Prototyping knit tensegrity shells: a design-to-fabrication workflow

Abstract

This paper describes a design-to-fabrication workflow for knit membrane tensegrity shells, a novel class of tensegrity structures that substitute discrete cables with a continuous machine-knitted membrane as the main tensile element. The workflow integrates (a) a simplified, simulation-driven design method for membrane tensegrity shells, (b) the conversion of the membrane geometry into digital inputs for Computer Numerical Control (CNC) knitting, and (c) the rationalization of the assembly connection details. This workflow begins by iteratively parameterizing reciprocal strut patterns, connecting the strut endpoints to form a mesh to represent the membrane, and implementing a dynamic relaxation algorithm to form-find the tensegrity shell geometry. The digital model then undergoes optimization procedures to negotiate between structural performance and fabrication constraints. Subsequently, the membrane geometry is extracted and converted into machine instructions to be CNC knitted with the following features: (i) integrated pockets to hold compressive struts at intended locations within the membrane, (ii) different types of yarn to create localized stiffer regions in response to stress concentrations imposed by the struts, and (iii) alterations of the shape of the membrane to adhere to the digital geometry. Several physical prototypes, including a 4-meter-diameter pavilion that was exhibited at the International Association for Shell and Spatial Structure’s Form&Force Expo 2019, serve as case studies for assessing the fidelity, benefits, and limitations of the proposed workflow.