Natural Ventilation, Cooling and Lighting

Unobstructed soffit was required on this project requiring a raised floor to cover the upturned beams and the space created below the floor facilitated IT distribution and sprinklers. The thermal mass of concrete and properties of normal weight concrete were critical to effectiveness of the natural ventilation on this project. Locations good for this application are the “west coast and desert regions of the United States, where the climate has daytime to nighttime temperature change of about 30 degrees, mechanical engineers can design cooling systems that take advantage of radiating cool from the structure….The efficacy of this method of cooling requires that the structural thermal mass elements must be directly exposed to the building occupants so that the structure can absorb heat gained during the day, and radiate (or release) heat during the night. Ceilings, floors, and walls that are covered with finishes will not be as effective in cooling a building….Concrete buildings naturally lend themselves to this strategy. (Kestner 2010) 

Concrete fill on steel deck used in steel buildings can also provide thermal mass, but if the underside of the deck is fireproofed, its ability to transfer heat will be greatly reduced, thus reducing the effectiveness....Structural CMU walls, if exposed, can provide sufficient thermal mass for come buildings. The architect will usually require units with architectural finishes, and will require precise lay-up and high quality workmanship.” (Kang 2006) 

Steel in buildings lend themselves to other another natural ventilation strategy called stack effect.  Stack effect requires pathway up the height of the building “unobstructed of walls or other large elements” and can benefit from using a “material that heats up quickly when exposed to sunlight…along exterior towers to draw hot air towards it.”  (Kang 2006)  Such an example is the Porticullis House which contains tall steel towers.

Structural Considerations for Innovative Systems

Sunshades and light shelves require support typically integrated with curtain wall systems and the structural engineer may provide design loads for the design build window supplier and then review design and calculations. Panels are about 5psf typically with frequent penetrations to roofing for armatures to resist uplift; alternatively, armatures could be designed to engage ballast to resist uplift increasing system weight to 20 psf to 25psf. And care in checking seismic forces should be taken when adding a system to an existing building. 

For radiant heating the primary concern is the coil,  One option is to have a topping slab to encase the coil adding weight to the system.  Encasing it in the structural depth of the slab taking lots of coordination with structural requirements of the slab especially at high shear and bending locations. Additionally coils limit future flexibility because coils are difficult to locate when needing to avoid damaging a coil when making a new slab penetration. 

Green roofs present heavier loads on the roof and consideration of drainage of water needs to be considered so as water cannot accumulate more than anticipated so as not to overload the roof. (Kang 2006, Gartner 2008)

For more design recommendations on implementation of these strategies, see the SEI Sustainability Committee’s book Sustainability Guidelines for the Structural Engineer and the SEAONC white paper “Structural Engineering Strategies Towards Sustainable Design,” full citations in the references section of Q2.

References

Kang, Grace and Alan Kren, (2006). “Structural Engineering Strategies Towards Sustainable Design,” SEAOC Proceedings 2006. Pp.473-490.

Kestner, Dirk, Jennifer Goupil, and Emily Lorenz, (2010). Sustainability Guidelines for the Structural Engineer,  Sponsored by Sustainability Committee of the Structural Engineering Institute of ASCE, Reston, VA: ASCE, 978-0-7844-1119-3.

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Q3a: Operational vs. Embodied Energy

How do operational impacts compare to embodied impacts on the carbon footprint of a building? 

Authors: Frances Yang, Rebecca Jones ; Contributors: Megan Stringer, Matthew Comber

The approximate average across several case studies (Perez, 2008; Ramesh, 2010; Junilla, 2006) is 20% of the total life-cycle energy, as often cited in the building practice.  However, as recorded in Figure 1, the results amongst these studies actually ranged from 5% to near 30%. 

Figure 3a-1: Ranges of typical embodied and operational energy proportions found in literature

This range resulted from differences in what was included or excluded, the building life assumed, and the operational performance.  Figure 2 shows the typical trend of both embodied and operational energy over a typical 60 year building life.  It illustrates that arriving at a 20/80 proportion of embodied to operational is very sensitive to building life.  (For more info on what structural engineers can do to account for building life in LCA, see Q4 and Q9.)

Figure 3a-2: Typical breakdown of embodied and operational impacts over 60 year building life

The studies referenced in Figure 1 concluded that operational performance was most dictated by climate and occupant behavior.  The resulting proportions of operational to embodied included intentional reductions in operational impacts from low-energy designs. (Perez, 2008; Ramesh, 2010; Junilla, 2006)  The fourth cited study of Figure 1 performed a particularly rigorous analysis to test this proportion. Basbagill ran a conceptual building design through all possible permutations of preset values within 31 variables that defined shape, massing, materials, systems, and dimensions. Figure 3 shows a random sampling of 5000 of these scenarios, from which a mean ratio of embodied to total life-cycle impacts of 18.66% is extracted (Basbagill, 2013).

Figure 3a-3: Embodied Impact as % Total, the remainder being the energy to operate the buildings. (Credit: Basbagill et al, used with permission)

The significance of Basbagill’s study is that it reveals even larger scatter that departs from the often cited 20/80 ratio. It illustrates that design decisions can affect the operational and embodied outcomes significantly, such that a singular focus on the operational portion may be too narrow a view.  At the same time, the average of the permutations in Basbagill’s study reinforces the expectation that most designs will land near the 20/80 split.

If operational impacts can be expected to be of such great significance, structural engineers should be aware of opportunities for structural materials (the embodied impacts) to participate beneficially to reduce operational impacts.  Therefore, perhaps a more meaningful question to ask is “What active and passive technologies in low operational energy buildings require special structural solutions?”

References

Basbagill, J., Lepech, M. (2013). “Characterizing Life Cycle Impacts of Conceptual Building Designs”. Energy and Buildings (in preparation)

Junilla, S., Horvath, A., Guggemos, A. (2006). “Life-Cycle Assessment of Office Buildings in Europe and the United States,” Journal of Infrastructure Systems.  ASCE Press. March, p. 10-17.

Perez Fernandez, Nicolas, (2008). “The influence of construction materials on life-cycle energy use and carbon dioxide emissions of medium size commercial buildings,” School of Architecture, Victoria University of Wellington, July.

Ramesh, T., Prakasha, R., Shuklab, K. K. (2010) “Life cycle energy analysis of buildings: An overview,” Energy and Buildings, 42, p.1592–1600

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