Structural design, digital fabrication and construction of the cable-net and knitted formwork of the KnitCandela concrete shell

Abstract

This paper describes the structural design, digital fabrication and construction of KnitCandela, a free-form, concrete waffle shell with KnitCrete, a falsework-less formwork approach using a custom prefabricated knitted textile as multi-functional, structural shuttering layer and a form-found cable net as the main load-bearing formwork. The digitally designed and fabricated textile provided integrated features for inserting and guiding elements such as cables and inflatables that helped shape the sophisticated mould. With a total weight of only 55kg, the 50m 2 formwork was easy and compact to transport. On site, the formwork was tensioned into a timber and steel rig, the pockets were inflated, and then coated with a thin layer of custom-developed, fast-setting cement paste. This paste served as a first stiffening layer for the textile, minimising the formwork's deformations during further concrete application. Fibre-reinforced concrete was manually applied onto the formwork to realise a 3cm-thick shell with 4cm-deep rib stiffeners. The novel approach, for the first time applied at architectural scale in this project, enables the building of bespoke, doubly-curved geometries in concrete, with a fast construction time and minimal waste, while also reducing the cost and labour of manufacturing complex parts.

Shell-supported footbridges

“…By following Musmeci's work in designing shellsupported footbridges, a way for realizing them meeting Musmeci's and Nervi's work was also proposed. The way of constructing the shell by first realizing a wire net in tension successively filled with mortar to obtain a light ferrocement formwork finally thickened to obtain the concrete shell is an important finding that has been successively proposed by also some other authors [7,25,35]. By instead designing a curved shell-supported footbridge, the main finding was in defining the conceptual design of the bridge, that is the intuition and definition of the shell form on the basis of the required boundary conditions.…”

“…Of course, todays the net can be made not only of steel wires but also of other fibres, i.e. basalt e/or carbon fibres [7,35,36]. Such a thin shell is however sufficiently light to be easily set-in-place by modern cranes (Figure 7a) for a span length even quite higher than that of the footbridge under consideration.…”

Opportunities and challenges for structural engineering of digitally fabricated concrete

“…KnitCandela was located inside a museum and classified as an artwork. Hence, the conducted structural verifications [96] (limitation of concrete stresses and deflections) had to prove the safety of the shell but without having to strictly follow any particular building code. The shell was considered unreinforced for these verifications (the contribution of the textile was neglected), but a small amount of glass fibres was added to the concrete mix provided by Holcim Mexico in order to ensure crack distribution and predefined capacity redistribution.…”

“…The concept was to explore how the range of buildable concrete shell geometries can be expanded by the use of novel computational design methods and a custom-knit textile as a lightweight stay-in-place formwork that allows the generation of a ribbed structure. In KnitCrete, the formwork is coated with a thin layer of fast-setting cement paste[96] that serves as a first stiffening layer for the textile, minimising formwork deformations during the application of further concrete layers. Finished KnitCandela concrete shell showing the soft textile interior and the smooth concrete exterior[96].…”

“…In KnitCrete, the formwork is coated with a thin layer of fast-setting cement paste[96] that serves as a first stiffening layer for the textile, minimising formwork deformations during the application of further concrete layers. Finished KnitCandela concrete shell showing the soft textile interior and the smooth concrete exterior[96].…”