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Free-flowing rooftops on the horizon

A breakthrough in flexible formwork could make complex curving geo­metric forms more affordable

Less than 1.5 tonnes of formwork has 20 tonnes of concrete moulded on to it. Top CNC cut timber formwork acts as the frame for the cablenet.
Less than 1.5 tonnes of formwork has 20 tonnes of concrete moulded on to it. Top CNC cut timber formwork acts as the frame for the cablenet.

What: Lightweight flexible formwork 

Where: ETH Zürich

Researchers in Switzerland have developed a lightweight flexible formwork which they claim could make complex curving geo­metric forms much cheaper and faster to build.

A team from Zürich’s university for science and technology (ETH Zürich) worked with industry partners to construct an ultra-thin, self-supporting, double-curved concrete roof inspired by the thin shell structures made famous by Spanish architect Felix Candela.

The 160m², 7.5m high structure was a 1 to 1 prototype (now dismantled) for a new rooftop apartment to be built in Dübendorf next year. It comprised an inner concrete layer, covered by heating and cooling coils and insulation, and a top layer of concrete covered by thin-film photovoltaics.

The inner layer of the shell was formed by spray-applying concrete to a stretched cable net with an underlying a polymer textile layer.

Researchers claim the construction technique can cut the cost of formwork, including the non-reusable custom-fabricated timber or milled foam normally associated with constructing complex curved geometric structures. Just 800kg of material, including a 500kg cable net and 300kg of textile, was required to support the 20 tonnes of wet concrete.

Professor Philippe Block from the Institute of Technology in Architecture at ETH Zürich told RIBAJ: ‘Positively or negatively double-curved geometries are normally prohibitively expensive to make and are therefore only used for projects with big budgets. Most of the cost is associated with building formwork and support scaffolding and the associated labour. We wanted to prove it was possible to realise these efficient forms, and create something as thin or thinner than what Candela did in Mexico, using an efficient and cost-effective lightweight forming system.’

  • Optimised viscosity concrete is then poured over the structure.
    Optimised viscosity concrete is then poured over the structure.
  • The steel cablenet formwork, position calculated by algorithm, is then installed.
    The steel cablenet formwork, position calculated by algorithm, is then installed.
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The cable net was designed to take on the desired shape under the weight of the wet concrete, based on algorithms developed in collaboration with the Swiss National Centre of Competence (NCCR) in digital fabrication.

An initial algorithm worked out the non-uniform pre-stresses that had to be applied to the net to cause it to settle in a specific way. A second set of ‘automated optimisation algorithms’ calculated the additional stresses required on different cables to adapt the net to real-world conditions involving inaccurate construction tolerances or movement in timber edge beams to which the cables were fixed.

‘The second set of algorithms sorted through about 90 trillion options to tell us precisely how to correct the net by tightening or loosening the turnbuckles on the edges to redirect the cables to hit the correct geometry and ensure we don’t overstress members,’ said Block.

The technique can be applied to any pre-stressed structure that involves structural dependencies between various links, he added, including hanging forms, cable stayed bridges etc. ‘A known challenge on large scale civil engineering projects, such as cable-stayed bridges or large cable roofs on football stadiums, occurs when the geometry does not match the design simulation,’ he explained. ‘There are so many cables, it is difficult to know which to pull on to effectively get you to the geometry you need without overstressing other elements.’

Scientists worked with cement producer Holcim Schweiz to determine the correct concrete mix, which had to be fluid enough to be sprayed and vibrated, yet viscous enough to adhere to the fabric shuttering without slippage.

Construction of the prototype took around six months, but refinements to the process should mean the actual apartment roof will be completed in just eight to 10 weeks.