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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Q3b: Reducing Operational Energy with Structural Innovation

As designers look to make buildings more sustainable, the comparison between operational and embodied energy highlights the importance of honing in on the operations of the buildings as it holds the larger energy footprint. 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?”

Considerations for participation of the structure in operational benefits are two-fold.  The first consideration is synergies between the structural system and the operations of buildings. “Synergies refer to the acting together of building parts or systems to the benefit of the building as a whole.” (Kang 2006)  The second consideration is for the unique detailing required for innovative systems that operate the building.

Most of the examples below address the first strategy to integrate structural elements into the mechanical system, providing savings in cooling and heating energy. Primary components of operational energy that are tracked are electricity for lighting and energy used for heating, ventilation, and cooling. The last example addresses the second strategy using special structural considerations for miscellaneous architectural and MEP strategies that reduce operational energy savings.

Structure and Heating/Cooling

Exposed concrete ceilings and walls can provide adequate thermal mass to support natural ventilation, also known as “nighttime cooling” (Kang 2006) One example of this is the San Francisco Federal Building. This strategy exposes underside of suspended concrete floor slabs to utilize their thermal mass to absorb heat in the daytime and passively cool the indoor space.  Then at night the concrete purges the stored heat into the cooler air flowing across the slab between automated window openings near the height of the slabs, as depicted in Figure 2b.

For the SF Federal Building, this strategy enabled elimination of A/C on the top 13 floors of the 18 floor office building.  HVAC ordinarily accounts for about 28% of operational energy use in commercial buildings using standard air conditioning and designed with good practice to meet energy standards. Thus, integrating the structure with the energy saving strategies in this building “... does more to reduce lifetime energy use than most other measures that only target embodied energy.” (Kestner 2010) 

Figure 3b-2: Section of raised perimeter bay for increased day lighting.  Also indicated by the arrows is the path of airflow for night-time cooling utilizing thermal mass of the slab.  Credit: Morphosis architects

Another example of this strategy from One Tooley Street in London, UK is described in Figure 3-3b.

Figure 3b-3: One Tooley St uses the hollow cores of columns as air ducts and thermal mass of the concrete to passively cool the air as is it drawn down and distributed to plenum space between the slabs and access floor before entering the occupied space.

Only a few studies have attempted to quantify the savings due to integration of structural elements into the heating and cooling system.  The energy modeling for San Francisco Federal Building showed an energy cost savings of approximately 20%.  Perez predicts a 25% savings for a naturally ventilated office building in New Zealand compared to an office building with typical HVAC. (Perez 2008)  In contrast, an MIT study shows up to 6% reduction in operational energy over the life of building can be achieved through a thermal mass strategy for commercial buildings. (Oschendorf 2010) This may be due to very different climates in New Zealand and the studied regions of the US.

For residential construction, the same MIT study shows that the average increase in insulating value and decrease in thermal bridging from the insulated concrete forms (ICF) compared to a wood-frame house resulted in 2-10% carbon savings over a home’s 100 year life,  This was due to reduced electricity and gas usage, but was highly dependent on climate and tightness of construction.  In energy terms, the ICF lowered operational energy by 4.7-8%.  This study further found that up to 20% operational savings can be achieved by using ICFs with even thicker insulation panels, increasing air tightness, and using thinner concrete walls. (Oschendorf 2010)  

Foundations and Heating/Cooling

Geothermal cooling and heating can be provided through conduit in foundation piles. The new GSA building in Seattle will be using 135 of its 205 structural piles as “’energy piles,’ meaning they have been combined with a geothermal system to provide heating and cooling. This will be one of the first projects in the region to combine both systems. The geothermal energy piles have been designed to provide 100% of the heating and cooling needs of the building. In combination with natural ventilation, the extremely low energy needs of this project is expected to earn it an ENERYG STAR score of 100, meaning it will be in the top 1% of comparable buildings in energy performance. (Zemtseff 2011)

Structure and Lighting

The SF Federal Building from above also provides examples of integration for reduced lighting loads. The design team raised the ceiling in the exterior bay and set column lines off the façade first bay to increase daylight penetration to the interior.  The structural engineer also intentional chose to use slag in the slab mix to create a whiter and more reflective exposed interior surface. (Kestner 2010)  The upturned beams already provided for an unobstructed soffit for the natural ventilation strategy also removed daylight obstructions. Increasing daylight penetration could have the effect of decreasing electricity required for lighting, since studies show lighting can be as much as 40% of the total operating energy consumption. (Perez 2008).

