Heron’s Head EcoCenter: Part 2 – Structural
As a follow-up to a recent post on Heron’s Head EcoCenter, we caught up with Alex Rood of Fulcrum Structural Engineering to discuss his contributions to the project. For those who have not read our recent post on the project, Heron’s Head EcoCenter is San Francisco’s first off-the-grid educational facility and laboratory for sustainable design sponsored by the non-profit organization Literacy for Environmental Justice. A project 10-years in making, the EcoCenter was finally completed in 2010, and incorporates many innovative sustainable design features including solar panels (both PV and solar thermal), living roof, rainwater harvesting, greywater reuse, living machine on-site wastewater treatment, SIP panels, super green concrete mix, recycled building materials, passive design, and the list goes on.
Site/soil: Located on a former industrial landfill in Hunters Point neighborhood of San Francisco, Alex’s first challenge was to design the foundation system that could sit atop the soil with extremely poor bearing capacity. To overcome the less than favorable soil condition, he decided to utilize 10” mat slab but he also had to incorporate a series of troughs for the indoor constructed wetland into the slab/foundation design. The challenge did not end there. The soil condition was so poor that he was told to anticipate as much as 10” to 12” of settlement over time. This meant utilities that would typically be laid below the slab now had to be routed within concrete troughs formed into the slab until they exited the structure above grade to make service connections. To make matters worse, he found out that below the site is a clay layer of landfill cap designed to remediate the contaminated soil, but the exact depth of this cap layer could not be determined. He had to proceed with caution to ensure that the weight of the structure would not cause the cap layer to fracture. In addition to solving all of these engineering challenges, it was his job to provide a cost effective foundation design to fit the tight budget of the project.
Concrete: Almost every modern structure in the world uses concrete to varying extent. While its longevity makes it a good building material, the environmental footprint of cement production is something Alex wanted to tackle on this project. According to Alex, experts estimate that cement production contributes to about 7% of carbon dioxide emission from human source. He started by specifying a green concrete mix containing 50% slag for cement replacement but once the job broke ground, he collaborated with the concrete sub-contractor and the supplier, Bode Concrete, to push this green concrete mix further, ultimately ending up with 80% slag and 100% recycled course aggregate. In the end, not only was he able to use the greenest concrete mix he, or Bode Concrete, had ever heard of, but the builder was able to provide concrete that was superior to the original specification in many ways. It ended up exceeding the compressive strength requirement by two fold, the concrete showed no cracks and will provide superior protection against rebar corrosion, all thanks to its high slag content.
Framing: Once out of the ground, the remainder of the structure was designed using Structural Insulated Panels (SIP). Having worked on many other SIPs jobs with the project architect, Toby Long, Alex knew that Toby would want to use SIPs for the project for many of its environmental preferable attributes. There are always challenges in designing SIPs structures in seismically active zones, especially when the building is not a simple box but he was able to use his extensive experience with SIPs to help Toby realize his design. Being the first SIPs project for the builder and the framing crew, he worked very closely with the builder and was often asked to offer creative solutions to troubleshoot issues arising in the field. Thanks to the countless hours of research by the LEJ project manager Laurie Schoeman, SIPs panels made with FSC-certified OSB panels were sourced. All framing lumber used in the job to amend the SIPs system was also all FSC-certified.
We asked Alex what was the best part of the project for him.
It was the collaborative effort with the concrete sub-contractor, Gerald Creed of OSM Co. and the local concrete producer, Bill Garland of Bode Concrete. In fact, it was Gerald who initiated the idea of exploring an even greener concrete mix than the 50% cement replacement originally specified. Bill had worked with a concrete mix with 75% cement replacement, and he has also used crushed returned course concrete aggregate in non-structural flatworks. Not only did we decide to combine both of these ideas, but we also decided to use the mix for the structural slab/foundation, while increasing the slag content by additional 5%.
Alex believes it was the collective knowledge and will to do the right thing for the environment that allowed them to take the leap of faith to push the boundary of the green concrete mix. Everyone went above and beyond their call of duty to come up with the greenest concrete mix they have ever worked with.