Mass Timber Begins to Take Hold in North American Commercial Construction Market

Carbon12, a 12-story luxury condominium building located in the uber-hip Williams Corridor of Portland, is tall for its surroundings. And if not for the three modern office buildings located cattycorner from it, Carbon12 would stand out far more in its neighborhood, a mix of craftsman bungalows and Victorian-era homes.

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What truly sets it apart from other buildings in the area is its construction: Carbon12’s is entirely framed with structural cross-laminated timber (CLT) and is currently the tallest structure in the US to use all-wood structural materials. The developer says it is “the most environmentally advanced, seismically prepared, and technologically sophisticated residential project in the United States.”

Cross laminated timber is essentially mass-timber plates made from smaller framing lumber laminated crosswise on their wide faces. Although CLT is relatively new to North America, this technology was first developed and used in Germany and Austria in the early 1990s. During the mid-1990s, more extensive industry research was completed, which led to its full commercialization in Europe for various building systems such as single-family and multi-story housing.

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Sustainable attributes of CLT are fairly obvious. It’s wood, so it’s a natural carbon sink—forever. And wood is also a renewable material that uses far less energy than steel to machine manufacture. Other advantages include:

  • Design flexibility – CLT can be used in walls, roofs or ceilings. The thickness of the panels can easily be increased by adding more layers and the length of the panels can be increased by joining panels together
  • Sustainability – CLT is a sustainable material, as it is made out of wood and does not require the burning of fossil fuels during production
  • Prefabrication – Floors or walls made from CLT can be fully manufactured before reaching the job site, which decreases lead times and could potentially lower overall construction costs, as more buildings utilize it and commercialization is fully established
  • Thermal insulation – As it is made out of multiple layers of wood, the thermal insulation of CLT can be high depending on the thickness of the panel
  • Seismic advantages – Particularly in the western part of North America, where earthquakes are still a threat, mass timber building offers seismic advantages over concrete and steel, as wood is able to flex during a seismic event, lessening the damage

In 2015 CLT was incorporated into the National Design Specification for wood construction, which was used as a reference for the International Building Code, which now allows CLT to be recognized as a code compliant construction material. These code amendments permit CLT to be used in the assembly of exterior walls, floors, partition walls and roofs. Also included in the 2015 IBC were char rates for fire protection, connection provisions and fastener requirements specific to CLT. To meet structural performance requirements, the code mandated that structural CLT products met the requirements specified by ANSI/APA PRG 320.

(Moholt 50|50, by MDH Arkitekter, photo by MDH Arkitekter)

Two years ago, Kebony wood was specified for Europe’s largest cross laminated timber structure, Moholt 50|50, which, comprising five blocks, houses more than 600 students at the Norwegian University of Science and Technology in Trondheim, Norway. With a strong environmental and community focus, Moholt 50|50 is a valuable addition to the area, which benefits the local community, rather than being simply an extension of the university’s existing accommodation.

In light of the pressures facing the construction industry with regard to energy consumption and climate change, the incorporation of wood in commercial buildings, both as a structural material and finish, is a fast-growing and likely permanent trend. Commercial projects like Moholt 50|50 and Carbon12 — in completely different ways — exemplify the enduring versatility and sustainability of wood as a building material. 

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