Critical mass: taller timber towers

6 July 2015

After decades of building only in steel and concrete, mass timber is now being used by architects to build ever-taller towers across the world. Greg Noone talks to Andrew Waugh, founding director at Waugh Thistleton; and Benton Johnson, engineer at Skidmore, Owings and Merrill, about how the oldest building material is poised to transform how we think of the modern high-rise.

Andrew Waugh is pleased with what he sees. "We're on site at the moment with the biggest single mass-timber project in the world," the Shoreditch-based architect says, standing just far enough away from the site to be heard clearly, but not enough to be fully removed from the cacophony of drills, sawing and shouting builders. "3.2 trees are being used here per person. That's what I like."

None of this would be possible without the advances made in timber construction techniques over the past decade. The project, currently named Dalston Lane, is being built primarily out of cross-laminated timber (CLT), an engineered wood product wherein several layers of treated spruce or pine are fixed together at right angles with glue to create prefabricated panels with high tensile strength. The innate durability of the panels means that architects can now build with wood at much greater scales than previously possible.

Waugh's firm has made its name by building with treated wood, otherwise known as mass timber, in commercial and residential spaces. Before Dalston Lane, the studio was known mainly for its work on the Stadthaus, a block of flats in Hackney built almost entirely out of mass timber in 2008. At nine storeys, it was the tallest wooden residential building in the world and, although it has since been surpassed, interest in how it was built remains strong.

"There is not a week gone by where we haven't had an enquiry from an architect or engineer about details or drawings, and we've always just given those out," says Waugh. "The implications of this building internationally have been phenomenal. I've got government officials from Oregon coming into the office next week, and we have a visit from the minister of housing of South Korea in a few months."

Timber in the frame

“A lot of the time, the conversation stops at discussion of the carbon footprint. We don’t tend to talk about enhanced air and water quality, which are by-products of the sustainable forests.” 

Indeed, building in mass timber has piqued the interest of firms eager to flaunt their environmental credentials and that of major media outlets normally removed from architectural discourse.

In November last year, USA Today hailed the construction of a ten-storey high-rise in Melbourne and a 14-storey tower in Norway as indicative of a growing interest among architects in sustainable solutions to demand for residential space. In May this year, citing designs put forward for a 34-storey high-rise in Stockholm's Västerbro neighbourhood and a 35-storey timber tower in Paris, The Times pronounced that "the concrete jungle [might] be turning into a wooden forest".

Such attention has built upon the efforts of an increasingly vocal minority within the architectural community, led by the likes of mass-timber evangelists such as Vancouver-based Michael Green, the reThink Wood group and Waugh himself, to promote the structural and environmental advantages of building in mass timber.

In 2013, this trend was considered by the Chicago branch of architectural giant Skidmore, Owings and Merrill (SOM) enough to merit a study on just how high architects could potentially go with a mass-timber building. The result was the Timber Tower Research Project, and SOM engineer Benton Johnson has been busy promoting the firm's findings ever since.

Primarily known for building steel skyscrapers like One World Trade Center and the Burj Khalifa, the firm has always been interested in new ways to increase the efficiency of structural systems. SOM's research indicates that a mass-timber high-rise can be just as structurally sound as its steel and glass variants up to 42-storeys, so long as reinforced concrete is used to secure the tower's structure at the points of greatest pressure. The Timber Tower study has also successfully showcased the priorities set to be incorporated in SOM's future designs. "Going forward, we're now looking at how we design these buildings with not only cost in mind but also their carbon footprint," says Johnson.

Wood for the trees

Cutting down the very trees that absorb CO2 to reduce a carbon footprint seems counterintuitive - until one considers that a building made out of mass timber continues to act as a carbon sink long after construction has finished. "It takes less carbon to transport mass timber to the construction site than it does concrete or steel, and the dry weight of wood material is about 50% carbon by weight, so it sequesters carbon in that material for the life of the building," explains Johnson.

