Upscaling of wood bilayers: design principles for controlling shape change and increasing moisture change rate

Abstract

Wood exhibits anisotropic swelling and shrinking upon changes of wood moisture content (MC). By manufacturing bi-layered structures with adapted grain orientation in the two bonded layers, humidity-driven actuators are generated, which have the potential to be used for autonomous climate adaptive building with tile. The present study deals with design principles for upscaling the size of the bilayers and for increasing the rate of MC change and, thus, rate of shape change. Wood bilayers with widths of up to half a meter were subjected to changes of relative humidity (RH). Moisture and curvature changes were recorded. Bilayers with different widths showed curvature exclusively along their length. Next to this, the performance was compared between bilayers with and without milled-in grooves. These grooves lead to shorter diffusion paths along fibre direction for increasing the rate of MC change. The highest rates of MC change were visible for the samples with the smallest width within the first hours after change of RH. Later on, all samples showed similar rates. The milling of grooves increased the moisture change rate substantially compared to the non-milled samples resulting in a higher rate of curvature change. The increase is especially pronounced for cyclic changes of RH. This study shows that, by applying material specific design principles, the shape change of wood bilayers can be adapted and the rate of the MC change can be increased by keeping diffusion paths short along fibre direction. These principles may facilitate the use of large-scale wood bilayers as lamellae in shading systems.

Modeling and design of thin bending wooden bilayers

“…More recently, novel and innovative concepts for application as self-actuation-capable, humidity-responsive structures have arisen in applied research for biomimetic architecture [ 1 – 8 ]. Bi-layered wooden structures, capable of complex and extensive shape changes in function of a given geometrical setup and change in ambient climate have been envisaged for diverse uses including shading elements [ 7 ], climate-adaptive facades [ 9 ], or motor elements [ 1 ]. The promising and sustainable principle found its inspirations in nature where anisotropic biological materials with inherent bi-layered and differential fiber structure use humidity changes to generate movement [ 10 – 12 ].…”

Wood and the Activity of Dead Tissue

“…[ 76 ] This principle has been used for reversible systems, for instance, for shading systems at building envelopes (Figure 6B). [ 75,77 ] The diffusion‐based bending response strongly depends on the change in relative humidity, the layer dimensions, and layer thickness ratios and the selected wood species. For an informative basis on the order of magnitude, an immediate relative humidity change from 85% to 35% results in a curvature of ≈5 × 10 −3 mm −1 after 6 h for a thin bilayer specimen of a length of 120 mm, consisting of a 1 mm thick “passive” spruce layer and a 4 mm thick “active” beech layer.…”

“…Grooved elements or coupled elements can be an improvement, but in particular the latter results in rather complex installations (Figure 6B). [ 75,77 ] By eliminating the principle of reversibility, recently, the shape changing potential of wooden bilayers upon humidity changes was utilized to develop curved cross‐laminated timber (CLT) in an elegant way. [ 79 ] Since wood has to be dried during processing from the saw mill to the timber product, one can utilize this processing step to implement information into the elements.…”

Fabricate 2020

“…Self-shaping systems where shape is generated from physical material programming to actuate based on external stimuli have already been developed at much smaller scales for medical applications, micro robotic applications, and meso-scale mechanisms with a wide range of functions (Studart and Erb, 2014;Tibbits, 2014;Duro-Royo and Oxman, 2015;Wang et al, 2017;Kara et al, 2018;Kotikian et al, 2019). In architecture, similar principles have been applied for self-regulating façade systems that respond continuously to changes in the environment such as temperature and moisture (Correa et al, 2013;Holstov et al, 2015;Sung, 2016;Correa and Menges, 2017;Vailati et al, 2017Vailati et al, , 2018Poppinga et al, 2018). Most shape-morphing structures are limited in scale due to the reduced stiffness of the material required for actuation and high costs of the material and processes to produce them.…”