Moisture content creates new opportunities

ARTICLE While the last generation of ‘timmermän’ in the 1920s were still building cottages, cabins and farmhouses from logs in rural Sweden, engineers on the continent were experimenting with glulam and shell structures. Despite the considerable differences, there are points of contact between traditional craftsmanship and modern computer-modelled structures that become visible with the benefit of time.

What carpenters building log cabins and structural engineers have in common is that they know their building materials and have a thorough understanding of their limitations and possibilities. Their material knowledge is intimate, and while the carpenter could assess timber quality with the naked eye and an axe, today we can physically, chemically and mathematically investigate specific material properties such as fibre structure, density, strength and elasticity. And both cases involve developing methods to circumvent or exploit the material’s properties.

What was once a constraint is now seen as an opportunity. »Winter felled and summer dried«, a carpenter in Dalarna declared in 1928, testifying to the practice of cutting the timber in winter, letting it dry for one or two summers and then building with it. It was a slow process that was subordinate to the properties of the material. However, the moisture content and movement of the timber, which used to be limitations, are now being used as an asset to produce new structures and shapes.

Digital models creates new technologies and designs

ITECH ’s Hygroshell pavilion, at the 2023 Chicago Architecture Biennial, presented a full-scale experiment on how the moisture content and drying process of wood can be utilised to create shell structures from laminated wood panels that are comparable to their concrete counterparts in the early 20th century. With digital models that take into account the anisotropic moisture properties of wood, Hygroshell was able to develop both new technologies and designs.

Using two cross-laminated panels with different moisture content and fibre direction, it was possible to create a shell structure that put itself together during the drying and shrinking process. A thicker active layer made of high-moisture-content boards was glued together with a thinner limiting layer of low-moisture-content boards. As the double-layered panel dries, it creates a curvature that follows the shrinkage of the lamellas, and this is what gives the structure its stability. The pavilion is an extreme example of how materials, form and manufacturing can be integrated and optimised.

Combining historical knowledge and modern technology

The finished result displays a unique combination of hyper-modern form generated by the material itself and solutions seemingly rooted in local tradition. The winged shape resembles a bird in flight, while the outer shell on the top of the pavilion and the outside of the curvature call to mind fish or dragon scales. The diamond-shaped scales reference alpine cladding in southern Germany and are as practical today as they ever were. The pavilion was completely prefabricated, transported in three flat packages to the construction site and joined lengthwise. As the wings unfold during the drying process, the pre-assembled scales follow the expansion of the curvature and form their tightly sealed shell, which adapts to the movement of the material relative to the moisture content.

Neither the technology nor the design would have been possible without today’s advanced analytical digital tools and models, or indeed without the special understanding of wood properties developed by master carpenters and refined by engineers. With today’s interest in and need for lightweight, durable, wide-span structures that can be easily transported and assembled, Hygroshell’s pavilion represents a further step towards a more cost-effective and sustainable solution that utilises and combines historical knowledge with modern technology.

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