MIT researchers design structures with tree forks

Researchers at the Massachusetts Institute of Technology have developed a unique strategy to use wood waste as load-bearing structural components in building structures, replacing traditional concrete and steel elements in construction.

The construction industry is one of the biggest contributors to global carbon emissions – the concrete and steel industries alone are responsible for 15% of total CO2 emissions. Thanks to its carbon sequestration abilities, wood is increasingly seen as a more environmentally friendly alternative to traditional building materials.

However, wooden replacements for traditional concrete and steel elements are made from straight sections of trees, with irregular sections such as knots and forks made into pellets and burned or ground to make garden mulch; both approaches release the carbon trapped in the wood into the atmosphere.

Sensing an opportunity to dig deeper into sustainability gains, Caitlin Mueller, an associate professor of architecture, as well as civil and environmental engineering in MIT’s building technology program, and her Digital Structures research group have developed a strategy to “recycle” this waste. into structural components.

“The greatest value you can give to a material is to give it a load-bearing role in a structure,” she says.

Mueller and his team focus on tree forks — those sections of a tree where the trunk or branch splits in two, forming a Y-shaped piece.

“Shaft forks are naturally designed structural connections that function like cantilevers in shafts, which means they have the potential to transfer force very efficiently due to their internal fibrous structure,” says Mueller. . “If you take a tree fork and cut it in half, you see an incredible network of fibers that intertwine to create these often three-dimensional load transfer points in a tree.”

The Digital Structures team has developed a five-step “design-to-manufacture workflow” that combines natural structures such as tree forks with the digital and computational tools now used in architectural design. “Many iconic buildings constructed over the past two decades have unexpected shapes,” says Mueller. “Tree branches have a very specific geometry that sometimes lends itself to irregular or non-standard architectural form – driven not by an arbitrary algorithm but by the material itself.”

The researchers sourced their tree forks from Somerville, Massachusetts, where several trees were downed near a school site.

The first step in the process was to create a library of digital materials for their collection of tree forks. Each tree fork has been isolated and 3D scanned to create high resolution digital representations.

The next step was to match the forks of the Material Library shafts to the Y-shaped nodes in an architectural design example where three straight members come together to support a critical load. The goal was to achieve the best overall distribution of all tree branches among nodes in the target design.

The third step in the process was to incorporate the designer’s intent or preference. To allow for this flexibility, each design includes a limited number of critical parameters, such as bar length and bending stress. Using these parameters, the designer can manually modify the general shape or geometry of the design or use an algorithm that automatically modifies or “transforms” the geometry.

In Stage 4, the researchers prepared the forks of the trees by recutting their end faces to better match the adjacent straight timbers, simplifying assembly, while trimming any remaining bark to reduce susceptibility to rot and fire. . A custom algorithm automatically calculates the cuts needed to fit a given tree fork into its assigned node and to remove the bark.

The last step is to assemble the available forks and linear elements to build the structure. The joints based on tree forks are all irregular, and combining them with the pre-cut straight wooden elements could be difficult; however, they are all labeled.

“All the geometry information is built into the joint, so the assembly process is really low-tech,” says Mueller. “It’s like a kid’s toy set. You simply follow the instructions on the gaskets to put all the pieces together.

The team temporarily set up their final structure on the MIT campus, though this is only part of the structure they plan to build eventually. “It featured 12 nodes that we designed and made using our process.” As activity on campus resumes post-pandemic, researchers plan to complete the design and construction of the full structure, which will include approximately 40 nodes and be installed as an outdoor pavilion at the site of downed trees in Somerville.

Photograph by Felix Amtsberg

Abdul J. Gaspar