Q1:Which is better, steel or concrete?

Which is better, steel or concrete?

Author: Frances Yang; Contributors: Steve Buonopane, Kate Simonen, Adam Slivers

Although many trade-sponsored studies promote one type of structural material as more sustainable than the other, the results across all concrete vs. steel studies we have seen reinforce the conclusion that the likely variation within each material is greater than the difference between materials (Weisenberger 2010, Masanet 2012, Arup 2010).

Figure 1-1: Results from literature review of 13 embodied carbon studies

This conclusion is influenced by two primary factors. First, all structures are truly composite systems of steel and concrete, with significant amounts of both concrete (foundations, fill in deck) and steel (rebar).   Second, differences in the goal, scope and functional units of various LCAs leads to greater differences in results than the difference between the concrete and steel figures of each study (Hsu 2010).  Variability across material types within a single LCA study are often less than 5%, yet variability of the same material types across multiple LCA studies are on the order of 25% to 35% (Ramesh 2010).

Figures 1-2, 1-3: Courtesy of S. Hsu at MIT

Another very important consideration in LCA is the full life-cycle.  Most studies only include cradle-to-gate or cradle-to-site impacts, meaning from extraction of the raw materials to leaving the product factory gate or completion of construction on site.  Truly holistic evaluation should include the full life-cycle.  A recent MIT study compared steel, concrete and wood frame commercial and residential structures, and quantified the carbon emission impact of building systems over their complete life cycle. This study found that the greatest differences in impact were in the operation phase of the life-cycle (Ochsendorf 2011). See Q2 and Q3 for ideas on how structural design and specifications can affect operational impacts.

So a more effective question the SE should ask is, “What can a structural engineer do, to lower the environmental footprint of the building within the structural system and material(s) selected?”  LCA can help you identify the material or system which has the most potential for effective sustainability improvements within your specific project constraints. Most often, the selection of the main structural system is based on traditional performance criteria—strength, fire resistance, durability, constructability, low vibration, noise control, aesthetics and cost—just to name a few. Sustainability criteria, including environmental impacts, are additional performance criteria that should be considered in the design process on all projects.

Material efficiency, designing for adaptability and deconstruction to enable reuse, using salvaged materials, integration of structural system with building systems to optimize energy use such as design to utilize thermal mass or to optimize daylighting potential, reducing cement where appropriate in concrete mixtures, and specifying more environmentally responsible sourcing -- structural engineers can implement all of these measures to some degree all buildings.  (See Q2 for more detail.)  As every building project has unique criteria and available resources, the best approach for SEs is to maximize these opportunities, and use them to inform materials selection, rather than the other way around.

At the same time, variability in accounting is a current problem in arriving at truly comparable results, and structural engineers need to do the following things to address this problem:

1. Be clear on the boundaries and accounting method used in LCA studies to insure final comparisons consider the full life-cycle impacts.  This first requires an understanding of the methodological differences that can impact LCA results.  See Q5 for the various accounting methods seen in life-cycle inventories and tools.
2. Focus on reducing impacts that are within their control through design and specification.  See Q2, plus Q3 for examples specific to operational energy performance, and Q9 for resilience.
3. Engage in creation of product category rules (PCRs) and environmental product declarations (EPDs).  See Q6 for more on PCRs, EPDs, and whole building LCA standardization
4. Recognize the broad range of environmental impacts related to material extraction, manufacturing, use and end of life.  See Q7 and Q8 for more information on LCA metrics.

References

Arup (2010) “Embodied Carbon Study: Study of Commercial Office, Hospital and School buildings,” The Concrete Centre, United Kingdom. 

Hsu, S. (2010) “Life Cycle Assessment of Materials and Construction in Commercial Structures: Variability and Limitations,” Massachusetts Institute of Technology, Cambridge, Massachusetts. 

Masanet, E., Stadel, A., and Gursel, P. (2012) Life-Cycle Evaluation of Concrete Building Construction as a Strategy for Sustainable Cities, SN3119, Portland Cement Association, Skokie, Illinois. 

Ochsendorf, J., et al. (2011) “Methods, Impacts, and Opportunities in the Concrete Building Life Cycle,” Massachusetts Institute of Technology, Cambridge, Massachusetts. 

Ramesh, T., Prakash, R., Shukla, K.K. (2010) “Life cycle energy analysis of buildings: An overview,” Energy and Buildings, 42 1592-1600. 

Seppo, J.; Horvath, A., and Guggemos, A. (2006) “Life-Cycle Assessment of Office Buildings in Europe and the United States,” Journal of Infrastructure Systems, March 2006 Issue. 

Weisenberger, G. (2010) “And the Winner is... A framing system’s environmental impact depends on more than just the choice of materials,” Modern Steel Construction, Aug 2010 Issue.

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