Generalized Quasi-Symmetric Nets: A Constructive Approach to the Equimodular Elliptic Type and to Flexible $m \times n$ Nets

Abstract

Quadrilateral meshes that flex with rigid planar faces are of interest to transformable architecture, rigid-foldable origami, and mechanical metamaterials. Their building blocks are flexible $3 \times 3$ nets, that is, Kokotsakis polyhedra with a quadrangular base, classified by Izmestiev. We study one of his types, the equimodular elliptic type, defined by three conditions on the flat angles, the third of them through Jacobi elliptic functions. We turn these conditions into equivalent algebraic ones, free of elliptic functions, which lead to the generalized quasi-symmetric nets (GQS-nets), polyhedra of this type that include the known elliptic quasi-symmetric nets. For every polyhedron of this type, we give a flexible polyhedron with the same flat angles and its flexion in closed form. From the simplest GQS-nets, we assemble arbitrarily large flexible $m \times n$ nets. Examples include nets that, at one position, form letters; further nets in the supplementary materials form digits or, seen along their rows, approximately follow the side silhouettes of a bunny and a fish. Unlike numerically computed mechanisms, these nets are exactly flexible. As an application, we design three such nets as transportable arch halls of 80 mm sandwich panels, which fold to fit through the door of a standard container or into the well of a low-bed semi-trailer, and we check them numerically under snow load and build a 1:20 model of one. Supplementary Mathematica notebooks verify the symbolic computations, a Python tool constructs the nets and checks them numerically, including for self-intersections, and further scripts recheck the halls.