Wooden skyscrapers, the future of tall buildings?

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‘Timber towers’ might conjure up images of flammable, Jenga-like structures, but they could represent the safe and eco-friendly future of construction

The world is in the midst of a tall building boom, with more skyscrapers being built now than ever before. But scratch below the surface and you find essentially the same steel or concrete structure that supported the first skyscrapers in 19th century New York and Chicago.

However, this could be set to change through the reinvention of one of our oldest construction materials – timber.

These developments are being led by Vancouver architect Michael Green and his proposal for the tallest timber-supported tower in the world – a 35-storey skyscraper that forms part of a six-tower development called Baobab in Paris. The building would be over three times taller than the world’s current tallest timber building, the ten-storey Forté Apartments in Melbourne, and the UK’s tallest, the nine-story Stadthaus. Other 30-storey-plus wooden towers are planned in Stockholm, Vancouver and Vienna.

Instead, the designs take advantage of recent innovations in “mass wood” to create vast solid timber panels that can support buildings to a much taller height than ordinary wood can.

Green’s Baobab uses Cross Laminated Timber, or CLT. This consists of several layers of timber board glued together at 90 degrees to form large structural sheets up to 40cm thick. The cross-lamination provides the material’s dimensional stability and strength.

For the Baobab project, CLT sheets would be manufactured and cut to exact sizes before being transported to site. “Their manufacture is very accurate, there’s very little work that needs to be done on site,” said Green. “Because of this, construction will be very quick. Someday I’d like to make a building where all you need is a giant allen key to put it together.”

Baobab’s structure consists of a series of these solid timber walls that travel the full height of the tower, along with timber columns and a central timber “core” housing elevators and stairs. However, not all of the structure is timber. Steel beams are used to provide additional strength and flexibility to resist sideways force from the wind, while concrete is used in the foundations and ground floor to provide a stable base for construction.

So what’s behind this shift? “The fundamental reason for me is climate change,” said Green. “We’re taking two materials - steel and concrete - that have high carbon footprints, and replacing them with a low-energy material.”

While the use of sustainable materials in construction is not new, the current battle for a more environmentally-friendly built environment focuses almost entirely on attempts to reduce the energy we use when operating our buildings: heating, cooling and lighting them. But buildings contribute to carbon emissions before they’ve even been occupied, from the mining of iron ore to create a steel beam, to the energy required to lift that beam into place on site.

It is only recently that we’ve started to grasp the huge contribution the energy used to create materials plays in a building’s carbon footprint. Concrete and steel are huge contributors to this. The production of cement – the primary ingredient in concrete - is responsible for 5% of all man-made CO2 emissions (to put this into perspective, the aviation industry is responsible for between 2 and 2.5%). Add to this the fact that high-rise are more material hungry than most due to the extra structure required to resist stresses from wind and earthquakes, and there is a real need to change the way towers are built.

Green is convinced timber is the solution, and not just because it is a low-energy material, but because it acts as a “carbon sink”.

Trees absorb CO2 through photosynthesis. When they die and decompose, or are burnt, this CO2 is released. Green believes we need to better manage this carbon-cycle, by harvesting trees early in their life and using the timber to create our cities. This, he says, would capture carbon within our buildings, whilst also creating a greater economic demand for wood, leading to more trees being planted, and more CO2 absorbed.

“If we manage our forests, if we harvest trees carefully, we can actually capture carbon as a building material. Then we can replant the trees.” He said.

Through this method, suggests Green, the timber used in Baobab would store an estimated 3,700 metric tonnes of carbon dioxide.

The problem with Green’s theory is that if we dispose of or burn the timber structure at the end of the building’s life, all the sequestered carbon is released. But here too, Green has a solution: disassembly and reuse.

The prefabricated construction of the building can be reversed once it is no longer needed, with the tower dismantled, and the CLT sheets used in other buildings. Even taking this into account timber has other environmental benefits. A timber tower would be far lighter than a concrete one, which would mean less material is required in the foundations.

While timber makes a compelling case environmentally, we all know that wood burns, while steel and concrete do not, raising questions about its safety. However, there’s a huge difference in terms of physical properties between mass timber, such as CLT, and the lightweight timber frames that are perceived as being susceptible to fire.

When mass timber is exposed to fire, the outer layer starts to burn and form charcoal. The charcoal layer forms a protective barrier, soaking up heat and insulating the interior timber. As exposure to fire increases, this layer grows, creating even more insulation, and further slowing the burning rate.

In timber towers the thickness of the charcoal layer that would form in a fire can be predicted. To protect the structural integrity of the timber, this same thickness of wood can be added to the CLT sheets. This would form a sacrificial timber barrier which would char in a fire, and in doing so, protect the material within.

“It’s mother nature doing what she does best,” says David Barber, a principal fire protection engineer at Arup in Washington DC, and an author on a report on the fire safety challenges of wood tall buildings. “In the aftermath of a forest fire you’ll see trunks of trees that are black and charred. The timber inside the charcoal layer is still alive – after a fire you will see regeneration. This is the same physical property we utilise in timber towers.”

Somewhat unexpectedly this even puts mass timber at an advantage against steel in terms of fire resistance.

Steel starts to lose some of its structural strength at around 550 – 600C (1022-1112F). Since typical fires burn at between 800 and 1000C (1472-1832F), steel buildings require fire protection. This can take the form of intumescent paint, which swells when exposed to heat, or other insulating materials such as gypsum or concrete cladding.

“CLT doesn’t need fire protection. The thickness and inherent mass of the wood provides the performance. This gives you an inherent reliability,” notes Barber.

While Green’s 35-storey Baobab may become the world’s tallest timber tower it is unlikely to stay so for long.

Skidmore, Owings and Merrill, architects of the world’s current tallest skyscraper, the 163-storey Burj Khalifa in Dubai, have designed a theoretical 42-storey timber tower in Chicago. This would be supported by a hybrid structural system of solid timber walls and floors with supplementary concrete beams. They’ve shown this hybrid system would reduce the building material carbon footprint by 60 – 75%, as compared to a concrete-only structure.

“This composite system has no theoretical height limit,” suggests Benton Johnson of Skidmore, Owings and Merrill and lead investigator on the study. “In practice, the limiting factor on height will be the economics of the project or municipal restrictions.”

Green has similarly lofty ambitions. He’s currently undertaking research to explore whether the 110-storey Empire State Building in New York could have been constructed with a timber structure, instead of steel. His solution, like Skidmore, Owings and Merrill’s proposal, is a hybrid system combining mass timber alongside steel and concrete.

“One of the unique problems with wood buildings is we need to keep the columns a bit closer together than you’d usually expect in a tall building,” he said. “The columns in the Empire State Building are already close together. If we replace these with wood we absolutely could build to 110 storeys.”

The research is only conceptual at the moment, suggests Green, but such heights are unlikely to be realised in the near future. Regardless, he is convinced that timber is the material that will transform skylines around the world. “When I said seven years ago we could build three storeys with wood people thought I was crazy. Today we’re seeing timber tower proposals every month.”

To read more, got through the link mentioned below:

http://www.theguardian.com/artanddesign/2015/jul/07/tree-houses-are-wooden-skyscrapers-the-future-of-tall-buildings