vol 7, issue 9

Wooden high-rise buildings

17 September 2012
By Mark Harris
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LCT One in Dornbirn, Austria

LCT One in Dornbirn, Austria: an eight-storey ‘hybrid’ building made  from both wood and concrete

The Open Academy school, Norwich

The Open Academy school in Norwich, the UK’s largest cross-laminated timber building

Eco-friendly engineered wooden panels are helping architects reach for the ply.

"Three years ago, when I said we could build 30-storey buildings from wood, the head of wood engineering at one of the UK's biggest engineering companies said that wasn't going to be possible, ever," says Michael Green, an architect based in Vancouver, Canada. "People just thought I was crazy."

Some still do. At first glance, wooden skyscrapers might seem like a recipe for disaster, a Hollywood B-movie of all-consuming fire or catastrophic collapse waiting to happen. But with the recent completion of an eight-floor wooden office block in Austria, a nine-storey residential project in London and several taller buildings on the drawing board, the 'plyscraper' is fast becoming reality.

For a start, forget pioneer-style log cabins or the timber-framed McMansions that are popular in much of the US. Today's high-tech high-rises are assembled from solid panels made from multiple layers of wood and other materials. The production process aims to remove natural variations in the wood, resulting in composite panels that can consistently meet the structural, thermal, acoustic and fire safety criteria for use in massive buildings. The panels are then cut to fit in computer-guided mills before being shipped to the building site and simply dropped into place by crane.

"We want to make buildings like car companies make cars, or computer companies make computers, using an industrial process and a systems approach," says Nabih Tahan, an architect with Cree, the Austrian firm responsible for LCT One, an eight-storey 'hybrid' building made from both wood and concrete. "To get good performance out of buildings, you can't keep making them chaotically piece by piece on site, cutting things in the rain. You need to design them right and have a step-by-step organised process."

Strength in glulam

LCT One has a central concrete core housing its lifts and utilities. From a concrete foundation, vertical posts of glulam (glue-laminated wood) rise up, floor by floor. Glulam is made by sticking together smaller pieces of wood to create structural members with a greater tensile strength than steel, and that can resist compression better than concrete.

Cree's glulam posts support hybrid floor panels made from more glulam beams embedded in reinforced concrete. The glulam adds strength for little weight, while the concrete gives fire protection and sound insulation. Cree claims that its off-site manufacturing system cuts construction time in half, guarantees quality and, most importantly, slashes the building's carbon footprint.

"It is much more sustainable to use wood than concrete and steel," says Tahan. Today, the construction industry accounts for about 6 per cent of energy use worldwide, and a similar proportion of greenhouse gas emissions. Concrete, for instance, emits nearly its own weight in carbon dioxide during production. The raw material for wooden skyscrapers, on the other hand, literally grows on trees, absorbing carbon from the atmosphere as it does so.

"Wood is the only building material grown by the sun and the only one that is truly rapidly renewable, if it's harvested and managed correctly," says Green. "In the past, we didn't look at forestry very favourably from an environmental point of view. What's changed dramatically is our ability to forest and harvest in a more sustainable way. Forestry companies have come on board with the idea that they need to be much more responsible, which in turn makes wood a more relevant material."

In building the world's tallest modern timber structure, the nine-floor Graphite Apartment building in Hackney, London, architects Waugh Thistleton calculated that they avoided producing 125t of CO2. If you also count the carbon sequestered in the wood itself, the building should remain carbon neutral for over 20 years.

The Graphite Apartments use even less concrete than Cree's system. Apart from a concrete ground floor, the walls and floors are made up of cross-laminated timber (CLT) panels. As its name suggests, CLT involves multiple layers of inch-thick wood planks, oriented at right angles to each other. Up to 10 layers can be glued together in a press, with even low-grade woods like spruce forming panels that are capable of supporting tremendous loads.

For the Graphite Apartments, hundreds of CLT panels were fastened together to carry the load of the entire building, removing the need for a central concrete core altogether. Dozens of CLT buildings are now springing up across the country. The Open Academy school in Norwich, completed in 2010, is currently the largest building in the UK made solely out of cross-laminated timber. Its 9,000 square metre structure locks up over 750t of carbon dioxide.

Ironically, all-wood CLT structures can end up looking very much like traditional concrete and steel structures. In the event of a fire, CLT panels are engineered to produce a blackened char that resists further burning. Even so, interior walls are often finished with fire-resistant gypsum plasterboard, while outside walls require thermal and acoustic insulation. Waugh Thistleton's latest CLT project, a seven-storey mixed use building on Regent's Canal in London, restores its eco appearance with sweet chestnut exterior cladding.

Hit the wooden heights

The race is now on to build ever higher, ever greener plyscrapers. Construction has begun on a 10-storey residential block made from CLT in Melbourne, Australia. On completion in October, the Forte building will boast rainwater capture to flush toilets, energy-saving LED lights and a locavore vegetable garden on each balcony. But like almost every wooden high-rise built to date, it also comes with a less desirable feature: a glass ceiling.

Most countries around the world regulate the maximum height of wooden buildings. In Russia, it's a mere four storeys, in Canada a more generous six and in the US (along with China, the world's biggest construction market), wooden structures can be no taller than five storeys.

