Climate

High-Rise Wooden Buildings May Help Make Cities More Sustainable

Tall wood construction is less carbon intensive than concrete or steel, but it will likely take a major cultural shift for builders and architects to embrace wood construction on a large scale.

And with the world’s population expected to grow to nearly 10 billion by mid-century, with two-thirds of them in cities, some scientists, architects, and builders say the key to avoiding a Sasquatch-sized carbon footprint may lie in the world’s forests.

“I like to say a tree is a solar-powered carbon-sequestering machine whose byproduct is one of the best building materials ever known to mankind,” said Scott Francisco, a founder of Wood at Work and advocate of what’s being called “tall wood” construction. “It produces this amazing material while taking carbon out of the atmosphere and turning it into the material you want to use.”

While more commonly used to frame single-family homes, wood is being used to frame an increasing number of multi-story buildings that are sprouting around the world. The first major US structure is in Minneapolis, where a seven-story office building opened in 2016. Canada’s University of British Columbia just built an 18-story dormitory out of wood, the tallest such building to date. An Austrian company soon hopes to top that with a 24-story tower in Vienna, and several other buildings ranging between 7 and 14 stories have gone up across Europe and in Australia.

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Using more wood instead of concrete-and-steel construction could shrink projected carbon emissions by anywhere from 13 percent to 20 percent, Yale University forestry professor Chad Oliver estimated in a 2014 study. If managed properly, turning trees into lumber and cultivating their replacements will soak up more carbon than leaving forests alone, he wrote.

“What surprised me when I did the first analysis was that I realized we were only harvesting about 20 percent of what we’re growing in the world,” Oliver told Seeker in an interview. About half of that 20 percent is burned for fuel, and a fifth of it is converted to paper.

“So essentially, we’re only using about 6 percent of growth to make wood construction out of,” he said. “If we boosted that from 6 percent up to around 70, 80, or 90 percent — leaving some natural areas and not touching them — we could build a whole lot of buildings and save from creating a whole lot of concrete and steel … But we also have to be concerned with biodiversity, which is why we’re suggesting only cutting excess growth.”

Oliver said responsible management using a combination of tax incentives, satellite surveillance, and longer growing cycles would let much larger amounts of timber be produced without sacrificing woodlands overall. He said he’s working with the United Nations Development Program and Turkey’s forest service — which controls about a quarter of that country’s land — to develop open-source tools to help manage forests sustainably.

Tall wood isn’t new. The pagodas of Asia were the skyscrapers of their day: China’s Wooden Pagoda of Yingxian, built in 1056, tops out at about 67 meters (220 feet). 

But the lumber used in multi-story buildings today differ greatly from the two-by-fours that frame the typical single-family home. Beams, floor panels, and wall sections are made of multiple pieces of wood laminated together, often with different layers arranged at right angles to each other. The pieces can be fastened together with ordinary screws or nails, or joined by steel plates.

The result is a material that’s high-strength but far lighter than steel or reinforced concrete. And while fire has historically been a concern for wooden structures, Francisco said the thicker the material, the harder it is to burn.

“We know if done properly, it’s very fire-resistant and very safe,” he said. “We just have to learn how to do that in today’s context.”

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And speaking of those ancient temples, they’ve withstood untold numbers of earthquakes over the centuries. That’s in part because wood’s flexibility helps them stand up to seismic shaking, said Shiling Pei, a civil engineer and assistant professor at the Colorado School of Mines near Denver.

“There’s other people arguing the life-cycle benefits and carbon sequestration,” Pei said. “But structurally, wood especially for seismic issues is light and naturally very resilient. It immediately cuts down your design demand compared to a concrete-and-steel building.”

Pei and his colleagues are preparing to test a two-story structure built from modern timber construction on a “shake table” at the University of California, San Diego as part of a National Science Foundation-backed effort to test wooden construction. He said earthquake-prone Japan has tested structures up to six stories and found only minor damage. And when there is damage, wooden buildings are faster and easier to repair, he said.

 “What we’re hoping is with the research and large-scale testing, the engineering community can gain confidence and this can become a common practice.”

So is the sky the limit for tall wood?  

Francisco said from “a purely engineering and structural perspective,” wooden towers could top 100 stories: Michael Green, the architect who designed the Minneapolis building, recently teamed up with a lumber company in Finland to redesign the Empire State Building in timber.

“If you think about it from what’s practical or cost-effective, that of course changes,” Francisco said. “There’s a point where practicalities diminish significantly, but I can’t tell you where that is.”

Oliver said growing cities get the most environmental bang for their buck out of mid-rise buildings, in the 8- to 12-story range. Building tall buildings from wood may be more expensive in some conditions, and not all cities have embraced the concept.

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Vancouver’s building codes required the cores and elevator columns in the 18-story dormitory to be made of concrete, Francisco said, while others might require a hybrid of wood and steel to meet local conditions like earthquake resistance.

Building smaller buildings first might be needed for people to get comfortable with the concept, he said.

“It’s a culture change,” said Francisco, “for building officials. It’s a culture change for architects and engineers to learn these new metrics and approaches, and it’s a culture change for the developer and the end user of the buildings.”