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Mass Timber Now for a Healthy Future
by: Sam Hitchon, AIA, LEED AP & David Masenten, AIA, LEED AP BD+C
TOPICS DISCUSSED:
Mass timber is a building method that’s been used in Germany and Canada for decades, and yet the construction industry is just starting to catch up here in the US.Mass Timber
The 2021 version of the International Building Code (IBC) is updating its classification of mass timber structures to allow mass timber buildings up to 18 stories in height with certain provisions.
In the early design phases of a six-story office headquarters in California, ELS explored using mass timber, as an alternative to a more traditional steel structure, for the following advantages: architectural aesthetic, performance, and sustainability potential. Designing with this innovative building material sends a forward-looking message that can set the project apart from its concrete and steel counterparts.
To understand how the latest technology related to our project, I attended the 2019 International Mass Timber Conference in Portland, Oregon. The conference is produced by both the Forest Business Network and the Wood Products Council, and has grown considerably in recent years as mass timber construction has become an accepted alternative method and gained traction in the code community. The conference highlighted where the industry is going and helped us better understand how to apply these advancements in production and design to the commercial office projects that we’re working on now.
Product Origin
The forest is the starting point when talking about a timber project. A large part of the sustainable credentials for mass timber comes from the forests where the timber is harvested. Using a sustainable forestry standard such as FSC is a start, but as Mark Wishnie, Director of Forestry and Wood Products at The Nature Conservancy, suggested at the conference, the best way to know if your material is truly sustainable is to know the forest it was sourced from and that sustainable practices have been used there.
Once the harvested timber makes it to the mill, plants scan individual logs, optimizing the number of useful boards that can be cut from each. The boards are finger-jointed into lengths required for the desired product. The boards are then layered, glued, and pressed to form standardized, uniform glue-laminated timber (glulam or GLT) beams and columns or cross-laminated timber (CLT) panels, which have the strength to match steel and concrete. Waste wood from milling can be reused in other processes.
The Nature Conservancy wants to see mass timber play a significant role in addressing climate change: the sustainable harvesting of wood products and the storing of embedded carbon in the form of mass timber buildings can help reduce the amount of greenhouse gas in the atmosphere. Eliminating the carbon generated by energy-intensive materials, such as the steel and concrete, is an added benefit, making a project more marketable and profitable while reducing the building’s carbon footprint.
A system of exterior shades over the timber structure increases occupant comfort and reduces cooling loads.
Applying What We Learned
Mass timber inherently has structural, architectural, and aesthetic properties. For our study, the ELS design team utilized a one-way structural system when establishing the floor plates and structural grid, with the large glulam girders running north-south and CLT panels in between. This system allowed expansive areas of glass on the primary south-facing façade, spanning from the floor to the underside of the CLT deck. The properties of the CLT meant we could cantilever the floor edges on three sides of the building and use them to support a system of exterior floor extensions and vertical fins to shade the building. The CLT panels and glulams would be pre-cut offsite to avoid oversized load requirements.
The aesthetic and biophilic potential of wood is a key advantage of designing with mass timber. Distinct species of trees, grades of wood, and available finishes offer a variety of aesthetics to choose from. Exposing the mass timber structure can eliminate additional layers of framing, drywall, and ceiling panels, which are typically used to hide steel and concrete construction. People relate well to natural materials and typically have a positive emotional response to wood. In the case of our office project, we can leave a substantial portion of the underside of the CLT deck, beams, and columns exposed. We introduced a raised access floor to provide flexible electrical and data connections to offices and workstations, along with an efficient HVAC VRF system that will reduce equipment in the open ceiling.
The visual appeal of the exposed wood provides a welcoming ground floor experience.
The Growing Mass Timber Industry
Mass timber buildings are cost-competitive with—and in some cases less expensive than—their concrete and steel counterparts. Benefits include:
- Mass timber structures are lighter, which reduces foundation size.
- CLT and glulam can be manufactured to exact sizes and penetrations. This eliminates the need for cutting onsite and reduces waste.
- Smaller construction crews and less staging area are required. Components are delivered to site to match the construction sequence and lifted directly from the truck to be bolted into place.
- Designs that use the exposed structure eliminate large areas of finishes, including drywall and hung ceilings.
For the right building type, mass timber can result in faster construction timelines, less expensive construction, and less waste.
As more projects are opting towards using a mass timber solution, local jurisdictions have become more willing to approve these types of projects. Our study was based under the 2016 California Building Code (CBC) as a Type IV, which allows mass timber buildings up to six stories for office use. The code acknowledges the ability of mass timber to exceed required fire ratings based in part on the process whereby mass timber builds up a char layer that insulates the interior of the member and allows it to maintain its required structural capacity in a fire event The International Code Council is studying changes to create three new types of construction based on developing knowledge of mass timber. The new 2021 IBC categories will allow mass timber buildings up to 18 stories depending on criteria set for each type. For California, these will likely be incorporated into the 2022 CBC release.
The visual connection to exposed wood creates interest at street level and differentiates the building from typical concrete and steel structures.
Designing in mass timber can be a win for everyone: for our clients that want a unique, signature building which supports wellness and operational goals; for the planet in mass timber’s potential to impact climate change; and for the design and construction teams to deliver projects more quickly and with less environmental impact, as mass timber enables reducing costs and time through technology and prefabrication.
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