One of the most famous images of the 20th century is the 1932 photograph of workers on the RCA Building in New York eating their lunch, perched on a steel beam, hundreds of metres up in the air. How things change. Workers are now forbidden from lunching so dangerously on site (that’s right: they should be wearing hard-hats). But here’s a thought: did you also know that in future that steel beam could be replaced by one made of wood?
Architectural practice Skidmore, Owings & Merrill (SOM) understands life at high altitude. It is best known for designing some of the world’s most famous skyscrapers, including the recently completed Cayan Tower in Dubai (the world’s tallest twisted tower) and the Burj Khalifa, the world’s tallest tower.
And now SOM is pushing the boundaries with a study, just published, entitled the Timber Tower Research Project. This describes a study that has developed a structural system for a 42-storey prototype building using mass timber.
At the same time in Stockholm a collaboration between Scandinavian firms Berg CF Moller and Dinell Johansson is proposing a 34-storey residential building that will also be built predominantly from timber. There is a concrete core for the lift shaft but, aside from this, mass timber will support most of the structure and outer pillars will be made of laminated timber.
“We are considering reinforcing the wooden pillars with steel pillars at the base to limit the size of the construction. The wood and steel will form a composite,” says Berg CF Moller architect Ola Jonsson. On this design, the mass wood elements will be mainly joined with long screws and the pillars will be joined with bolts.
“We will use clean wood joints as much as possible,” says Jonsson.
These proposals, if they are taken up, could turn the industry on its head. For more than a century, steel and concrete have been specified for tall buildings, not only because of their high material strengths, enabling them to support heavy loads, but also because of their non-combustible properties.
Architects and engineers, seeking more sustainable methods, are beginning to challenge this received wisdom. Benton Johnson, the project engineer who led the timber research project at SOM’s Chicago office, says the need to address global population trends was a key motivator for the study.
Around the world, the number of people living in cities is projected to double to seven billion by 2050. Tall towers not only make the best use of limited space; with smart systems and shared services they can also be considerably more energy-efficient than individual residences.
The trouble is that the high levels of carbon that are required to produce the steel and concrete for skyscrapers effectively wipe out any carbon savings made once the towers are inhabited.
By contrast, timber is defined as a “carbon sink” because trees absorb carbon from the atmosphere as they grow. And the energy consumed for turning timber into structural- grade wood is considerably less than for producing steel or concrete.
Ola Jonsson says that an additional benefit of using timber is that it is more lightweight, requiring less structural material as a whole.
“To build sustainable buildings we need to limit the amount of material that mainly supports its own weight. Concrete is a fantastic material but it is heavy,” she says. “The total weight of the [timber-framed] building is radically less and limits the amount of foundation needed.
"Neither SOM’s prototype building, nor the Scandinavian proposals avoid steel and concrete entirely. The Stockholm tower block specifies a reinforced concrete core for the lift shaft in the heart of the tower.
SOM’s timber frame relies on concrete joints for added strength. It has a concrete topping slab on the roof, while the substructure and the first two floor levels above ground are also concrete.
Both teams concede that it would be possible, but technically more difficult, to increase the proportion of timber in their designs. Larger timber beams would be required, adding to the cost.
“We wanted to design a product that would be competitive in the market. We are using timber, but not too much. The design has to be as efficient as possible,” says SOM’s Johnson.
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The concrete in the SOM prototype performs a variety of functions. As well as strengthening and stiffening the structure, it also adds weight. “If you divide the weight of the prototype building by its volume, you have the density of balsawood,” comments Bill Baker, SOM partner and structural engineer who was involved in the project. “The concrete is holding this building down.
”Baker concedes that it would be possible to build taller than 42 storeys using a timber frame system, but probably more “onerous”.
“Here in the US, building to 30 storeys is considered to be the sweet spot (for developers). We wanted to go beyond this height just to be sure, but we’re not testing this system to the limit,” he adds.
A common perception reflected in most building codes is that timber buildings are more vulnerable to fire. But Ola Jonsson argues that timber frames are more fire resistant than steel or concrete ones.
“This is due to 15% of wood mass being water, which will evaporate before the wood actually burns. In addition, logs get charred which protects the core,” she adds.
There are also likely to be concerns that wooden floors have unwelcome acoustic properties. This could necessitate thicker ceilings, further adding to the height and cost of the structure. To overcome this problem, SOM specifies that the floors are covered with a thin lightweight gypsum concrete. “Keeping the depth of the panels as low as possible helps keep the cost down,” says Johnson.
The proposals in the USA and Stockholm are tantalising, but there are still hurdles to overcome before either scheme can reach reality.
The Stockholm tower has been entered into a competition run by HSB, one of Sweden’s largest housing cooperatives. Not only must it pass the assessment of the client, but it must also win the approval of the local population, who are being asked to vote for one of three potential designs for the site.
In the USA, SOM is now building up a shopping list of further research that will be required to develop its concept. It is seeking cities and municipalities that are prepared to alter their building codes.
“Current codes do not allow for tall timber buildings over 65ft (20m) as they require the structural system to be non-combustible,” says Baker. “We need to move from prescriptive to performance-based codes.”
Johnson adds that contractors will need to be open minded and prepared to take on a different perspective. But does he believe that timber tower building systems could become mainstream?
“Clearly. We’ve got to cover a lot of bases first, but what we’ve seen so far looks good.We’ve got something with legs,” he says.
SOM: Benchmarking against concrete
The team at Skidmore, Owings & Merrill has developed a prototype concrete-jointed timber-framed building, where mass timber elements are connected with steel rebar reinforcement through concrete joints.
The prototype is based on, and benchmarked against, the Dewitt- Chestnut Apartments in Chicago, a concrete-framed 42-storey apartment block designed by the practice in the 1960s.
On the prototype a typical floor consists of 80% timber and 20% concrete. The entire building is roughly 70% timber and 30% concrete by volume when substructure and foundations are taken into account.
SOM says that the embodied carbon footprint of a high-rise timber structure could be approximately 60-75% less than that of a reinforced concrete structure.
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