Mass Timber: The Tall Building of Tomorrow
Mass Timber: The Tall Building of Tomorrow
One of the most exciting new advancements in building science is coming from one of its oldest techniques - wood. Specifically, a growing category of wood products known collectively as ‘mass timber’ is quickly emerging onto the scene and changing the way the industry thinks about the future of tall buildings. Mass timber also provides an alternative construction methodology that can effectively address climate change as construction costs rise, cities look for more housing density, and more stringent building and energy codes are adopted.
What is mass timber?
Mass timber is a category of engineered wood products fabricated from the dimensional, sheet, or strand lumber constituents generally associated with light frame construction. These pieces are combined into larger structural elements that behave more like heavy timber. Many mass timber products, such as laminated veneer lumber (LVL), laminated strand lumber (LSL), parallel strand lumber (PSL) and glue-laminated timber (glulam) have been widely adopted in the market for some time, having proven useful in the world of light-frame wood construction. This is due to their high strength-to-weight ratio and relative low cost when compared to steel members of similar strength.
More recently, other mass timber products such as cross-laminated timber (CLT), dowel-laminated timber (DLT), nail-laminated timber (NLT), mass plywood panels (MPP), and timber concrete composites (TCC) have emerged into the market. These newer products, rather than simply replacing individual structural members like their established counterparts, aim at replacing entire structural systems like floors, roofs, and walls, with massive solid timber volumes.
What are the benefits of mass timber?
The chief performance benefits of mass timber come from its fire resistance. Because of the wide cross-sectional area of mass timber, a fire only chars the face of mass timber to a shallow and predictable depth. As it does this, it drives the moisture in the wood towards the center of the member rather than allow it to escape. This allows the fire to ‘self-extinguish’ before the members become structurally compromised. As such, the mass timber elements can be left exposed on the inside of the building without additional fire-rated finish layers typically required in wood construction.
Shake plate tests of the seismic performance of mass-timber buildings have also yielded promising results. Because mass-timber buildings are typically around 30% lighter than steel and 60% lighter than post-tensioned concrete counterparts, they experience considerably less lateral loading from seismic activities making them particularly exciting in earthquake-prone regions.
Additionally, because the majority of the structural components are fabricated off-site with a high degree of precision, builders have found that airtightness is easier to achieve with these interlocking components than in more conventional building typologies.
The economic advantages of mass timber stem from the fact that the bulk of the structure is pre-fabricated off-site and arrives on site ready to be assembled. This means fewer trades and less on-site construction, which have allowed mass timber buildings to often be constructed in 25% less time than steel and concrete buildings.
Additionally, because many mass timber structural systems can also be left exposed as the building finish, many developers and landlords have reported decreased tenet improvement costs for their buildings. The aesthetic warmth provided by the exposed wood has been shown to increase occupant comfort and wellbeing and provides a point of market distinction for these buildings.
Finally, as the market and society more broadly become increasing conscious of climate concerns and sustainability, building with wood presents a distinct opportunity. For starters, by reducing the concrete associated with typical construction methods (combined with reducing the typical foundation size due to the reduced building weight) greatly reduces the estimated carbon footprint of the building. The wood itself also functions as a carbon sink, capturing carbon from the atmosphere during its growth cycle as a tree and then sequestering that carbon into a building rather that re-releasing the carbon into the atmosphere as would happen if the tree were to naturally decompose or burn. In fact, a recent study estimates that using wood as a building material could save “4 to 31 percent of global CO2 emissions and 12 to 19 percent of global FF [fossil fuel] consumption by using 34 to 100 percent of the world’s sustainable wood growth”. Incentivizing more wood construction is an important part of a long-term strategy of removing excess carbon from our atmosphere.
Why use mass timber now?
While mass timber buildings do comply with current building codes, they have until now been required to conform to the existing building codes either though the existing Type-IV Heavy Timber or Type V Light Frame Construction typologies. In either case, their heights have been limited to 6 stories and 4 stories respectively – a far more restrictive limit then what the testing data would suggest is safe for this building material. However, that is set to change soon. In 2019, the International Code Council (ICC) announced approval of code changes as part of the 2021 International Building Code (IBC) that will provide for 3 new subcategories of tall mass timber structures. The three new types: Type IV-A, Type IV-B, and Type IV-C allow for different levels of exposure for mass timber structural elements and allow for building heights of 18 stories, 12 stories, and 9 stories respectively. This code amendment will allow for the creation of a new market category for tall midrise buildings without the high construction costs associated with other non-combustible construction types. In the last few years we have also seen the number of mass timber fabrication facilities jump considerably and they are expected to continue to grow in the coming years making mass timber increasingly more cost competitive.
Sources
American wood council https://www.awc.org/pdf/education/des/ReThinkMag-DES610A-MassTimberinNorthAmerica-161031.pdf
https://www.woodinstitute.org/mod/book/view.php?id=336&chapterid=388
Oliver, C.D., Nassar, N.T., Lippke, B.R., & McCarter, J.B. (2014, March 28). “Carbon, Fossil Fuel, and Biodiversity Mitigation with Wood and Forests,” Journal of Sustainable Forestry, 33(3), pp. 248–75, DOI:10.1080/10549811.2013.839386