Using wood's natural ability to flex when drying to create curved structures

Using wood’s natural ability to flex when drying to create curved structures
Urbach Tower, a 14 m tall building demonstrator with high curvature CLT components produced using self-shapping manufacturing technology. Credit: ICD/ITKE University of Stuttgart

A team of researchers from Laboratory for Cellulose & Wood Materials and the Institute for Building Materials, ETH Zurich in Switzerland and the University of Stuttgart, in Germany, has found that modeling the contraction of wood while drying can be used to create curved structures. In their paper published in the journal Science Advances, the group describes their simulations and a test structure they built using their ideas.

As the researchers note, manufacturing complex, curve-shaped wooden objects takes a lot of energy and produces a lot of waste. This is because machines are typically used to cut or grind off undesired parts of the wood. In this new effort, the researchers have found a way to take advantage of wood's natural ability to absorb and release moisture to make similar structures.

Wood absorbs moisture and then releases it, often leading to warping—wobbly tables and chairs made of wood that have been exposed to high humidity are ample evidence. In the , this ability is considered to be a problem to be overcome. But the team in Switzerland thought it might also be an advantage if used in a certain way.

The researchers began with the idea that it should be possible to predict the way wood warps and by how much. To find out if that was the case, they created that accounted for such factors as the type of wood and how much moisture it contained. They next reasoned that if they could get many pieces of wood to bend predictably using only their natural warping tendencies, they could use them to create desired shapes. They created computer models of wood planks made of different types of wood and ran them through simulated wet and dry periods. The researchers report that their models showed that it should be possible to create complex curved structures without resorting to machining. To that end, they designed a 14-meter they dubbed the Urbach Tower and then built it at a real site in Germany.

Time-lapse video of large-scale wood bilayer actuation. Credit: Grönquist et al., Sci. Adv. 2019;5: eaax1311

To create such curved structures, the researchers started with "wet" wood of different types—in this case, European beach and Norway spruce. A layered plank was made by gluing thin planks together. As the planks dried, one of the layers would shrink, while the other did not, forcing the entire plank to bend without cracking. After the desired shape was obtained, the would be bonded to prevent further change—and then combined with other planks that were bent in specific ways to build an entire structure.


Explore further

Urbach Tower offers view of self-shaping architecture

More information: Philippe Grönquist et al. Analysis of hygroscopic self-shaping wood at large scale for curved mass timber structures, Science Advances (2019). DOI: 10.1126/sciadv.aax1311
Journal information: Science Advances

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Citation: Using wood's natural ability to flex when drying to create curved structures (2019, September 16) retrieved 16 October 2019 from https://phys.org/news/2019-09-wood-natural-ability-flex.html
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Sep 16, 2019
Yes, but does the structure tear itself apart when it gets wet again?

The reason why wood is steam-bent and glued, nailed, or pegged into shape rather than allowed to buckle naturally for the desired shape is exactly because the aid humidity changes enough over the year that your chairs and tables would start walking around on their own. Your lazy chair would get a whole lot lazier in the winter when the relative humidity dips below 20%, and downright perky in the summer when it goes up to 40-50%

Steam bending melts the lignin in the wood, allowing you to re-set the internal tensions. With this method, the wood is bent because of internal tensions, so it's inherently unstable. It's like two springs fighting against each other.

Sep 18, 2019
Wood can be waterproofed easily enough and if they can get it to curve to shapes that normally require machining means the inherent structure can be preserved and the resultant structure will probably be stronger. But wood will still burn, which is why they don't build many buildings of it these days.

Sep 18, 2019
Wood can be waterproofed easily enough

I.e. encased completely in plastic. Not really a good option. Moisture diffuses through eventually, or the coating gets compromised and the structure warps anyways.

and if they can get it to curve to shapes that normally require machining


Wooden structures usually don't "require" machining, because wood is a composite material that is highly oriented. Taking a block of wood and machining a shape out of it compromises the grain structure - so parts that are produced by machining rarely serve a point. They're mostly done as cheap imitations.

Where curved wood is used for a proper reason, these days it's most often done by laminating veneers (plywood) in a form and letting the glue set it in shape. What these guys are suggesting is doing the same thing, but without the form, by letting the wood warp itself to approximately the right shape.

Sep 18, 2019
As the planks dried, one of the layers would shrink, while the other did not, forcing the entire plank to bend without cracking. After the desired shape was obtained, the wood would be bonded to prevent further change..

What if, as the desired shape was obtained, the second plank started warping to counter the first?

Sep 18, 2019
Finally! A woodworking article on Phys.org!

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