Researchers at the Massachusetts Institute of Technology have developed a unique strategy to use waste timber materials as load-bearing structural components in building structures, thereby, 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 together are responsible for as much as 15 per cent of total CO2 emissions. With its carbon sequestration capabilities, timber is increasingly being considered as a more environment-friendly alternative to traditional building materials.

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

Sensing an opportunity to further the sustainability gains, Caitlin Mueller, an associate professor of architecture, as well as civil and environmental engineering in the Building Technology Program at MIT, and her Digital Structures research group have developed a strategy for ‘upcycling’ those waste materials 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 her team focus on tree forks — those sections of a tree where the trunk or branch divides in two, forming a Y-shaped piece.

“Tree forks are naturally engineered structural connections that work as cantilevers in trees, which means that they have the potential to transfer force very efficiently thanks to their internal fibre structure,” explains Mueller. “If you take a tree fork and slice it down the middle, you see an unbelievable network of fibres that are intertwining to create these often three-dimensional load transfer points in a tree.”

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

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

The first step of the process involved creating a digital material library for their tree fork collection. Each tree fork was isolated and 3D scanned to create high-resolution digital representations.

The next step was to match the tree forks in the material library to the Y-shaped nodes in a sample architectural design where three straight elements meet up to support a critical load. The goal was to get the best overall distribution of all the tree forks among the nodes in the target design.

The third step in the process was to incorporate the intention or preference of the designer. To permit that flexibility, each design includes a limited number of critical parameters, such as bar length and bending strain. Using those parameters, the designer can manually change the overall shape, or geometry of the design or use an algorithm that automatically changes, or ‘morphs’ the geometry.

In step 4, the researchers prepared the tree forks by re-cutting their end faces to better match adjoining straight timbers, simplifying assembly, while cutting off any remaining bark to reduce susceptibility to rot and fire. A custom algorithm automatically computes the cuts needed to make a given tree fork fit into its assigned node and to strip off the bark.

The final step involves the assembly of the available forks and linear elements to build the structure. The tree-fork-based joints are all irregular, and combining them with the pre-cut, straight wooden elements could be difficult; however, they’re all labelled.

“All the information for the geometry is embedded in the joint, so the assembly process is really low-tech,” says Mueller. “It’s like a child’s toy set. You just follow the instructions on the joints to put all the pieces together.”

The team has installed their final structure temporarily on the MIT campus, though it is only a portion of the structure they plan to eventually build. “It had 12 nodes that we designed and fabricated using our process.” As activity on campus resumes after the pandemic, the researchers plan to finish designing and building the complete structure, which will include about 40 nodes and will be installed as an outdoor pavilion on the site of the felled trees in Somerville.

Photography by Felix Amtsberg

Architecture News & Editorial Desk

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