"This glass ceiling on building height around wood is absurd," says Green. "These structures will perform just as well as steel and concrete ones. We have to recognise that the world is urbanising at an incredible rate and we need more big, tall buildings."

A few countries, including the UK, Norway and New Zealand, put no automatic limit on wood structures, instead relying on performance-based criteria that do not discriminate between materials. In the birthplace of the skyscraper, however, US building codes specify that all skyscrapers must be made from steel and concrete.

"These are huge industries and it's in their interest to sell their products," says Cree's Tahan. "In the past, they were able to influence the codes and the way we build to include more concrete and steel. But when you step back and look at what we now know about climate change, there needs to be a new way of doing things for large volume buildings."

Green thinks that he has one. Working with timber fabricators and consulting engineers, he has released a part-manifesto, part-how-to guide providing detailed specifications for building wooden skyscrapers up to 30 storeys high. "Cree's system is proprietary and, as a result, has been very slow to take off," he says. "I wanted my solution to be an open source system. This is important enough, about the global context and response to climate change, that it needs to be available to all architects and engineers worldwide."

Green is making his system, called FFTT (Finding the Forest Through the Trees), available for free under a Creative Commons license. FFTT is a tilt-up system to balloon-frame engineered wood panels into medium rise (up to 12 storeys) buildings, or high-rises (20-plus storeys) with the addition of steel beams. The system includes suggested materials, cost analyses, structural, acoustic and fire performance data, and full floor plans and cross-sections.

"The truth is, it's easier and cheaper to work with these materials than it is with steel or concrete," says Green. "The parts show up, pre-cut to size. They go in very quickly using basic equipment, like screw guns, that any wood construction company is used to working with."

Finding the forest through the trees

FFTT can be used for both office and residential projects, and can accommodate different varieties of engineered wood, including laminated strand lumber (LSL) and laminated veneer lumber (LVL) . FFTT's open source nature means that it can also be modified in the future as new wood products arrive.

One such innovation could be interlocking cross-laminated timber (ICLT), the brainchild of Ryan Smith, director of the Integrated Technology in Architecture Centre at the University of Utah. Smith was working on how to use the vast quantities of pine trees in the American west that had been killed by the voracious mountain pine beetle. Tens of millions of acres of forest in the US and Canada have fallen victim to this tiny beetle, which kills trees but leaves them suitable for harvesting for lumber.

"We want to find new ways to deal with the wood so it doesn't go to waste," says Smith. The affected trees are generally smaller than those traditionally used to make CLT panels, and US timber companies lack the expensive presses necessary to sandwich, glue and cure the wood into panels. Smith suggested using computerised mills to cut dovetail joints into the layers themselves, then assemble the panels like a 3D jigsaw without using any glues or fasteners.

"We've done structural testing on three- and five-layer ICLT panels and have already built our first structures from them," says Smith. "By cutting out the glue, we've improved environmental air quality, reduced their embodied energy and made something that almost any timber fabricator can produce. This product is ideal for the really aggressive carbon-neutral folks."

Further testing of ICLT panels is needed but, even once their physical characteristics are confirmed, they are more likely to find themselves in the regulatory doghouse than the next generation of wooden skyscrapers. "America is a really litigious society," says Smith. "It's really difficult to get things passed quickly. Structural insulated panels [plywood sandwiched around rigid foam] were invented here in the 1950s but didn't make it into the building code until the late 1980s."

Projects using non-approved products can still get made, albeit with the onerous legal liability falling squarely on the shoulders of designers, engineers and architects. Some companies, like Cree, have decided to pass on building wooden high-rises in the US altogether. "It's not worth trying to overcome these obstacles now," says Tahan. "We've decided to do things within the code and try growing things that way. Only 10 per cent of commercial buildings in America are currently built out of wood. There are tens of thousands of buildings up to five storeys that would meet all existing code requirements if we built them out of wood."

The future of tall wood, then, seems to lie in Canada, Europe and Oceania. Cree is working on a utility company headquarters in Austria that will be the world's largest wooden office building, with two concrete cores and an integrated hydro-electric power system. Green is hoping to announce a 16 to 20-floor residential development in downtown Vancouver before the end of the year, and plans are floating around for ambitious CLT skyscrapers in both Norway and Russia.

"Wood is waking the sleeping giant, capturing the imagination of architects and engineers who have this opportunity to redefine the face of architecture," says Green.

"It's not going to change overnight, it might take 40 years before wooden skyscrapers become commonplace, but it will happen."

Further information

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Glossary: A World of Wood

CLT - Cross-laminated timber. A panel made from (usually) three to seven layers of boards stacked crosswise and secured with glue, fasteners or interlocking dovetails. First developed in central Europe by the forestry industry to break into residential housing markets dominated by brick and concrete construction.

Glulam - Glue-laminated lumber. A structural composite beam where individual lumber is end jointed and glued in layers. Sometimes a direct replacement for steel and concrete members.

LVL - Laminated veneer lumber. Layers of wood veneers laminated together using an adhesive, and cured in a hot press. The grain of the wood runs parallel to the long end of the panel. Similar to plywood but much larger in scale.

LSL - Laminated strand lumber. A structural composite manufactured from'strands of wood blended with an adhesive. Strands are oriented parallel to the length of the beam and then compressed using a steam injection press.'Made from cheap flakes and chips of'wood.

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