the designer and the structural engineer in the same way that energy modeling initiated a discussion between the designer and the mechanical engineer.”
The comment referred to the newly added credit option for whole building life-cycle assessment (LCA), which is prompting many design teams to learn more about LCA. If you are one of those people, you are in luck!
The LCA Working Group of the SEI Sustainability Committee has amassed the first-ever Top 10 list of questions most commonly asked about sustainable design and LCA, and how they pertain to structural engineering. The answers, authored by our member experts, come from focused literature review of more than 20 case studies based on LCA, with current and geographically specific applicability. Specifically, the Top 10 FAQs are:
- Which is better — steel or concrete?
- What are some of the most effective things I can do as a structural engineer?
- How do embodied impacts compare to operational?
- How much of total embodied comes from structure?
- Does it matter if I use LCA data from different sources?
- How much do things seen in LEED (recycled content, local sourcing, etc.) matter?
- What’s the difference between embodied energy and embodied carbon? Can I just scale embodied carbon results to embodied energy?
- What do the other LCA metrics (besides embodied carbon and embodied energy) mean and how important are they?
- What is the environmental impact of seismic damage? How can comprehensive life-cycle thinking impact how we design for disasters?
- Which is better — retrofitting an existing energy-hogging building or building a new super-efficient building?
The answers can be found on our committee blogsite at http://structureandsustainability.blogspot.com/2012/03/top-10-questions-ses-have-about.html.
We ordered the questions roughly based on how frequently we hear them. We realize that answering some of the latter questions first will help prime our readers to better digest the answers nearer the top of the list. Thus, in this month’s article we will address Q7 and Q8, to first acquire some fluency with the metrics and methodology of LCA.
Q7: The terms “embodied energy” and “embodied carbon” are often confused or misunderstood as interchangeable. The confusion likely comes from how the building industry realizes how much operational energy use dominates resulting carbon emissions. Operational energy and carbon have a tendency to trend the same, the singular difference between them being the fuel source that provides the energy and how much carbon it emits in the process of turning it into energy. However, when it comes to materials, additional carbon sources and sinks can throw the two out of proportion. These include chemical reactions, absorption during growth of plant-based materials, and occurrences at end-of-life. Thus, in some cases, one can be a proxy for the other, but with structural materials, there are more factors that distinguish them apart. More specific examples and further detail on carbon and carbon accounting protocols appear on the blogsite.
Q8: Beside energy and carbon, other popular LCA indicators quantify the potential for acidification, eutrophication, ozone depletion, and smog formation. Along with global warming potential and non-renewable energy resources, these are the ones recognized in the LEEDv4 credit option. However, you many also run into indicators for other impacts such as eco-toxicity, human health, fossil fuel depletion, water use, and land use. The quantification of these impacts is based on characterization of the inputs (i.e. resources) and outputs (i.e. emissions) that occur over the life-cycle stages of the material or product (extraction, production, transportation, end of life, etc.), found in the life-cycle inventory, or LCI. A graphical representation of how this is done can be found on the blogsite. A few key points to remember:
- This characterization is based on scientific models representing our current understanding of consequences in the environment. As this is imperfect, the characterization carries an inherent uncertainty, so those using LCA results to compare options should consider this inherent uncertainty before concluding that one option is better than the other. Furthermore, different indicators have variable degrees of uncertainty. For example, global warming potential is relatively better understood than eco-toxicity.
- LCA indicators are just indicators and cannot be used for predicting future conditions or occurrences. Whether the fate of the inputs and outputs follows the model often depends on the local conditions, interaction with other substances that have entered the environment, and how those change over time.
- LCA is based on our current, best, scientific understanding of the fates of chemicals and substances in the environment. There are many characteristics of healthy communities and environments that cannot be captured by LCA very well, such as biodiversity, social impacts, and the younger fields within human health and toxicology.
Still, the multiple indicators of LCA are useful for realizing relative impacts and trade-offs that may occur as consequence of our choices.
More answers to the Top 10 are provided at http://structureandsustainability.blogspot.com/2012/03/top-10-questions-ses-have-about.html. In subsequent articles, the SEI Sustainability Committee will provide a brief summary of the lessons learned from LCA literature review and what it can tell structural engineers about what they can contribute to sustainable design.