Stefanie Pender

Robotic fabrication · MADWORKSHOP & USC · 2020

The Arroyo Bridge

The completed Arroyo Bridge, an orange steel lattice pedestrian bridge spanning a wooded canyon in Los Angeles
The finished span across the arroyo in Los Angeles.

An 80-foot single span across a ravine near Los Angeles, designed by R. Scott Mitchell and built out of a six-year research initiative that began as a student design-build studio at USC in 2014. Its branching lattice, drawn from the vascular structure of leaves and the layered branching of the surrounding eucalyptus, was effectively unbuildable by conventional means: an asymmetrical superstructure of over 600 discrete parts joined by connections no two of which are the same. A six-axis arm held each member in position while master welders made the joins, at the Autodesk Technology Center in Boston. The finished sections were trucked across the country and installed over the ravine.

Role
Supported robotic fabrication at the Autodesk Technology Center, Boston
Team
Designed by R. Scott Mitchell. MADWORKSHOP and the USC School of Architecture, with the Autodesk Technology Center, Boston
Numbers
80 ft span · 600+ discrete parts · ~18,000 lb of steel · held to within ~1 mm of the digital model · ~1,000 lb waste, recycled · four 35 ft pilings hand-bored into the hillside
Read more
Dezeen · Fast Company · The Architect's Newspaper · Metropolis · Autodesk: Collaborative robotics · Mitchell, “The Arroyo Bridge,” ACADIA 2022
The bridge modelled as an orange steel lattice, sitting inside a white point-cloud survey of the ravine it spans
The structural model set into a survey of the ravine. Every member is sized and angled to its own position in the span.

Why a robot

Every node in the lattice meets at a different angle, so almost no two cuts or welds are the same. That's the case where a jig-and-fixture workflow collapses and a programmable arm starts to make sense. The setup cost is the same whether the next part is identical to the last one or not.

It stayed a collaboration rather than a handover. The robot did what it is good at, holding a one-off part at an exact angle for as long as it took, and the welders did what they are good at.

Overhead view of the Boston shop floor covered with hundreds of cut steel tubes, with an ABB robot arm on a linear track among them
The shop floor mid-production. Hundreds of members, each cut for one specific joint.

Getting it there

The bridge was built in sections in Boston and assembled on site in Los Angeles, which puts every accumulated fabrication error into a handful of field joints.

Workers and a forklift loading welded steel lattice sections of the bridge onto a flatbed truck outside the shop
Loading finished sections for the trip west.
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