Scott System
Case Study
Arching Over Seattle: EPS Foam Formwork for the Union Street Pedestrian Bridge
Project
Overview
The Union Street Pedestrian Bridge, part of Seattle’s Waterfront initiative, provides an accessible connection between Western Avenue and Alaskan Way, bridging the city’s downtown and waterfront. The design features flowing arches on the bridge’s underside and integrated artwork by Norie Sato, turning a functional structure into a rhythmically engaging architectural space.
Scott System partnered with PERI to deliver the custom EPS foam formwork that made those arches possible.
The Challenges
Casting continuous concrete arches with conventional wood or steel formwork often results in visible seams and imprecise radii. A custom solution was required to achieve a smooth, seamless finish that integrated with PERI’s shoring system.
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The Vision
Schemata Workshop designed the bridge with arched soffits to create visual softness and movement. These arches frame the pedestrian experience, establishing a distinct rhythm along the waterfront connection.
Artist Norie Sato’s work further grounds the infrastructure in its regional context, drawing inspiration from the Pacific Northwest’s natural environment.
Proposed Solution
Scott System engineered custom CNC-milled EPS foam formwork that integrated seamlessly with PERI’s forming system. EPS foam’s lightweight nature and precision allowed for the creation of large-scale, seamless curvatures.
Hard Shell Polycoat was applied to the casting face to ensure surface integrity and a design-accurate finish. Close engineering coordination ensured the void formwork aligned perfectly with project shoring.
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Components for Success
- EPS Over Wood and Steel: The choice of EPS foam as the forming material directly addressed the primary limitation of conventional alternatives. Wood and steel impose seams and produce imprecise radii on flowing geometry. EPS, CNC-milled to specification, eliminates both problems and delivers a casting surface consistent with the arch’s full length.
- Hard Shell Polycoat: Coating the casting face with Scott System’s Hard Shell Polycoat protected the surface through the pour and contributed to the quality of the finished concrete. The coating is a standard part of Scott System’s EPS formwork process for exposed architectural applications.
- Engineered Integration with PERI: The formwork was not designed in isolation; it was designed to work within PERI’s system. Scott System’s engineering team treated PERI’s shoring and forming solution as a given and engineered the void formwork to fit within it. That coordination removed uncertainty from the installation process and kept the project on track.
- Geometry Without Compromise: The arched profiles required precision across the full span of the bridge underside. CNC fabrication made that precision repeatable, producing arch geometry that matched design intent without field adjustment.
Products Used
Arched EPS Foam Void Formwork: Custom-engineered, CNC-milled for continuous arch geometry
Hard Shell Polycoat: Protective coating applied to casting face
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The Outcome
The Union Street Pedestrian Bridge successfully delivers smooth, continuous geometry through custom EPS engineering. The finished structure serves as a highlight of the Waterfront Seattle initiative, merging architectural precision with infrastructure.
The Union Street Pedestrian Bridge reflects what becomes possible when custom EPS formwork engineering, forming system collaboration, and architectural precision align around a shared outcome.