Desired deformation

Copyright: ICD/ITKE University of Stuttgart

Copyright: ICD/ITKE University of Stuttgart

Since last week there is a unique wooden building in the Remstal near Stuttgart: a tower made of self-formed spruce boards. The method, which has been developed at Empa and ETH Zurich, uses the natural swelling and shrinking of wood under the influence of moisture and thus enables a new and unexpected architecture for the construction with the renewable and sustainable resource of wood.

The 14-metre-high tower near Urbach sits enthroned in the middle of the Remstal valley as an exhibit for the Remstal Garden Show 2019. It consists of a total of twelve wooden panels and owes its curved form to a method developed at Empa and ETH Zurich. Curved wooden constructions are not new, but how the individual parts of this building were created is groundbreaking. The two researchers Markus Rüggeberg and Philippe Grönquist from Empa’s Cellulose and Wood Materials lab use the natural behaviour of wood and its reaction to humidity.

Rüggeberg explains that the team was faced with the challenge of upscaling to meter-sized components, which they have now successfully mastered: "Now we can even predict how strongly such large wood elements bend under the influence of moisture," says Grönquist, who is dedicating his doctoral thesis to this topic. The Empa/ETH team and colleagues from the University of Stuttgart used these findings to calculate exactly how which wood will bend during the drying phase. The principle is actually simple, said Rüggeberg. Two layers of wood are glued together. The orientation of the fibres is important. When the moisture content in the wood changes, one layer swells or shrinks while the other remains rigid. However, since both layers are firmly connected, the wood bends. And depending on the angle at which the fibres of the wood lie on top of each other, a different bend occurs.

Deforming wood is not new. In contrast to the "natural" method of Empa and ETH Zurich, however, heavy machines are generally used to press the wood into the desired shape using a great deal of energy. As part of the Urbachturm architectural project, researchers leave the bending to the wood itself, i.e. without any additional force being applied. The corresponding wooden parts are dimensioned and glued together in such a way that the desired curvature is created during drying, according to the calculation. After drying - and when the wood has been "shaped" - several curved components are laminated. This second bonding prevents further deformation and at the same time provides the necessary wall thickness of the elements.

The revolutionary method was developed at Empa as part of an Innosuisse project together with Blumer-Lehmann AG and researchers from the Institute for Computer-Based Design and Building Production at the University of Stuttgart. The project runs until the end of August; further results are to be published at the end of the year. "The tower shows in a unique way which innovations are possible in timber construction", says Rüggeberg. "Wood as a sustainable and ecological resource is thus gaining more and more attention. Ideas for follow-up projects already exist.

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