The combination of daylighting and views, reduced operational energy, and use of recycled content in the cement earned the SF Federal Building 10 points under the LEED rating system.  Five of these points were from energy reductions enabled by structure, exemplifying how structural engineers can have a greater role to play in green building design if they expand beyond their traditional scope and approach the task with an understanding of whole-building and life-cycle building performance.  (Ratchye 2009)

Miscellaneous Strategies

Miscellaneous strategies requiring structural coordination include sunshades and light shelves, solar arrays / photovoltaic (PV), radiant heating, and green roofs. These elements are often all used in combination to get to net zero energy buildings, such as those meeting the Living Building Challenge, the most stringent green building certification currently in existence.

For more detail on structural considerations of these systems, go here. 

Another Look at Embodied Versus Operational Comparison

When thinking specifically in terms of global warming potential, designers should keep in mind that the greenhouse gas emissions that arise from construction materials, in other words, the embodied carbon of our buildings, enters the atmosphere well before the operational carbon emissions.  Thus, although they seem dwarfed by operational carbon over the life of the building, reduction in embodied impacts carries its own importance due to the benefits of acting sooner rather than later to address climate change (Webster 2012).  Thus, these reductions should be pursued concurrently with reduction in operational impacts. (See Q2 on the effectiveness of strategies aimed at reducing embodied impacts and Q8 for the difference between embodied energy, carbon, and other environmental impact metrics.)

Lastly, the proportions of embodied vs. operational impacts generally assume a 50 year or more building life.  The proportions would change drastically if this building life is shortened or extended.  (See Q9 for the environmental impacts of disasters and Q2 for LCA studies on ways that extending the life of structure reduces embodied impacts.) 

Author: Rebecca Jones ; Contributors: Kate Simonen, Frances Yang, Steve Buonopane

References

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

Gartner, Mikael (2008). “Structural Implications of Green Roofs, Terraces, and Walls,” SEAOC 2008 Convention Proceedings

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.

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

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.

Webster et al. (2012) Structure and Carbon: How Materials Affect the Climate written by the Carbon Working Group of the Structural Engineering Institute’s Sustainability Committee, ASCE.

Zemtseff, Katie.(2011) “$72M HQ for corps pushes the green building envelope,” Seattle Daily Journal of Commerce, June 27.

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

A rule of thumb often stated within the building design community is that the structure accounts for approximately 50% of a building's life cycle embodied energy.  

Author: Terry McDonnell; Contributors: Frances Yang, Kate Simonen, Rebecca Jones

This article examines three main scenarios for the amount of embodied energy due to structure compared with the entire building;

1. Structural Embodied Energy During Construction
2. Structural Embodied Energy Over Building’s Single Lifespan
3. Structural Embodied Energy Over Building’s Rebuilt or Repaired Lifespan

Structural Embodied Energy @ Initial Construction

There are numerous studies of structural embodied energy during the initial construction phase.  The following is a brief summary of those studies:

• In the embodied energy study by Kofoworola and Gheewala shows that the concrete and steel components of a 38 story reinforced concrete office tower in Thailand can be as high as 71% of embodied energy of a building through construction. (Kofoworola 2007)
• In the embodied energy study by Treloar et al shows that the concrete and steel components of a 15 story steel office tower in Australia can be as high as 90% of embodied energy of a building through construction. (Treloar 2001)
• The international building consulting firm Arup studied multiple pieces of literature and their own building designs in order to gather a data set of embodied energy that was bounded by “cradle to site”.  Calculations included all impacts of the extraction of the raw material, factory production and delivery to site.  Their analysis shows two interesting points;

           1) That the type of structure is not changing the significance of structural contribution to
           embodied energy.  Figure 4.1 shows a 50%-60% contribution independent of building type
           (Arup 2010).
           2) That the structural system may vary the contribution of embodied energy much more.
           Figure 4.2 shows that the structural system selected produces a range between 30%-60% of
           structural embodied energy compared to the entire building (Arup 2010). 

Figure 4-1:  Prepared by Arup (Kaethner 2012) Shows embodied energy vs. type of building use.

Figure 4-2, Prepared by Arup (Kaethner 2012), shows embodied energy vs. type of structural system.

Structural Embodied Energy Over Building’s Single Lifespan

Going further, one can look at the percent of embodied energy (or sometimes embodied carbon) of the structural materials throughout the entire life span of a building, including all operational impacts.  In most cases this will reduce the structural related percent vs. the total, especially during very long life spans.  Most building designs are benchmarked for 40-50 year life spans, but the exact amount is often determined during conversations between the building Owner and design team.   The following are studies and analyses performed in order to determine this value:

