Pringle : equitangential bending active frame and minimal surface robe-net

Membrane surfaces usually follow the principles of traditional formfinding methods. In analogy to soapfilm experiments, anticlastically curved, tension-only surfaces are formfound by predefining closed boundary conditions. In the realization, these boundary conditions represent essential structural elements, but mostly they are neglected in terms of architecturally designed, space defining elements. In 2009, the unrealized, competition winning entry (Filz) for a roof cover for the Guatemala relief map raises the architectural demand for an integrated solution, where the bordering frame-like structure and its enclosed membrane surface form an architectural and structural unity. This paper presents the follow-up research, which led to the experimental structure of the “Pringle” in 2017, a minimal surface within an actively bent, annular stripe (Filz and students at the University of Innsbruck). Such a closed elastic stripe with overlenght is frustrated by geometry, forcing it to buckle. Its non-planarity would increase both the curvature and torsion depending on the magnitude of its overlenght but the resulting shapes are determined by minimizing the total elastic energy. The tension forces from the filling, minimal surface could be absorbed by using the strong axis of the annular stripe while its weak axis allowed for elastic bending into the selforganized shape. In retrospective, the analysis and the comparison of results of different setups and configurations of the structure have been analyzed and in a collaborative accomplishment by Filz, Shahzad, and Niiranen at Aalto University, Finland.