Sustainable construction efforts for wooden skyscrapers reach new heights » Yale Climate Connection

At the University of Toronto, across from the football stadium, workers are building a 14-storey building that will house classrooms and faculty offices. What's unusual is the way they built it – by bolting together huge beams, columns and panels made of faux wood panels.

As each wooden element is transported via flatbed truck, a tall crane lifts and holds it in place while workers connect it with metal connectors. In its semi-finished state, the building resembles flat-panel furniture being assembled.

The tower uses a new technology called mass timber. In this construction, steel beams and concrete are replaced by large man-made wooden elements that can stretch more than half the length of a football field. Although still relatively uncommon, it is growing in popularity and is starting to appear in skylines around the world.

Today, the tallest large wooden building is Milwaukee's 25-story Ascent skyscraper, due to be completed in 2022. . According to a report by the Council on Tall Buildings and Urban Habitat, 70% of existing and future buildings are located in Europe, about 20% in North America, and the remainder in Australia and Asia. If you include small buildings, at least 1,700 large wooden buildings will have been built in the United States alone by 2023.

Mass timber is an attractive alternative to energy-intensive concrete and steel, which together account for nearly 15% of global carbon dioxide emissions. While experts still debate the role of mass timber in combating climate change, many believe it is better for the environment than current construction methods. After all, it relies on wood, which is a renewable resource.

Large pieces of wood also provide a different aesthetic and can make a building feel special. “People are tired of steel and concrete,” said Ted Keswick, a construction scientist at the University of Toronto's Mass Timber Institute, which promotes mass timber research and development. With its warm, soothing appearance and natural variations, wood can be more visually pleasing. “People actually enjoy looking at the wood.”

Same wood, stronger structure

Of course, using wood in large-scale construction is nothing new. Industrialization in the 18th and 19th centuries led to the need for large factories and warehouses, which were often “brick and beam” buildings—frames of heavy wooden beams supporting exterior brick walls.

However, as buildings grew taller, builders turned to concrete and steel for support. Wood frame construction is mostly limited to houses and other small buildings made from the standard-sized “dimension” lumber you stack at Home Depot.

But about 30 years ago, German and Austrian builders began experimenting with the technology of using this readily available wood to create large wooden components. They use nails, dowels and glue to combine smaller pieces of wood into large, strong, solid pieces without the need to cut down large, old trees.

Engineers, including Swiss-based German engineer Julius Natterer, pioneered new ways of using these materials for construction. Architects including Austria's Hermann Kaufmann began to gain attention for mass timber projects, including the Ölzbündt Apartments in Austria, completed in 1997, and Brock Commons, an 18-storey student residence at the University of British Columbia, completed in 2017.

In principle, bulk wood is similar to plywood, but on a larger scale: smaller pieces of wood are layered and glued together under the pressure of a large, specialized press. Today, beams up to 50 meters long, usually made of glulam or glulam, can replace steel members. Panels up to 50 cm thick (usually cross-laminated timber or CLT) replace the concrete of the walls and floors.

These wood composites are incredibly strong – stronger than steel by weight. But the massive timber elements would have to be bulkier to achieve the same strength. The taller the building, the thicker the wooden supports have to be; at some point, they just take up too much space. Therefore, for taller mass timber buildings, including the Ascent skyscraper, architects often turn to a combination of wood, steel and concrete.

Historically, one of the most obvious issues with using large amounts of wood in high-rise buildings has been fire safety. Until recently, many building codes restricted wood construction to low-rise buildings.

While buildings do not have to be completely fireproof, they need to be able to resist collapse long enough to give firefighters a chance to control the fire and allow the occupants to escape. For example, materials used in traditional skyscrapers are required to maintain their integrity during a fire for three hours or more.

To demonstrate the fire resistance of mass timber, engineers placed timber elements into the gas chamber and monitored their integrity. Other tests fired up models of large wooden buildings and recorded the results.

These tests gradually convinced regulators and customers that large quantities of wood could resist burning long enough to be fire safe. Part of the reason is that a layer of char tends to form early on the outside of the wood, insulating the inside from much of the heat of the fire.

