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Timber dome as a slot-connected shell structure

Thanks to new software developed by TUK’s Digital Timber Construction (DTC) group headed by Professor Christopher Robeller, complex timber structures and components can now be simply designed, fabricated and slot-connected together out of individual pieces just like a jigsaw puzzle. The computer program calculates the number and shape of each piece and decides how they are fitted together. It takes into account factors such as structural analysis, geometry and jointing, which ultimately determine the strength and stability of the end product. Then a CNC milling machine cuts the timber pieces to the required shapes, which are then fitted together manually in accordance with the assembly instructions.

Cross laminated timber prototype
The TUK team first used this process to create a dome with a diameter of 4 m. It took only a few hours to assemble the dome from 58 pieces. Spruce cross laminated timber was used for the individually manufactured segment plates. This standardised material is relatively cheap and has a very good strength to weight ratio. The Xshaped connectors are made from hardwood: industrially manufactured standard elements had not previously been considered for free-form or shell structures. The software calculates how, where and in what direction the connectors should be positioned to produce stable and strong segments. The plates, which butt up to one another at a shallow angle, are drawn together to close the gap by the shape of the beech connectors as they are inserted.

Connections between neighbouring plates at very shallow angles of between 150° and 175° are particularly complicated to achieve. Earlier methods involving metal plate connectors cannot be used for free-form structures because of the individually shaped elements. Positive-fit timber connections, such as box joints, additionally require a large number of screws to resist bending moments and out-of-plane shear forces. The wedge-shaped beech connectors have proved ideal in terms of design, manufacture and construction: in combination with precisely CNC-prefabricated segment plates, they enable quick and efficient assembly and straightforward recycling of the monomaterial structure. The computer models used by the researchers can currently simulate actual structures with column-free spans of 15 to 30 m and the use of different types of wood such as birch. The results so far appear to be very promising. The group is looking to collaborate with further partners, architects or clients to implement the idea on a first architectural project.

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DETAIL 4/2021

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