A hybrid flax fibre/CLT roof for a prototype pavilion in Germany enables column-free spans and validates the structural application of a bio-based alternative to timber
A first-of-its-kind hybrid cross-laminated timber (CLT) and natural fibre construction system forms the wave-like roof of an experimental building designed by students at the University of Stuttgart in Germany.
The Hybrid Flax Pavilion was developed by the university's Institute for Computational Design (ICD) and Institute of Building Structures and Structural Design (ITKE) for a garden show in Wangen im Allgäu on the banks of the Argen river.
The 380m² exhibition building features a seating and display area and an inner courtyard ‘climate garden’, all surrounded by a full-height glazed facade.
The gently undulating circular roof is formed from alternating ‘spokes’ of CLT panels, and CLT panels reinforced with robotically wound flax fibre, to enable an expansive column-free space.
The lightweight solution was developed to show how significant demand for timber, a scarce resource in some parts of the world, needed to deliver low carbon buildings, can be met using alternative natural materials.
Professor Jan Knippers, head of the ITKE, said: ‘The hybrid flax pavilion showcases how fibres from annually growing plants, in this case flax, can be used as structurally active components. Structural support provided by the fibre body made it possible to reduce the thickness of CLT panels from 320mm to 120mm, thus reducing the consumption of precious wood to approximately one third.’
The primary framework for the roof comprises 44 CLT panels, 20 of which are hybrid flax fibre/CLT panels spanning a 8.6m space between linear supports.
The fibre ‘cage’ for each panel is screwed into the underside of the CLT and mostly handles tension loads, while the timber manages compression forces and creates a surface for a roof enclosure. The resulting structure has the strength and stiffness needed to support high snow loads encountered in the foothills of the Alps.
The hybrid panels require multiple layers of flax fibre reinforcement to achieve the necessary structural performance. A primary ‘spine’ layer aligns with the main beam direction; a ‘fan’ layer gradually disperses loads to the edges and visually dominant ‘lattice’ layers create a uniform fibre mesh. In addition, two corner layers boost reinforcement in structurally-critical areas.
A 5-axis robot milled the panels to include chamfered edges that continuously change in angle to match the varying orientations of the fibre connections, plus holes for facade connections.
An integrated and iterative design process saw multiple academic and industry specialists collaborate in software to design the hybrid components, taking into account interfaces with conventional building elements, such as the facade and roof, and related geometric and constructional requirements.
This enabled adjustment and optimisation at every stage of development and reduced the design, manufacturing and construction process to just 12 months. Full-scale load tests were carried out to calibrate finite element models and check structural integrity. Onsite assembly of all 44 roof elements was completed in eight days.
According to Knippers, the flax/CLT prototype showcases what regenerative architecture, based on annually-renewable plants, as an alternative to timber, could look like. ‘Here we use fibres made of flax, a plant that is cultivated in central Europe, but in other parts of the world it could be hemp, jute, coco, sisal or others,’ he said.
The use of flax is significant to this region of Germany, having historically been processed by the local textile industry, and an old spinning mill was renovated as part of the garden show.
Other innovative features in the pavilion include a geothermally-activated floor slab made from recycled concrete and CO2-reduced cement, designed to ensure year-round indoor comfort with minimal heating. The climate garden provides a source of natural cooling and ventilation.