In a remarkable 240-page report simply entitled Tall Wood, architect Michael Green makes the case for wood as the ideal building material for skyscrapers and their structures. Green argues that it is imperative that society builds tall buildings out of wood rather than less-sustainable, more-polluting materials, and he claims that wood is a sorely underrated building material that is capable of much more than is typically asked of it.

His report (PDF) sets out a blueprint for the design of wooden structures up to 30 stories high, capable of performing in "high seismic areas" like his native Vancouver. Unusual for the construction industry, he's shared the report under a Creative Commons license. Ars looked into the report in detail and spoke to Arup structural consultant and wooden buildings advocate Hans-Erik Blomgren for another perspective.

A PR problem

In both the construction industry and the broader public, wood suffers from something of an image problem. Among the industry preconceptions tracked by Green are the notions that wood is both weaker and more expensive than concrete, that it is susceptible to fire (and that fire-protecting it is expensive), and that it will not withstand earthquakes. Ask Joe Public and one is liable to hear that wood rots, burns, shrinks, and gives off unpleasant gas, Green reports.

Such associations haven't materialized from the ether, Hans-Erik Blomgren argues. They come from people's direct experience with older wooden structures. "In commercial settings, some of the performance issues of older wood buildings such as poor acoustics—sound transmission and structure-borne noise between floors, creaking and bouncy floors—are something that building occupants won't tolerate in new build construction," Blomgren told Ars.

Timber, the massive kind

But both Green and Blomgren argue that the wooden building technologies available today are a far cry from the creaky wood-framed structures of the past. Crucially, Green's manifesto does not argue that the light-frame (or stud) construction commonly used in housing in the US should be applied to taller buildings. Instead, Green is pushing for the wider adoption of what is increasingly known as "mass timber" (or "massive timber"), a family of technologies that includes Cross Laminated Timber, Laminated Strand Lumber, and Laminated Veneer Lumber—products composed of thin layers of wood that are not only stronger than conventional timber, but thanks to their uniformity and straightness, are also simpler to design and build with.

Green calls his mass timber construction approach FFTT—a non-technical initialism that stands for Finding the Forest Through the Trees. "The acronym speaks to the idea that much of the sustainable building conversation is focusing on minutiae," Green writes in his report. "While even the minutiae contributes and is important, the big systemic change ideas are what we believe will be necessary for the built environment to tackle the scale of the climate change and housing demand challenges facing the world."

Old wood...

Building tall buildings out of wood isn't a new idea, and even without modern wood composites, wooden buildings have stood the test of time, even in areas of seismic activity. Green briefly points to the example set by Japanese pagodas built up to 19 stories tall which have proven adept at withstanding earthquakes and typhoons over centuries. The 122-foot (32.45-meter) tall pagoda at Huryu-ji was built during Japan's Asuka period (538 to 710) and is thought to be one of the oldest wooden buildings in the world. It is still standing today. A 1997 article in The Economist claims that only two or so of Japan's 500 pagodas have collapsed in 1,400 years. Some have even claimed that none have ever succumbed to earthquake or typhoon.

Blomgren looks to more recent history (as well as closer to home) for examples of wood's inherent soundness. "You might think I'd say it is not suitable as a building foundation; however, one of my favorite little known stories about the Brooklyn Bridge is that the caissons used to found the towers on bedrock down deep below the seabed are constructed out of vast quantities of untreated southern yellow pine timber," he told Ars. "These caissons are of course still in place at the bottom of the towers and functioning wonderfully. So yes, wood makes a fine foundation material in the right circumstance."

...New tricks

Ironically, Michael Green's FFTT proposal employs concrete foundations up to grade (or ground) level, though it's not required thereafter. Above the foundation, Green summarizes his design approach as "strong column—weak beam," with large panels of mass timber used to form the building's vertical structure, its lateral (sideways) load-resisting shear walls, and the floor slabs at each level. But the weak beam elements are not made from timber, and instead are steel beams designed to lend the structure ductility and added resilience when the wind blows or the ground shakes.



"On a weight to strength ratio, engineered wood products generally match, and in some cases exceed, the performance of reinforced concrete," Green writes, before arguing that wood in mass timber form meets "basically all" desirable properties of a construction material from cost effectiveness and constructibility to durability, water ingress protection, and fire protection. Indeed, Green's report contains analyses of all these criteria and more, and sets out a peer reviewed theoretical model of how it could work. Green deliberately steers clear of suggesting how such buildings might look, instead offering up a structural design to which a variety of architectural facades might be applied.

When it comes to the economic and technical competitiveness of mass timber, Blomgren echoes Green. "Right out of the gate there is evidence that massive wood systems do compete on cost with concrete and steel," he said. "The reasonable expectation is that as the supply chain and number of manufacturers for massive wood systems grow, it will only become more cost competitive."

