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Design and Structural Optimization of Topological Interlocking Assemblies

ACM Transactions on Graphics (Proceedings of SIGGRAPH Asia), conditionally accepted, 2019

Figure 1: A topological interlocking assembly (a) designed with our approach to conform to an input freeform design surface (b). The 3D printed prototype (c-e) is stable under different orientations.


We study assemblies of convex rigid blocks regularly arranged to approximate a given freeform surface. Our designs rely solely on the geometric arrangement of blocks to form a stable assembly, neither requiring explicit connectors or complex joints, nor relying on friction between blocks. The convexity of the blocks simplifies fabrication, as they can be easily cut from different materials such as stone, wood, or foam. However, designing stable assemblies is challenging, since adjacent pairs of blocks are restricted in their relative motion only in the direction orthogonal to a single common planar interface surface. We show that despite this weak interaction, structurally stable, and in some cases, globally interlocking assemblies can be found for a variety of freeform designs. Our optimization algorithm is based on a theoretical link between static equilibrium conditions and a geometric, global interlocking property of the assembly-that an assembly is globally interlocking if and only if the equilibrium conditions are satisfied for arbitrary external forces and torques. Inspired by this connection, we define a measure of stability that spans from single-load equilibrium to global interlocking, motivated by tilt analysis experiments used in structural engineering. We use this measure to optimize the geometry of blocks to achieve a static equilibrium for a maximal cone of directions, as opposed to considering only self-load scenarios with a single gravity direction. In the limit, this optimization can achieve globally interlocking structures. We show how different geometric patterns give rise to a variety of design options and validate our results with physical prototypes.


Figure 2: A variety of patterns supported by our tool for designing TI assemblies. The surface tessellations can be generated by lifting 2D tessellations (see the boxed images) using conformal maps (the left four columns), manually designed by users (top two patterns in the rightmost column), or created as a surface Voronoi diagram (bottom pattern in the rightmost column).

Figure 3: TI assemblies of various shapes and their corresponding feasible cones (except those that are globally interlocking).