‘Timber towers’ might conjure up images of flammable, Jenga-like structures, but they could represent the safe and eco-friendly future ...
“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. “The timber products we’re using have a real financial value. When these buildings have finished their life, I can imagine the salvaging of these materials for generations. I expect them to last for centuries.”
Predicting how we might use materials decades from now is challenging. But even so, we can envision a scenario at some point in the future where these timber panels would be disposed of, and their captured carbon released. 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.”