Lightweight Timber Research Structures

Entrant: 
Andrew Barrie Lab & Batchelar McDougall Consulting

Category: 
11. Innovative timber manufacturing & technology award

Photographed by: 
Andrew Barrie / Dylan Waddell / Kanade Konishi

In recent years, staff and students [at a NZ university] have been developing lightweight timber structures fabricated from relatively small structural elements with relatively complex machined joints.

These timber elements occupy something of a gap in the spectrum of CNC fabrication options available in New Zealand: highly specialised CNC machines are used to fabricate heavy primary structural members, while smaller, widely available milling machines are used to mill sheets into interior elements such as cabinetry. However, thinner, non-repetitive structural and finishing timber elements are typically still made by hand on the construction site. We have devised a ‘hack’ methodology for efficiently modeling and milling such elements en masse using standard CNC machines. This involves developing digital processes for modeling elements and preparing cutting files, as well as the development of a jig system that allows over-length elements to rapidly be secured in the milling machine, milled, and removed for assembly.

Functionally, these structures have fulfill a variety of roles – garden pavilions, outdoor classrooms, and rest structures for mountain bike trails. Each project in the ongoing series had only a small budget: $10,000 for design, materials, fabrication, shipping, and installation.

Each structure has effectively been a full-scale prototype for the fabrication technique as it has become increasingly ambitious. Development focused on timber-to-timber jointing techniques that could be used to provide structural rigidity. The first structure employed a relatively simple partial lap joint. The subsequent structure absorbed techniques from traditional Japanese carpentry, developing a CNC-milled cogged lap joint, which is more complex but stronger than a simple lap joint. Drawing further on Japanese traditions, the next structure developed mortise and tenon joints, which allowed structural elements to occupy the same plane. This required techniques allowing the timber to be turned over in the jig and precisely relocated to avoid a loss of accuracy.

These early structures in the series used Abodo screening timber, which is light and very stable timber that facilitated very accurate milling. As with most digitally machined structures, they relied on very stable timbers (Abodo, natural hardwoods, CLT, LVL) that are either expensive or not suitable for outdoor use. Lower quality timbers are problematic as the natural behavior of the timber—warping, twisting, shrinking—makes the required precision difficult to achieve. Our next step, therefore, was to further develop the technology to allow the combined use of unstable timber and precision machining, the next structure updated an ancient Japanese wedged joint technique to seamlessly absorb inaccuracies or deformation in the timber elements. As a step towards reinvigorating unique Maori building technologies, another pavilion adapted the post-tensioned heavy timber construction of pre-colonial Maori whare. Where whare employed hand-adzed wall-beam joints and flax tensioning ropes, this structure combines digital milling and the standard agricultural wire fence strainers.

Our goal for these projects was lightweight, sustainable structures that expand the possibilities for timber construction, a core part of our response to the current climate emergency.