Mass timber received a major approval in 2021 when the International Code Council revised the International Building Code – a model for jurisdictions around the world – to allow mass timber building heights up to 18 stories. With this change, more and more localities are expected to update their regulations to routinely allow high-rise wood buildings instead of requiring them to obtain special approval.

However, there are other challenges. “The real problem is moisture, not fire,” said Steffen Lehmann, an architect and urban sustainability scholar at the University of Nevada, Las Vegas.

Moisture control is a must in all buildings, but it is absolutely critical with mass timber. Wet wood is easily spoiled by fungi and insects such as termites. The builder was careful to protect the wood from moisture during transportation and construction and deployed a comprehensive moisture management plan, including designing heating and ventilation systems to prevent moisture buildup. To provide additional protection against insects, the wood can be treated with chemical insecticides or surrounded with mesh or other physical barriers where it contacts the ground.

Another issue is acoustics, since wood transmits sound very well. Designers use methods such as soundproofing, leaving space between walls and installing raised floors.

Potential advantages of mass timber

Fighting global warming means reducing greenhouse gas emissions from the construction industry, which accounts for 39% of global emissions. Diana Ürge-Vorsatz, an environmental scientist at Central European University in Vienna, said large quantities of wood and other bio-based materials could be an important part of this effort.

In a 2020 paper published in the Annual Review of Environment and Resources, she and colleagues cited a timber industry estimate that the 18-storey Brock Commons in British Columbia avoided the equivalent of 2,432 tons compared with similar buildings. CO2 emissions from concrete and steel. Of this, 679 tonnes were saved as the manufacturing process of wood produces less greenhouse gas emissions compared to concrete and steel. An additional 1,753 metric tons of carbon dioxide equivalent are locked in the building's wood.

“If we use bio-based materials, we have a win-win situation,” says Ürge-Vorsatz.

But much of the current enthusiasm for the climate benefits of mass timber is based on some big assumptions. For example, accounting often assumes that any wood used in mass timber buildings will be replaced by the growth of new trees, which will absorb the same amount of carbon dioxide from the atmosphere over time. But some environmental groups believe that if old trees are replaced with new plantations, the new trees may never reach the same size as the original trees. There are also concerns that increased demand for wood could lead to more deforestation and less land for food production.

Research also tends to assume that once wood enters a building, the carbon is locked away permanently. But not all wood from felled trees ends up in finished products. Branches, roots and lumber mill waste may decompose or burn. When a building is demolished, if the wood ends up in a landfill, the carbon disappears in the form of methane and other emissions.

“A lot of architects are scratching their heads,” said Stephanie Carlisle, an architect and environmental researcher at the nonprofit Carbon Leadership Forum, who wonders whether mass timber always has a net benefit. “Is that true?” She believes the climate benefits are real. But understanding the extent of these benefits requires more research, she said.

At the same time, mass timber is at the forefront of an entirely different mode of construction known as integrated design. In traditional construction, an architect first designs the building and then hires multiple companies to handle different parts of the building, from laying the foundation to building the frame to installing ventilation systems.

In integrated design, the design phase is more detailed and involves individual companies from the beginning, Kesik said. The way the different components assemble and work together is predetermined. The precise size and shape of the elements are predetermined and even pre-drilled holes can be made for connection points. This means that many components can be manufactured off-site, often using advanced computer-controlled machinery.

Many architects prefer this because it gives them greater control over building elements. Because much of the work is done ahead of time, buildings tend to be constructed faster on site, up to 40 percent faster than other buildings, Lehmann said.

Keswick said large wooden buildings tend to be manufactured more like cars, with all the individual parts shipped to the final site for assembly. “When this massive timber building came on site, it really looked like an oversized piece of Ikea furniture,” he said. “Everything comes together.”

This article originally appeared in Knowable magazine, a nonprofit publication dedicated to making scientific knowledge accessible to all. Sign up for Knowable magazine's newsletter.