BTA’s post-tensioned timber frame prepares to move from lab to real life

Swiss researchers at ETH Zurich have successfully designed a building frame made out of timber, providing a cost-competitive and environmentally sustainable alternative to the concrete frames that have dominated the construction industry for over a century.

The innovation, known as post-tensioned timber frame or PTTF, was developed as part of Climate-KIC’s Building Technologies Accelerator (BTA) programme. Researchers are now looking for partners in Europe to use the PTTF structure in new construction projects following its successful deployment in ETH Zurich’s House of Natural Resources.

PTTF offers several advantages to traditional concrete frames. One significant benefit is speed during construction and the accompanying cost savings: the structure – made up of hardwood columns and beams – is easy to construct. It took just four days, for example, to build one storey at the ETH House of Natural Resources with a workforce of two people. PTTF is also an environmentally sustainable product, offering reductions in carbon dioxide emissions in the double-digit percentage range.

Flavio Wanninger, a researcher at ETH Zurich who has spent the past four years designing the model for the construction of post-tensioned timber frames, believes the structure could prove to be an industry game changer.

“Every construction company knows how to pour concrete; it’s the way it has been done for a century,” Wanninger says. “I see a lot of potential here in Europe for PTTF, mainly in the speed in which you can erect buildings.”

 In BTA’s view, the search for new PTTF customers comes at a timely moment.

“The building industry is becoming very attentive to what we’re doing in terms of innovations in the market that can really reduce C0₂ emissions,” explains Katrin Hauser, Manager of the BTA programme. “If we can find technologies like PTTF that also help our clients in the market to reduce their costs, then we will be successful.”

Designed for the market

PTTF is best suited for office or residential buildings that are up to 10 storeys high, or for adding additional storeys to existing buildings. The structure could also prove interesting in earthquake zones due to the self-centring system that snaps back to its initial position after being distorted.

Both Hauser and the ETH researchers stress that cooperation with the industry is key to the product’s success. From early on, PTTF has benefited from the support of industrial partner Häring & Co. AG. Researchers adapted the structure to suit Häring’s requirements, including cost considerations, according to Wanninger.

“They made sure that the system we built can be put on the market,” Wanninger says.

ETH Zurich researchers are now interested in teaming up with other partners in the industry across Europe, as well as pursuing new construction projects for PTTF with Häring. As PTTF moves from the lab to the market, BTA is playing a critical role in helping ETH researchers reach out to new partners and further develop their product for a wider launch.

The structure is one of BTA’s most advanced products, and was already used in the construction of ETH Zurich’s House of Natural Resources. The new office building is part of BTA’s Europe-wide network of so-called Living Labs, which provide researchers with the unique opportunity to use and monitor their innovations in a place where people live and work.

Hauser believes that this is the stage where BTA can gain the trust of decision makers. Prospective clients and partners can see for themselves that PTTF is a viable structure rather than academic theory. In addition, architects and other industry players are providing valuable feedback upon visiting the ETH House of Natural Resources, further contributing to the product’s development.

“We have a long-term vision,” says Michael Klippel, a researcher at ETH Zurich who is also working on developing PTTF. “If we get a small piece of the market, that would already be a big success.”

Build it like Lego

The concrete industry itself provided the original inspiration for the timber frame construction. While many construction companies pour concrete on site, others prefer to use prefabricated concrete columns and beams that have a tendon inside to hold them in place. In 2006, researchers at the University of Canterbury in New Zealand replaced concrete with timber. ETH Zurich researchers, in turn, adopted the timber system for the European market in 2010.

The structure, Wanninger explains, is as straight forward as building with Lego. The columns are made of ash glulam (glued laminated timber), while the beams are produced from a combination of ash and spruce glulam. The hardwood (ash) provides the crucial strength and stiffness required for connecting areas. A steel cable within the timber member connects the beams to the columns and is tightened to pull the elements together. This system has a lateral stiffness, therefore eliminating the need for a concrete core, according to Wanninger.

“The whole structure at the ETH House of Natural Resources is stabilized by the frame,” he explains.

Chopping C0₂ emissions 

 Hardwood has not been widely used up to now in the building industry in Europe. ETH researchers admit it’s not the easiest material to work with. Hardwood’s higher density and strength makes it more difficult and expensive to process than softwood. It’s also more sensitive to changes in moisture levels, though there are new technologies and products available on the market to address this issue.

 Nevertheless, hardwood is become more appealing as a building material as the number of hardwood trees in European forests is on the rise, according to ETH researchers.

 “At the moment the lifecycle of hardwood is that you cut it and burn it,” explains Wanninger. “We want to offer an alternative: you cut it, build with it, and then burn it decades later.”

This helps the environment because the timber can store C0₂ for the lifetime of the building. Hardwood has other notable, environmentally friendly features. It is widely available in local forests, lessening C0₂ emissions from transportation. As well, building with the PTTF system emits far less C0₂ – ETH researchers estimate the reduction is in the double-digit percentage range – compared with traditional concrete and steel frames.

“We are using a naturally-growing material that permanently stores C0₂ over the lifetime of the building,” says Klippel. “This is a major advantage of this material.”

Further information

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