Cadenza

Cadenza is a material-efficient helical stair that showcases an innovative tectonic concept based on post-tensioned prefabricated concrete steps. This pilot project addresses the complex fabrication challenges of free-form stairs through the use of reusable polymer-extruded 3D-printed formworks. The paper presents the construction system and design-to-fabrication workflow for Cadenza. Each step features a slender 20 mm thick stepping surface that cantilevers 1 m from an off-centre stringer and is reinforced with structural ribs on the underside. Filigree topological features are achieved with an ultra-high-performance fibre-reinforced concrete mix. The steps are pre-assembled around temporary support and post-tensioned using an innovative system consisting of three helical shape-memory steel bars contained in the stringer, allowing the stair to be installed as a single element. With precise load-transferring interface details designed for dry-connection assembly, the modularity and ease of disassembly of the stair align with circular economy principles. The project demonstrates the potential of reusable 3D-printed formworks for free-form geometry, enabling the serialisation of bespoke designs with radically new aesthetics that efficiently use high-performance materials. 3D printing unlocks complex topologies and shape freedom for concrete structures, enabling not just expressive designs but also significant material efficiency, with Cadenza achieving circa 60% savings compared to a conventional concrete stair.