ReciprocalShell: A hybrid timber system for robotically-fabricated lightweight shell structures

Reciprocal timber system has been widely studied, however it has never been directly applied to the segmented timber shell structure as a cross-bracing of a polygonal topology. For the first time, this paper presents an innovative hybrid timber system developed for design and construction of the robotically-fabricated lightweight timber shell structures. The timber system integrates two configurations of wood beams: polygonal framing and reciprocal bracing. While, the polygonal topology of facets enables a constant distance offset for the thickness of the shell, the reciprocal configuration allows for cross-bracing of polygonal frames where diagonals within the polygons cannot directly connect corners due to geometric constraints resulted by the free-form surface structure of the shell shapes. Joining the cross-bracing elements in the center of the polygons with a reciprocal node reduces the complexity of the connection system at joints while demonstrating the high load-bearing capacity of nodes for withstanding structural loads, compared to connecting 5, 6 or 7 beams in a single point. The article discusses the application and limitations of the timber system while presenting the design-to-assembly process of a case study of the small-scale shell demonstrator with the maximum span of 7.5 meters made of 144 wood elements for each polygonal and reciprocal configurations. The results show that the timber system has a great capacity for the rapid and precise assembly and disassembly of prefabricated timber structures. Generation of similar but different solid elements, allowed for the development of a custom CAD data interface for the automated production of numerous pieces, where simple joint details are applied for both alignment and attachment of beams, reducing the design complexity and facilitating the construction phase. As a result, the fabrication process was completely carried out with only a saw blade in a multi-axis robotic fabrication set up that enables the rapid, precise, and accurate cuts and grooves. Both timber configurations generate a uniform distribution of beam size, meaning that the production process created only a minimal amount of offcuts that allows for the use of simple and cost-efficient, short solid wood pieces.