The HiLo roof is built with a cable-net and fabric formwork system. This system is designed to dramatically reduce the material waste that is typically involved in the construction of concrete shells. It improves on traditional formwork structures for doubly curved surfaces, which would be comprised of custom timber carpentry or milled foam, by using mostly reusable components. The cable net is spanned within a reusable timber boundary supported by conventional scaffolding. The cable net is designed to deform under the weight of the wet concrete into the desired shape of the HiLo roof. This is achieved by the non-uniform distribution of forces in each one of the cables, a distribution that is planned by computational methods developed by the Block Research Group.
A full-scale prototype of the HiLo roof was built in the Robotic Fabrication Lab of the Institute of Technology in Architecture ETH Zürich. The development and realisation of this prototype focused on the first layer of NEST HiLo's thin concrete shell and represents a key milestone for the project, as it demonstrates the viability of the proposed lightweight, flexible formwork system to form a complex concrete structure. The prototype was built with the following objectives:
To develop and test the cable net components in real scale.
To test the feasibility of the system and the on-site logistics.
To test the concrete spraying of a large and doubly curved surface through the carbon-fibre reinforcement.
The prototype demonstrated the use of the advanced computational form finding tools developed by the Block Research Group, more specifically the Best-fit Thrust Network Analysis. The constrained form finding and optimization methods used in the design of the cable-net were able to negotiate structural requirements, architectural and fabrication constraints, such as respecting the glass facade lines, while minimizing the number of cables and nodes.
The prototype was a successful development of the cable-net and fabric formwork system, perhaps most importantly of the node component. The node was designed to ensure that the cable had the required degrees of freedom for the shaping of the net. The node also facilitates the placing of the fabric and the textile reinforcement in their intended locations while providing a guide for the correct concrete thickness at any given point in the doubly curved shape. The node design also provides target points for the measurement of the as-built shell.
While the construction of the timber edge-beam and scaffolding supporting structure can never be perfect to the millimetre, the prototype successfully demonstrated the possibility of overcoming these imperfections by adapting the forces in the cables with the purpose of steering the shape of the shell, from an imperfect starting point towards the desired shape, by means of a control system. In collaboration with the Automatic Control Laboratory at ETH, an algorithm was implemented with the purpose of determining the amount of tension to be applied at each boundary cable, to best direct the shape of the cable-net towards the intended design. In just one round of control and re-tensioning, the shape deviations from the intended design were reduced by 50% to an average of 40mm. This is an exciting proof of concept that has many potential applications in the construction industry, from cable-stay bridges to large stadia.
The prototype was also a test of the spraying of a thin layer of concrete through the carbon-fibre reinforcement onto the fabric shuttering. The spraying technique utilised resulted in a solid concrete shell, which varied in thickness from 3 cm at the boundaries to 12 cm at the support locations). Spraying onto the fabric shuttering also gave the desired pillowing effect on the underside of the shell structure, which will provide an exciting and complex shape to the interior of the unit.
The development and construction of the HiLo roof prototype represented an ideal demonstration of the capabilities of the computation framework for architectural and structural design “compas” developed by the Block Research Group within the NCCR in Digital fabrication. From the form-finding process to the design of the node component and the control system, compas was indispensable as the main library for geometrical operations, algorithms and datastructures.
Check out how the unique concrete shell was designed, fabricated and constructed! Here!
Check out the amazing documentation by Zitronenwolf of different stages of the construction prototype:
Design and Engineering
- Block Research Group, ETH Zürich
- Bollinger + Grohmann Ingenieure
- Jakob (cables, cable-net rods)
- Bruno Lehmann (cable-net nodes and connections)
- Blumer Lehmann (edge beam)
- Dafotech (steel supports)
- Bieri (fabric cutting and sewing)
- Marti Construction SA
- Bürgin Creations
- Solidian (carbon fibre reinforcement meshes)
- Proserve (fabric)
- Doka (scaffolding elements)
- Holcim Schweiz (concrete)
- Institute of Structural Concrete, RWTH Aachen
- Mathematical and Physical Geodesy, ETH Zürich
- Automatic Control Laboratory, ETH Zürich
- ETH Zürich
- Holcim Schweiz
- NCCR Digital Fabrication
Cristián Calvo, Alessandro Dell'Endice, Philippe Fleischmann, Ursula Frik, Naida Iljazovic, Alexander Kobald, Juney Lee, Michael Lyrenmann, Ammar Mirjan, Mariana Popescu, Andreas Reusser, Matthias Rippmann, Alexander Nikolas Walzer, Hongyang Wang