Contrast that with the environmental record of concrete and steel. In total, the manufacture of both comprises around 8% of the world's greenhouse gas emissions. According to the US Environmental Protection Agency, global energy supply consists of 26% of total CO2 emitted into the atmosphere, with another 17% as a result of deforestation.

According to Johnson, the potential for mass timber to help lower emissions is a conversation that has hitherto been muted. "A lot of the time, the conversation stops at discussion of the carbon footprint," he says. "We don't tend to talk about enhanced air and water quality, which are by-products of the sustainable forests that this timber is felled from, or enhanced soil protection. That's something you don't really see in a concrete quarry."

For Waugh, it is for this reason that designing with mass timber is so vital. "When you build with wood, the construction process is completely predictable," he says. "To obtain your material, you plant some seeds and watch it grow out of the ground. In the end, you're not digging open-cast mines and heating the end result to 1,500°C for five days."

Part of that undoubtedly means debunking the myths over the vulnerability of mass timber buildings to fire. Every freewheeling arsonist knows it's easier to set light to a block of wood than it is a steel girder but, for Johnson, this issue should not be an overriding concern so long as the architect plans effectively for the eventuality.

"Fire safety is provided in building codes, and abiding by those is very achievable in timber construction," he explains. "Like other buildings, you compartmentalise fires into separate rooms and choke them off. You can also enhance the amount of fire rating that a material is going to get through charring or additional encapsulation."

According to Waugh, the inherent fire-retardant properties of mass timber buildings have already been demonstrated by several recent incidents.

"Wood is actually very popular with fire brigades because the ensuing blaze is very predictable," he says. "Timber will burn at 0.76mm a minute, but you don't know when steel will buckle or concrete is going to explode. There was a recent fire at the University of Nottingham where a big timber building burnt down. All the wooden columns were standing after 17 hours of fire, while the steel and concrete surrounding it was just destroyed. Now they're rebuilding in timber - at the insistence of the fire service."

“There is not a week gone by where we haven’t had an enquiry from an architect or engineer about details or drawings, and we’ve always just given those out. The implications of this building internationally have been phenomenal.” 

Deserving few

Despite evidence like this, Waugh is infuriated by what he sees as a failure of ambition among the wider architectural community.

"We're always taught that architects and engineers are these forward-thinking professionals, natural optimists," says Waugh. "Yet it seems that so much of the profession is mired in the particulars of form, or what colour the building is, rather than actually understanding what the implications of their building will have on the planet and on the future of humanity. And architects that are interested in the environment? Marginalised. They do bird-watching huts."

In the US, Johnson has seen a different problem emerge. Currently, inflexibility in US building codes drastically limits experimentation in mass timber.

"Unlike other parts of the world, our codes are more geared towards a prescriptive approach, meaning if you do A, B and C, then you're allowed to build your building. So right now, we're generally limited to mass-timber projects in the four to six-storey range," says Johnson.

Overall though, he is hopeful. "When you introduce the concept of mass-timber construction to other architects and engineers, there's a lot of interest in it," he says. "And when you see that kind of interest, you kind of feel like it's only going to be a matter of time before it catches on. The challenge really is not convincing them, but the owners and developers."

Just how much time remains uncertain. Half of the world's population now lives in cities, with that figure forecast to increase to three quarters by 2040 - three billion people will need a new home by then. A wholesale adoption of mass timber in 35 years is hard to imagine, yet the example of the Stadthaus and the multitude of plans for other timber developments at least demonstrate the potential for more imaginative combinations of materials.

Whatever happens, as the world continues to warm and the effects are increasingly felt, we will end up with the buildings that we deserve.

Artist’s impression of Dalston Lane, designed by Waugh Thistleton...
Skidmore, Owings and Merrill’s study found mass-timber high-rises could be structurally sound...
The firm’s own Dewitt-Chestnut Apartments in Chicago was used...
Like the Stadthaus before it, once completed, Dalston Lane will become...

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