Modeling the Ross Island Bridge

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The Ross Island Bridge is not one of Portland's more celebrated bridges, perhaps because of the awful approach from the west side, and the single narrow sidewalk. Built in 1926, the bridge carries the four lanes of US Highway 26 across the Willamette.

More optimistically, it earns a solid 8 on the Historical Bridges significance scale, above the lowly Morrison or Burnside bridges. And it looks great with the fresh coat of paint it received last summer.

Part of the Ross Island Bridge’s distinctiveness is its shape: It looks like an arch, but it’s technically not. Instead of the center span leaning against itself, as in an arched bridge, the two halves of the center span are cantilevered—each half of the center span is balanced over its support by the outer portions of the span, as in the human example below:

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The cantilevered structure allowed builders to work toward the middle from each side, without any formwork or scaffolding in place under the center span. (Or, apparently, safety gear of any kind!) Source: Oregon Historical Society

For our SketchUp model, this meant recreating a ton of vertical and diagonal structural elements supporting the roadway and tying together the cantilevered spans. Fortunately, the main portion of the bridge is symmetrical end-to-end and side-to-side. I only had to draw a quarter of the bridge, then used SketchUp's nested component capabilities to create the rest. I also made extensive use of Smustard's Path Copy plugin to create the repeating details on the bridge deck and guardrails.

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Joel is a designer at Fat Pencil Studio