Practicing ArchitecturePracticing Architecture
Lumbering into the Sky: Building Tall with Wood
One of the oldest building materials may become the newest
By Nalina Moses
When Vancouver architect Michael Green shared his plans for a 30-story timber frame tower last year, to some it seemed more like a pipedream than a press release. But today, he's not the only dreamer. Environmental concerns are inspiring architects around the world to search for ways to build tall (and thus dense) with wood. It's common knowledge that a tall building is more sustainable than a comparable number of smaller independent structures. The steel and concrete systems used to frame tall buildings, however, generate significant carbon emissions, and can’t be renewed and re-harvested like wood.
Vertical construction is so closely linked to steel framing that a “wood tower” at first sounds like an oxymoron. Traditional timber construction, which uses beams and columns cut from solid wood, can typically reach eight to 10 stories; wood-framed churches in Norway, temples in Japan, and mill buildings in North America have all reached these heights. “My office is in an eight-story heavy timber frame building that is 105 years old here in Vancouver,” says Green.
More recently, wood towers have returned to the spotlight. In 2009, London architecture firm Waugh Thistleton, led by Andrew Waugh, used a custom-designed timber frame system to build the nine-story Murray Grove apartment building in London. In 2011, Austrian architect Hermann Kaufmann used a similar framing system for the eight-story LifeCycle Tower (LCT) One in Dornbirn, Austria. Now a current Association of Collegiate Schools of Architecture design competition, Timber in the City: Urban Habitats Competition, highlights these and other innovative wood framing technologies.
Modern timber framing isn't about yelling “Timber!” and cleaning up sawdust. This emerging, experimental technology relies on engineered components that laminate wood in layers like plywood, press them into shape like fiberboard, and also incorporate selective non-wood components. The system developed by Green, called Finding the Forest Through the Trees, uses engineered wood columns, exterior cladding panels, and load-bearing partitions and a nonstructural exterior curtain wall that can be built from a variety of materials.
LCT One was built with a similar method, the Cree System. Cree is an Austrian company that recently established an office in San Francisco for North America product distribution. Cree's Chief Sustainability Officer Nabih Tahan, AIA, says the company will partner with different local manufacturers to build timber components to Cree's specifications, which integrate framing, insulation, windows, and exterior wood cladding in a single panel. Relatively few other North American companies produce these kinds of “closed” panel systems, but exceptions include Bensonwood in New Hampshire and Durfeld Constructors in British Columbia.
These new timber frames incorporate non-wood elements like steel beams and concrete foundation walls. “On lower heights we use only concrete foundations and then only wood for the structure,” Green says. “For tall buildings we use steel ledger beams. Combining the best of all materials is typically the best way to go, but where possible we move to less steel and less concrete to reduce our energy and carbon footprint. Will we get rid of steel and concrete? Not any time soon.”
Carbon to carbon
The reason cited most often for building wood towers is sustainability, since wood is a natural, renewable material that sequesters and absorbs carbon as it exhales oxygen. The manufactured wood components used in new timber frames aren't carbon-neutral, of course, as they require energy for fabrication and transportation. While some systems use wood dowels to hold smaller wood pieces together, most rely on adhesives and binders that can contain volatile compounds. These techniques are becoming more carefully legislated.
Steel is so naturally strong in both tension (in beams) and compression (in columns) that wood structural systems—after they’ve been fabricated and transported for use in very tall buildings—have a long way to go to catch up. Carol Willis, founder and director of the Skyscraper Museum in New York City, says that most steel used in the United States is already recycled, narrowing the sustainability gap between the two systems.
What's clear is that wood components require far less energy to fabricate and construct than comparable concrete systems, the norm for large-scale construction in many parts of the world. “The difference between a 20-story mass timber tower and a 20-story concrete tower is roughly a net savings of 4,500 tons of carbon,” Green says. “That accounts for the storage of the carbon in wood, as well as the elimination of the emissions of the concrete.”
Wood has additional advantages in terms of its broader lifecycle costs as well. Because it's not as thermally conductive as steel and concrete, so timber structures have reduced heating and cooling costs. Timber frame buildings also are simpler to take apart than steel and concrete ones, reducing demolition costs.
There have been technical advances that improve fire ratings for engineered wood framing components. “The American Wood Council recently performed a fire test on cross laminated timber, which achieved a fire rating over three hours,” says Cees de Jager, president of the Binational Softwood Lumber Council, which is cosponsoring the Timber in the City competition. “Further, cross laminated timber was recently adopted by the International Code Council into the heavy timber category.”
While Green's 30-story wood tower seems particularly well-suited to the forest landscape of British Columbia, his ambition is global. “These ideas make enormous sense in China, [as they] are increasingly stepping into climate change discussions,” he says. “I expect no place on earth will adopt these ideas faster than China, where the need is huge.”
That wooden feeling
Wood has different properties than steel and concrete, so special details are required when using it in tall structures. “Each material has the opportunity to underperform if not properly detailed,” says Green. Steel and concrete frames are naturally stiff, while wood frames often need to be braced with concrete decks or steel beams. Green says that acoustics in timber frame buildings are a special challenge, and that he and his consultants are testing new details.
One characteristic of wood which hasn’t yet been fully exploited with these new framing technologies is its rich natural grain and color. Frame components often use scrap and secondary wood that doesn't have a fine consistency. Additionally, wood is vulnerable to moisture, mold, and fire, and often needs to be enclosed in more mundane materials like gypsum board to meet safety codes. It’s redundant to finish wood beams and columns with protective cladding, and then again with wood panels, but often that’s what required to give the interiors and exteriors of timber frame buildings a wooden feeling.
Michael Green's 30-story wood tower, the tallest to be engineered, has been placed on hold. Now he's working with a client to select a site in Vancouver for a 20-story wood tower. Willis isn’t convinced that a 20-story building qualifies as a true skyscraper, and isn’t sold on wood being the next leap in sustainable tall building evolution.
But Green believes wood buildings will eventually surpass 30 stories. “That would be the natural progression, just as steel and concrete progressed a century ago,” he says. “We know we can achieve great heights. How high is the job of dreamers, and highly capable architects and engineers.”
Michael Green’s proposal for a 30-story timber frame tower in Vancouver. Image courtesy of Michael Green Architecture.
Construction of LifeCycle Tower One in Dornbirn, Austria. Image courtesy of Architekten Hermann Kaufmann.
LifeCycle Tower One, designed by Architekten Hermann Kaufmann. Image courtesy of Architekten Hermann Kaufmann.