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Knit Tensegrity Shell
Knit Tensegrity Shell is a project investigating tensegrity structures that use a membrane made from knitted fabric as the tensile element. Tensegrity is a self-supporting structural system that achieves static equilibrium through the distribution of minimal, discontinuous compressive elements within a network of tensile elements.
This research project was executed while I was a researcher at the Digital Manufacturing & Design Centre at Singapore University of Technology & Design.
To materialize these ultralightweight structures, a design-to-fabrication workflow was developed that integrates 1) a simplified, simulation-driven design method for membrane tensegrity shells and 2) computer-assisted conversion of computer-aided design geometry into CNC knitting software data.
The first challenge was modeling this type of structure, a simple shell composed of three radially symmetrical "petals". Because membrane tensegrity structures are form-active systems, a form-finding approach using dynamic relaxation in Kangaroo Physics was implemented.
The strut patterning and form-finding process was informed by and calibrated with physical models. The Kangaroo form-finding parameters were adjusted using measurements of length, height, and radius of curvature of 3D scans of knit shell physical models with generic strut patterns.
By parameterizing the 2D strut pattern, stretching properties, and support conditions, fluid exploration of shell designs could be conducted.
This computational design process was further augmented with an FEM-driven optimization framework to find structurally performant shell designs. FEM analysis within Grasshopper was executed using the K2Engineering plugin to report the maximum deflection and tensile stress undergone by the structure under self-weight. These structural performance measures were combined as an objective criterium for Opossum, a surrogate model-based optimization plugin for Grasshopper, with strut pattern parameters as the search space. The final shell design selected for a large-scale prototype is seen above.
Side quest: optimizing shell shape was also investigated by quantifying the variation in Gaussian curvature and setting this at the objective criteria for Opossum.
Besides the structural performance, two major fabrication constraints based on the knitting machine used in this research had to be factored in the pattern search for human-scale prototypes of knit tensegrity shells. The objective criterium was defined as the sum of the number of intersections between course-wise (pink) lines aligned with the upper/lower edges of the pockets (green rectangles) and the deviation in petal width from 1.25m. Opossum was instructed to minimize this value (i.e. reach zero intersections and under 1.25m width). This could be seen as a "fine-tuning" of the structural pattern found in the previous step.
The selected tensegrity shell geometry was then digitally processed in preparation for fabrication on a CNC knitting machine. CNC knitting technology allowed for the tensegrity membrane to be knitted from different types of yarn to vary the stiffness of the membrane in response to stress concentrations, and knitted with multiple stitch types to integrate pockets to hold the compressive struts and customise the shape of the membrane.
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A human-scale pavilion was exhibited at the International Association for Shell and Spatial Structure’s Form&Force Expo 2019 held in Barcelona.