  Geometrically complex masonry structures (e.g., arches, domes, vaults) are
traditionally built with expensive scaffolding or falsework to provide
stability during construction. The process of building such structures can
potentially be improved through the use of multiple robots working together in
a cooperative assembly framework. Here a robot is envisioned as both a
placement and external support agent during fabrication - the unfinished
structure is supported in such a way that scaffolding is not required. The goal
of this paper is to present and validate the efficacy of three cooperative
fabrication approaches using two or three robots, for the scaffold-free
construction of a stable masonry arch from which a medium-span vault is built.
A simplified numerical method to represent a masonry structure is first
presented and validated to analyse systems composed of discrete volumetric
elements. This method is then used to evaluate the effect of the three
cooperative robotic fabrication strategies on the stability performance of the
central arch. The sequential method and cantilever method, which utilize two
robotic arms, are shown to be viable methods, but have challenges related to
scalability and robustness. By adding a third robotic agent, it becomes
possible to determine a structurally optimal fabrication sequence through a
multi-objective optimization process. The optimized three robot method is shown
to significantly improve the structural behavior over all fabrication steps.
The modeling approaches presented in this paper are broadly formulated and
widely applicable for the analysis of cooperative robotic fabrication sequences
for the construction of discrete element structures across scales and
materials.