Big, but not that big

In his report, Green makes it clear that pilot projects are required to find and test the limitations of tall mass timber buildings. When the question of mass timber's limitations is put to Blomgren, he responds cautiously:

"It is hard to answer this question without having a small complex that it infers the dominant structural materials get a free pass here because they are perceived as having limitless capabilities," he said. "I'm inclined to say the biggest limitation of wood is society's perception of it. Yes, a psychological limitation. We have a hard time accepting that a material so pedestrian, familiar, and organic can do great things for us. If I'm an advocate for anything, it is simply that timber should have equal footing—a place at the table—with other structural construction materials."

That said, Blomgren is happy to discuss the technical challenges posed by the use of wood in building structures, including mass timber, though he claims most of these limitations can be overcome with good design.

Perhaps the most fundamental is that, above a certain size, wood's inherent strength and stiffness limitations make it more or less impossible to work with. But such uses are extreme, incorporating long-span highway suspension bridges, blast-resistant buildings, and super-tall skyscrapers: uses which Blomgren estimates constitute less than one percent of the built environment.

Blomgren also highlights wood's tendency to expand and contract as it absorbs and releases moisture: a dynamism that has to be considered during design. Wood can split, splinter and potentially fail completely where structural design has fallen short. And there's its tendency to decay due to fungal infection or insect infestation when misapplied to inhospitable environments or when not adequately treated (making the use of untreated southern yellow pine in the foundations of the Brooklyn Bridge all the more surprising).

From manifesto to portfolio

Though Green calls for an FFTT pilot project to more practically test the costs and construction implications of his principle, the more general use of mass timber is already on the rise. Both Green and Blomgren cite Waugh Thistleton's nine-story mass timber Stadthaus in London as an exemplar of mass timber construction, which employs Cross Laminated Timber walls exclusively, achieving a two-hour fire rating.



Along similar lines are Sweden's eight-story, 134-apartment Limnologen (actually four buildings) in Vaxjo; the eight-story H8 in Bad Aibling, Germany (the mass timber structure was completed in three weeks); and the seven-story E3 in Berlin, which is fire-rated to 90 minutes.

Green argues much taller mass timber buildings are possible, yet construction of even these would be prohibited in British Columbia, and Blomgren's Seattle, due to local building code restrictions on so-called "combustible buildings." Such codes were largely written with light-frame construction rather than mass timber in mind. Russia limits such buildings to three stories, while the UK (perhaps surprisingly, given the conflagratory events of 1666) has no specified height limit.

Ultimately, both Green and Blomgren are adamant that mass timber is the future, but Blomgren argues that to get there will require quantifying the sustainable merits of wood and educating the construction industry and public about the high tech merits of mass timber. "Wood is not new, but the adhesives, glulam [glued laminated timber] presses, cutting equipment, carbide tipped screw fasteners, Digital documentation (BIM) to automated manufacturing, and prefabricated construction are all very state-of-the-art," he argues, before citing Metropol Parasol and the Centre Pompidou Metz as two Arup projects that, he suggests, will alter people's perceptions of the capabilities of timber construction.

On a more cautious note, Blomgren describes the delivery side of manufactured wood products as starved. "To get the ball rolling on this issue there needs to be some policy shifts and developer incentives created to open up a market," he told Ars. "Once this happens I’m quite certain that the manufacturing supply chain will grow."

The starved supply chain is not for a lack of timber. The mountain pine beetle outbreak seen in British Columbia and parts of the United State is effectively turning forests from CO 2 stores into CO 2 sources. These resultant emissions from dead tree decay could be reduced if the dead wood, which is perfectly serviceable for engineered wood products, was simply put to use.

Ultimately, Blomgren and Green agree that climate change should be one of the greatest motivational factors in a shift toward wooden buildings. "In a rapidly urbanizing world with an enormous demand to house and shelter billions of people in the upcoming decades, we must find solutions for our urban environments that have a lighter climate impact than today's incumbent major structural materials," Green writes.

Blomgren's emphasis is somewhat different. A healthy market for wood from sustainable sources increases the need for managed forests. "Wood is 49 percent carbon by dry weight, all of that carbon has been extracted from the atmosphere," he points out. "Properly managed forests can make a material difference in the carbon issue."

Whether mass timber will make the leap required to become, as Blomgren hopes, a building material on equal footing with traditional materials used for medium and larger buildings remains to be seen. To do so will, in many cases, require a policy shift. Rhetorically answering his own question of why we should build with wood, Green points to Canada's 397.3 million hectares of forest and woodland (10 percent of the world's forests), but points out that less than one percent of Canada's forests are harvested annually. "Economically, we suggest that the question of relevance should be 'why not wood?'"