2020 | Wood: Past and Future | Brennen Donnelly and Jake Elbrecht
At the intersection of forestry practices, architectural design, and carbon impact is an opportunity to create a new carbon bank: our built environment. This research investigates each of these parameters to expand new and existing carbon banks.
Last year’s Research Fellowship, focusing on Paths to Carbon Zero, recognized that wood-based construction represents an opportunity for lower embodied construction in our buildings. While generalized carbon values for wood products can be found in environmental product declarations, they exclude vital terrestrial carbon pools associated with upstream forestry practices. 2020/2021 Fellows Brennen Donnelly and Jake Elbrecht choose to further explore how forest management practices impact the carbon storage potential of forests and how designers can advocate for these practices, the impact a shift to wood-based construction could have on annual U.S. carbon emissions, and how to empower designers to build at scale with engineered wood products rather than conventional mineral-based materials. The topics summarized below discusses each of these explorations and the major findings from the research.
Though largely under the purview of the forestry industry, it is important that designers understand the implications behind different forestry practices to appropriately leverage their design power in sourcing timber for wood products. The graphs on the right show how better managed forests sequester and store approximately 15-20% more carbon overtime than typical forests harvested with minimum guidelines. Characteristics of better managed forests include greater widths along streams , fixed green-tree retention rates, and max harvest opening sizes.
As designers advocate for wood-based construction, research was needed to ensure that it does not simultaneously advocate for the deforestation of U.S. forests. A series of decisions led to the conclusion that 24% of U.S. forests are currently suited for the production of engineered wood products. That 24% sees an annual increase of 4% by volume after harvests and mortality is subtracted from gross growth. These findings show that there is room for additional timber harvesting to support more wood-based construction without promoting deforestation.
Recent additions to the 2021 International Building Code will allow mass timber construction to reach a height of twelve stories while mostly utilizing the wood as the primary fire resistance. As stick frame construction dominates wood-based construction up to a height of six stories, mass timber can safely and efficiently range from six to tweleve stories.
Data by CBECS shows that this range of building heights makes up approximately 16% of annual construction, or ~370 million square feet. An analysis of the quantity of wood needed to support this level of yearly construction found that current harvest rates would need to be tripled, but that it would still not exceed the rate of growth in forests.
A combination of better forest management practices, sequestering and storing carbon in our buildings, and avoiding carbon emissions associated with mineral-based materials can see a net difference of 18 million metric tons of CO2e annually.
Throughout the fellowship, answering the question ‘how do we design out of wood, at scale’ was seen as just as important as the question ‘why should we build out of wood, at scale.’ Timber Tool was developed to help designers size mass timber elements and build intuition when designing with these largely unfamiliar wood products. Like other structural materials, mass timber elements have a large collection of interconnected parameters that dramatically influence their shape and depth. This tool couples those parameters to simplify and visualize the member sizing for the user. Timber Tool is best used as an asset during the early design phase, getting designers out of the mind set and away from conventional concrete and steel rules of thumb. Quick studies and analyses can be performed, freeing up more time for the design of beautiful buildings built out of mass timber. The tool can be accessed at: https://www.timber-tool.com