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Self-shaping timber could rival CLT for structural use

Words:
Stephen Cousins

New technique cuts costs by predicting the curves timber will form during drying – eliminating heavy manufacturing processes

The 14m high Urbach lookout tower installed in situ near Stuttgart.
The 14m high Urbach lookout tower installed in situ near Stuttgart. Credit: ICD/ITKE - University of Stuttgart

A corkscrew-shaped timber tower, erected at Urbach near Stuttgart in Germany, is the first example of an innovative method of construction that involves ‘programming’ wood to shape itself when it is drying.

All cross laminated timber components for the 14m-high tower were designed and manufactured flat then transformed autonomously into predicted curved shapes when dried in a kiln, in a process developed by the Institute for Computational Design (ICD) at the University of Stuttgart.

The technique exploits timber’s natural tendency to shrink as its moisture content decreases. Computer modelling simulations, developed by ICD in collaboration with Swiss technical university ETH Zurich, were able to predict the final shapes to within a few millimetres of tolerance, in line with conventional timber forming techniques.

Dylan Wood, research associate at ICD, told RIBAJ: ‘The digital predictive models developed by our Swiss research partners are able to predict the geometry to a point where the technique becomes industrially applicable. Installed panels are scientifically proven to have comparable structural performance to regular curved CLT.’

The locally-sourced spruce timber boards for the tower were first sorted based on moisture content, quality and grain direction, then glued together into 5m by 1.2m spruce wood bilayer panels to be dried in an industry standard process.

  • The amount of curve evidenced on kiln drying the CLT was predicted via software to within millimetres.
    The amount of curve evidenced on kiln drying the CLT was predicted via software to within millimetres. Credit: ICD/ITKE - University of Stuttgart
  • The tower is formed from Spruce CLT but other timbers can undergo the same kiln-curving process.
    The tower is formed from Spruce CLT but other timbers can undergo the same kiln-curving process. Credit: ICD/ITKE - University of Stuttgart
  • The timber boards were glued together into 5m by 1.2m spruce wood bilayer panels to be kiln-dried.
    The timber boards were glued together into 5m by 1.2m spruce wood bilayer panels to be kiln-dried. Credit: ICD/ITKE - University of Stuttgart
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The resulting curved parts were then overlapped and laminated together to lock the geometry in place as larger CLT panels.

‘Timber is naturally a variable material, so the end prediction always comes down to how carefully we are able to sort boards that make up the final CLT panels and how much information we have about them.  With digital technology we know more and more about each board we build with and can incorporate this into our design models. We worked with a timber manufacturing company/sawmill, Blumer-Lehmann, to record moisture levels and other information about the wood needed to fine tune the models,’ said Wood.

The process can be adapted to different species of tree and individual components can span up to 15m with a radius of 2.4m and structural thickness down to just 90mm.

  • Credit: ICD/ITKE - University of Stuttgart
  • The timber boards were glued together into 5m by 1.2m spruce wood bilayer panels to be kiln-dried.
    The timber boards were glued together into 5m by 1.2m spruce wood bilayer panels to be kiln-dried. Credit: ICD/ITKE - University of Stuttgart
  • The timber boards were glued together into 5m by 1.2m spruce wood bilayer panels to be kiln-dried.
    The timber boards were glued together into 5m by 1.2m spruce wood bilayer panels to be kiln-dried. Credit: ICD/ITKE - University of Stuttgart
  • The timber boards were glued together into 5m by 1.2m spruce wood bilayer panels to be kiln-dried.
    The timber boards were glued together into 5m by 1.2m spruce wood bilayer panels to be kiln-dried. Credit: ICD/ITKE - University of Stuttgart
1234

Self-shaping timber could challenge traditional industrial processes used to bend wood into different shapes using heavy machinery, which can be elaborate and energy-intensive.

In addition, it offers the potential to create more sophisticated and experimental structures, says Wood: ‘When building curved components with increased structural performance, construction tends to default to less sustainable materials like concrete, metal, or plastic, that are more moldable. But putting wood into that category has huge potential to make designs more economic and materially efficient. Beyond that, there are opportunities to create entirely new wood forms, for example tubular timber columns.’

However, while the technical limitations of the process ‘could be solved relatively easily,’ the biggest challenge to widespread application is posed by industry expectations around working with wood. ‘It’s a completely different way of thinking – how do you convince a carpenter, who has trained for many years only to touch timber when it is dry, to work with it when it’s wet?’ he concludes.

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