• An oft-cited study by R. Cole and P. Kernan based on the Canadian construction industry estimates that a 50,000 sft, 3 story office building using steel, concrete, and timber structural systems still produces a 25% to 33% range of the building’s total embodied energy. (Cole 1996)
• The carbon consulting firm dcarbon8, in a more recent case study conducted in 2007, calculated the total cradle-to-grave embodied carbon emissions (which are indicative of embodied energy) attributable to a warehouse building’s structure to be 57% of the total. (Werner 2012)
•  A similar study performed by Fernandez, N. Perez goes in depth to analyze an actual 55,000 sft, 6 story office building located in Christchurch, New Zealand.  Using the actual concrete design, and alternates of steel and timber, the author concludes that the life cycle structural embodied energy accounts for 30% of the concrete and timber designs, and 44% within the steel design alternate. (Fernandez 2008)
• Taking an actual four story, 80,000 sft office building located in Chicago, IL, a group of building designers studied four different structural systems using two separate material quantity calculations within the ATHENA EcoCalculator for the purpose of determining the structural material embodied energy.  By adding in non-structural components and using TRACE 2000 energy model each design assuming a 50 year life span.  The total embodied energy of the structure for this low rise building was determined to be between 6%-10% of the total cradle to grave energy.  This is lower than expected but remains a significant amount. (Stek 2012)

Structural Embodied Energy Over Building’s Rebuilt or Repaired Lifespan

Common practice in conducting an environmental life-cycle assessment (LCA) on a building includes a consideration of the impacts stemming from first construction of a building through its life span.   But in special circumstances, such as buildings in earthquake prone locations, the repair of damage or demolition and re-construction (sometimes referred to as multiple lives) of a building may significantly affect the overall structural embodied energy.  A comprehensive LCA includes impacts related to demolition but rarely includes the potential additional environmental impacts that could stem from repairing or demolishing and rebuilding a building after a natural disaster included in an LCA, if ever at all.  Methods are being developed to include seismic performance in LCA analysis.

One such study by Degenkolb attempts to include the affect of seismic systems within active earthquake locations in order to achieve a more accurate sense of a building’s full life-cycle impacts.  In this paper, an LCA study using the EnvISA methodology is performed.  This analysis measures common structural unit costs rather than a straight LCA inventory.  Anticipated seismic losses are then paired with the cost LCA database which forms the basis of EnvISA.  Their conclusions show that a more robust structural system can provide approximately 18% - 25% depending on the structural system selected, and how well that system can limit damage to the non-structural building elements over a 50 year life span. (Comber 2012)

The difficulty in all of the seismic evaluating studies is that they are to date based upon proprietary software.  Without knowing more about the system, the data sets, assumptions, inventories, boundary conditions, and actual measuring algorithm are different.

This brief paper shows that the often quoted rule of thumb that the structural materials are 20% of the total embodied energy is in reality subject to a lot of fluctuation.    Most cases presented show values higher than 20%.  As modern energy codes continue to reduce the operational energy, the percent of total embodied energy due to the structure will only increase.  In addition, the high rise building type has a much greater percentage of structural embodied energy out of the total.  Studies show that in buildings over 200 feet high (often times much more) the embodied energy can be 4 times or more of the embodied energy percentage of a shorter sized building. (Kaethner 2012)

Therefore it is relevant and altogether prudent to continue measuring and trying to decrease the environmental impact of structural materials.

References

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

Cole R., Kernan P. (1996) “Life-Cycle Energy Use in Office Buildings”, Buildings and Environment, 31 (4): 307-317

Comber, Poland, & Sinclair (2012) “Environmental Impact Seismic Assessment: Application of Performance-Based Earthquake Engineering Methodologies to Optimize Environmental Performance”, Victoria University School of Architecture, Wellington, New Zealand

dcarbon8 (2010) “Footprint Measurement and Reduction Study for Development Securities”

Fernandez, N. Perez (2008) “The Influence of Construction Materials on Life-cycle Energy Use and Carbon Dioxide Emissions of Medium Size Commercial Buildings”, Victoria University School of Architecture, Wellington, New Zealand

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

Kaether, Burridge (2012) “Embodied CO2 of Structural Frames”, The Structural Engineer.

Kofoworola, Gheewala (July 2007), “Environmental life cycle assessment of a commercial office building in Thailand”

Stek, DeLong, McDonnell, Rodriguez (2012) “Life Cycle Assessment Using Athena® Impact Estimator on Buildings: A Case Study”, SEI Congress 2012.

TRELOAR, G. J., FAY, R., LLOZOR, B. & LOVE, P. E. D. (2001a). Building Materials Selection: Greenhouse Strategies for Built Facilities. Facilities, Vol. 19, No. 3/4, pp,139 – 149

Werner, Burns (2012) “Qualification and Optimization of Structural Embodied Energy and Carbon”, SEI Congress 2012.

Other papers to research

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.

CTBUH Journal, Tall Buildings and Embodied Energy, 2009 Issue III

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