Wood is a renewable resource and a popular, sustainable construction material. Complex architectural designs featuring curved or twisted structures, however, pose increasing challenges for the wood construction sector. Reshaping wood currently utilises large machines that consume substantial amounts of energy to press the building elements into the required shapes.
In a study recently published in Science Advances, researchers from ETH Zurich and Empa presented a process that has the potential to replace these inefficient mechanical reshaping processes. In collaboration with colleagues from the University of Stuttgart, they have developed an approach that makes solid wooden construction elements bend themselves into a pre-set shape without the use of external mechanical force.
The self-shaping process is based on the natural swelling and shrinking of wood caused by the moisture content of the material. When damp wood dries, it contracts more strongly perpendicular to the grain than along the grain. This warpage is normally an undesirable effect, but the researchers have taken advantage of the property by gluing two layers of wood together so that their grain directions are opposed to each other. This bilayer panel constitutes the foundation of the new method.
“As the water content of the bilayer panel decreases, one layer shrinks more than the other. Both layers are firmly glued together, so the wood bends,” explains Markus Rüggeberg, who is affiliated with both Empa and ETH and led the study. Depending on the thickness of the layers, grain direction and moisture content of the wood, the researchers can use a computer model to calculate the precise warpage of the building component during drying. They have dubbed this process “wood programming”.
Wooden bilayers are produced flat, with a high moisture content and opposed grain directions (L, R, T) to ensure suitable warpage during drying. The warping angles are calculated using a material model. (Illustration: Philippe Grönquist / ETH Zurich)
Once a bilayer panel has reached its target shape, it can be glued together with similarly curved panels in a process called lamination. This allows the research team to obtain the required material thicknesses for use as cross-laminated timber, which always consists of multiple layers. A wooden component produced in this manner remains dimensionally stable even in fluctuating humidity.
“Our approach enables a wide range of shapes and curvature radii. The concept of programming wood opens up new architectural possibilities for this regionally available, renewable construction material,” says Philippe Grönquist, the lead author of the study who works at both institutions and has dedicated his doctoral thesis to this topic. The researchers have submitted a patent application for their method.
Grönquist P et al. Analysis of hygroscopic self-shaping wood at large scale for curved mass timber structures. Science Advances (2019), doi: 10.1126/sciadv.aax1311