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Structural pipe, tube creates natural fit for Portland footbridge

When the Barbara Walker Crossing Footbridge was first conceived in 2012, its main function was to spare hikers and runners on Portland’s Wildwood Trail the bother of dodging traffic on heavily traveled West Burnside Road.

What it became was a testament to aesthetically minded architecture, blending utility and beauty for a community that prized (and demanded) both.

Elegant Complexity
The bridge, completed in October 2019 and dedicated that same month, is a 180-ft.-long pedestrian walkway that is curved in plan and designed to blend into the surrounding forest.

It was fabricated off-site by now-defunct Supreme Steel of Portland, cut into three main sections, and transported by truck to the site.

Meeting both the visual and architectural requirements meant using material that would accomplish all the project’s highly distinctive objectives, artistically and structurally. And that meant using pipe—in this case, 3.5- and 5-in. corten (ASTM A847) structural steel pipe, which is designed especially for structures that need to be welded or bolted. Some of the pipes are exposed (another key corten feature), and some are painted green to match the forest canopy.

Ed Carpenter, a designer and artist who specializes in large-scale public installations, said he had several objectives in mind when conceptualizing the bridge. Among them: The bridge should fit into the forest context, it should be a continuation of the feeling and experience of the trail, and it should be as delicate and transparent as possible.

That design necessitated a tri-chord truss to support the structure.

“Since one of my most important design objectives was to make the bridge delicate and transparent, I needed the most efficient materials possible and the most efficient structural system—so, the tri-chord truss,” said Carpenter, also an outdoors enthusiast who has run on Portland’s vast trail system for more than 40 years. “You could build that out of other materials, but steel tube or pipes was simply the logical choice.

From a practical construction point of view, achieving all of that was no small feat. Stuart Finney, a structural engineer at the Portland office of engineering firm KPFF who worked as project manager on the bridge, said that successfully welding all of the components at the T-Y-K connections, at which all the supporting pipes converge, proved to be perhaps the trickiest aspect of the entire endeavor. In particular, all the different angles requiring different types of welds (like fillet and groove) presented the construction team with a stiff challenge.

“Essentially every joint was different,” said Finney, who’s practiced his craft for 20 years. “They had to get every joint perfect so that all these pipes connected together at a single node and they could get sufficient weld around all the pipes.

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