Cross-Laminated Timber as a Sustainable Building Material for Tight Budgets
Although the use of wood as a building material can be evocative of environmental degradation through deforestation, the rise of mass timber has flipped the narrative. Buildings cause nearly 40% of global carbon emissions, and one way to address this is to switch to less carbon-intensive materials.
One such material is mass timber, a prefabricated type of lumber containing a large proportion of sequestered carbon previously circulating in the atmosphere. Provided that it’s sourced from a sustainably managed forest, mass timber can continue to serve as long-term carbon storage. Conversely, leaving trees to decay emits previously sequestered carbon back into the atmosphere.
Cross-laminated timber (CLT), a type of mass timber, is especially promising due to its strength and can be used in lieu of concrete, a building material with a generally large carbon footprint. According to Forell/Elsesser Engineers Principal and Structural Engineer Allen Nudel, CLT is more prevalent in areas like the Pacific Northwest, where wood from sustainably managed forests is produced locally, but has yet to catch on in regions like the Bay Area, despite the concerted push for more sustainable buildings.
Forell/Elsesser is part of the design-build team alongside Blach Construction and ELS Architecture and Urban Design for the Millbrae Recreation Center project in Millbrae, CA. It’s a City-owned project that will yield 24,000 square feet of community space and is being built with 3-ply CLT panels to frame the second-floor and roof. It’s a CLT-steel hybrid system with the CLT panels spanning between steel beams, girders and columns.
In addition to the sustainability aspects, another advantage of CLT is that designing an aesthetically pleasing building requires less effort due to wood’s inherent beauty. In the case of the Millbrae Recreation Center, the CLT will be left exposed, which also means less money has to be spent to cover up the structure.
“We are seeing more interest in net-zero energy and carbon-neutral buildings, but it does require more money upfront,” said Nudel. “In the case of this project, changing from concrete to CLT, did not increase the cost significantly. We want to use materials that are considered more sustainable but are about equal in cost. And so that's the trick — we can't spend more money because we're on a tight budget. This particular project had a fixed construction cost that we couldn’t exceed.”
The City of Millbrae, wanting a sustainably constructed recreation center built quickly, had originally proposed pursuing CLT as a design enhancement that was ultimately embraced by the chosen design-build team. In addition, the hybrid CLT-steel approach was selected as a way to keep costs down. So far, the project is adhering to the tight budget and is expected to be complete in June.
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CLT is hardly the only sustainable building material. However, it is already gaining ground in places like Vancouver and France, where regulations encourage the use of wood in buildings. Other carbon-neutral materials such as new concrete formulations are still in the experimental phases and not necessarily ready for prime time, Nudel said. He said that CLT could be used for any type of building, though it’s better suited for rectilinear patterns versus curved designs. Another limitation is that sourcing CLT or mass timber, more generally, must be done through trusted vendors to ensure that it is, in fact, made with wood from sustainably managed forests.
Additionally, for a CLT project to pencil, timing is of the essence. For example, when lumber prices were soaring during the height of the pandemic, the added costs may have made mass timber prohibitive. However, as lumber prices have since dropped a bit, Nudel said the timing was suitable for the Millbrae Recreation Center to be built with CLT.
“We feel like we're early adopters, but we're just early adopters in this area — there's a lot of CLT work going on in Canada and in the Northwest U.S.,” said Nudel. “So it seems experimental and new, but it's really not.”
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