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 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.