“Timber Tower” Design Passes Strength Test

With little to no exception, all urban high rises have steel, concrete, or stone structures- since 1950, the former two especially. However, exciting new developments could lead to these buildings’ ranks being joined by more organic counterparts: wooden towers. Also known as plyscrapers, this new breed of structure will not only add variety to skylines but also, in the right environments, drastically cut down the environmental costs of buildings. Steel and concrete production cause high carbon dioxide emissions in both their production and, when a building is demolished, their disposal. Wood, if sustainable deforestation models are followed, has the potential to both reduce and store emissions, so major players in the construction industry are exploring its usage.

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Part of SOM’s structure being tested

One group experimenting with timber is architecture and engineering firm Skidmore, Owings, and Merrill (SOM). SOM have been the dominant force in skyscraper design for decades, their claims to fame including the Sears Tower, Freedom Tower, and Burj Khalifa, and are currently running tests on their design for a template wooden skyscraper. Their most recent trial, done in cooperation with Oregon State University, showed that the structural system they have developed can support eight times its design load- a massive success that will hopefully pave the way for actual construction. Though SOM can claim much of the credit for this success, all current “plyscraper” projects and experimentation have their genesis in the research of Vancouver architect Michael Green, who, in a selfless and progressive gesture, made his findings available for free in the hope of encouraging timber construction.

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A diagram by Green showing the construction of a small wood tower

At the core of Green’s model for tall wood buildings are mass timber panels, a wood product in which planks are layered at right angles before lamination. This type of assembly allows each panel to have very large dimensions and strength in all directions. Still, mass timber panels are not as strong as concrete, so the floor plan of a wooden tower would look slightly different than today’s typical skyscraper. These almost always rely on a solid concrete core and thinner exterior columns to bear the brunt of the dead load. Green postulates that a tall wooden building, while retaining a timber core, would need additional thick load-bearing walls on or near the tower’s exterior. While this strategy has some drawbacks, like decreased flexibility or window space, it spreads the mass of the structure, making it more resilient in the face of lateral loads (like wind) or seismic activity.

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A plan for a wooden skyscraper with load-bearing walls on either side of the double core

Of course, even if the structure can hold itself up in normal circumstance, any reasonable person will worry about the threat of fire and decomposition- but Michael Green assures us that in these respects a wooden building is no more unsafe than a steel-and-concrete one. Treating the wood, though expensive, removes the risk of decomposition for as long as the building’s lifespan. Fire resistance is another positive feature of the mass timber panels, which can be assembled to have an outer layer that, rather than slowly burning, quickly chars, which protects the wood beneath it, at least until a point at which even a steel building, its beams and columns weakened by heat, would also collapse. Despite this protective feature, Green advises that sprinklers should be incorporated liberally throughout the design.

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A diagram displaying the “sacrificial” charring layer. Click to Expand.

Green has incorporated his research into a design for a thirty-meter-tall apartment block in Vancouver. As the interior wood is vulnerable to the elements during construction, most of the components are prefabricated offsite so that large chunks of the structure can be quickly assembled and sheltered. While this edifice is projected to be the tallest wooden building in North America, it may soon be dwarfed by a project by SOM, who, in designing their structural system, picked up where Green left off. Their project involves the use of “supplementary reinforced concrete” at certain points in the structure, like joints, where there will be high stress. This sort of construction allows SOM to scale back on the number of load-bearing walls that Green recommended, while increasing the height to over thirty stories.

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Green’s Vancouver block, currently under construction

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SOM’s concept for a higher “plyscraper”

Plyscrapers should be a boon to the environment- as long as they are constructed intelligently and with some restraint. They should be built only in cities near heavily forested areas which are compatible with sustainable logging- if these factors are not strictly regulated, the environmental gains could be negated by the transportation of the material or the destruction of forests. There are other hurdles, such as the high cost of the scheme and the current lack of zoning regulations concerning mass timber panels. While such regulations are present in Canada, they are not in the US, so tall wood construction in this country could be stalled for several years while the government draws up laws. Nevertheless, research into wooden structures will certainly continue and hopefully revolutionize skyscraper construction in the near future.

 

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