Skyscrapers dominate the skylines of our major cities, offering more urban density and greater flexibility than smaller buildings. However, concrete- and steel-based tall structures require huge amounts of energy for their construction, which comes at a significant environmental cost. This can be mitigated by incorporating technologies such as solar power, passive cooling systems and efficient lighting into the design, but what if we could go even further and build skyscrapers using sustainable materials? Herein lies the impetus behind recent research into the efficacy of wooden skyscrapers.

Before considering the technical hurdles of constructing tall buildings from wood, perhaps the first question which should be asked is: what are the specific benefits wood can offer over concrete and steel?

Sustainability, writ large

The single most compelling argument in favor of building wooden skyscrapers is the fact that, providing the timber is sourced responsibly, they represent an opportunity to create a sustainable building on a truly grand scale, cutting down on overall CO2 output as a result.

As a recent lengthy report on the subject by Michael Green Architects (MGA) entitled "Tall Wood" [PDF] asserts:

Over the last twenty years, as the world’s understanding of anthropogenic climate change has evolved, we have seen the large impact that buildings contribute to the greenhouse gases causing climate change. Concrete production represents roughly 5 percent of world carbon dioxide emissions, the dominant greenhouse gas. In essence the production and transportation of concrete represents more than 5 times the carbon footprint of the airline industry as a whole. It is clear that the very fundamentals of what materials we build our buildings with are worth re-evaluating.

The "Timber Towers" [PDF] report produced by Skidmore, Owings & Merrill (SOM) cites the potential to compete with reinforced concrete and steel, while reducing the carbon footprint by 60 to 75 percent.

About those technical hurdles …

Structural strength

To cope with the heavy load, stresses, and vibrations a skyscraper undergoes daily, it needs to be built from material far more durable than normal timber. The SOM and MGA reports both agree that the solution to constructing tall buildings from wood rests on the use of "Mass Timber."

SOM's report defines Mass Timber as solid panels of wood, engineered for greater strength through the lamination of different layers. These panels can range up to a size of 64 x 8 feet (20 x 2.4 m), and a thickness of a few inches, to 16 inches (40 cm), or in some cases even more.

SOM's researchers prefer to add concrete connecting joints when building with Mass Timber, while MGA utilizes steel to reinforce the mass timber panels. Whichever reinforcing method is chosen, the result is a very tough building material which is worlds away from the timber framing used to build many homes, and suitable for tall buildings up to 30 stories in height, even in high seismic areas like Vancouver.

Indeed, MGA reports that in a weight-to-strength ratio, engineered wood products can often match, and in some cases even exceed, the performance of reinforced concrete.

Fire

If the thought of a very tall building constructed from wood gives you fire-filled nightmares, then you're not alone. Fire is perhaps the single biggest hurdle concerning the widespread adoption of wood as a building material for skyscrapers.

However, according to MGA, the implementation of fire safety measures such as automatic sprinkler protection, sophisticated alarms and smoke control, in addition to treatment of the wood itself, should enable a wooden skyscraper to be relatively safe in a fire scenario.

Gizmag spoke with a representative of the reThink Wood initiative, who explained that if combustion of the wooden structure does occur, its heavy mass ensures that the wood chars at a slow and predictable rate. This helps protect the wood from further degradation, maintains the structural integrity of the building, and reduces its fuel contribution to the fire, which in turn lessens the fire’s heat and flame propagation.

The wood could also be "encapsulated" in gypsum board to retard the spread of fire even further.

The reThink rep also told us that some wood products, such as the large beams used in heavy timber construction and cross laminated timber (CLT), may even perform better in a fire situation than non-combustible materials such as steel and concrete, under certain conditions.

Here we stumble into another hurdle, though. Even if the fire question is satisfactorily answered, most building codes don't allow tall buildings to be constructed from "combustible material," and part of the challenge facing proponents of wooden skyscrapers will be to seek to redefine these codes.

Cost

According to the best projections by MGA, wooden skyscrapers could be built for either the same cost as, or a sum comparatively close to, their concrete and steel brethren. Including the encapsulation method (the most expensive projected model), a hypothetical wooden skyscraper based in Vancouver was projected to cost US$300 per sq ft (roughly $3000 per sq m) for a wooden structure, as opposed to $292 sq ft for a concrete and steel structure.

The future is (almost) here

Architectural companies are beginning to wake up to the promise of wooden tall buildings, and Swedish firm Berg C.F. Møller is just one recent example of this. The company has proposed a 34-story wooden skyscraper for HSB Stockholm's architectural competition.

Berg C.F. Møller's residential building will pair wooden construction with sustainable technologies, which includes an energy-saving, glass-covered veranda for every apartment. Solar panels on the roof will satisfy some of the building's power needs, while a communal winter garden will provide residents with an opportunity to grow their own vegetables.

Could Berg C.F. Møller's vision prove a viable blueprint for getting the wooden skyscraper right? It's still too early to tell at this stage in the proposal's development.

One point on which we can be relatively sure though, the emergence of wooden skyscrapers is beginning to look increasingly likely. It now appears to be more a case of not if, but when, they will appear in our cities.

Sources: MGA, SOM, reThink Wood, Berg C.F. Møller