SAISC projects
Oregon Department of Transportation
New Arch for a New Age
A new steel arch in Portland replaces a prominent crossing of the Willamette
By Ian Cannon, PE, Eric Rau, PE and David Goodyear, SE, PE
ABOVE: The new Sellwood Bridge over the Willamette River near downtown Portland, Ore., replaces a more-than-90-year-old span that had become unusable.
BELOW( left to right): Ian Cannon( ian. b. cannon @ multco. us) is Multnomah County’ s transportation director and program manager of the Sellwood Bridge project, Eric Rau( eric. rau @ tylin. com) is a bridge engineer with TYLI and David Goodyear( david. goodyear @ tylin. com) is TYLI’ s chief bridge engineer and the lead bridge engineer for the Sellwood Bridge project.
OPPOSITE PAGE: Plan and elevation drawings of the new bridge.
Two out of 100
That was the National Bridge Inventory( NBI) sufficiency rating that the 90-year old Sellwood Bridge received in 2005 after the latest round of engineering studies, emergency repairs and additional load restrictions. Multnomah County, Ore., the owner of the bridge, was keenly aware that shoring up the old bridge was no longer an option.
Constructed in 1925 to replace the Spokane Street Ferry, the Sellwood Bridge spans the Willamette River just south of downtown Portland. It was designed by Gustav Lindenthal, a noted bridge engineer of the time and – along with the nearby Ross Island and Burnside bridges – was built with funds from a $ 4.5 million local bond measure.
Lindenthal was hired to redesign the Sellwood Bridge as a result of cost overruns on the Burnside Bridge. The result was a unique and efficient four-span continuous steel truss costing a mere $ 541,000. At 32ft wide, the bridge was extremely narrow: two lanes, no shoulders or median and one 4-ftwide sidewalk. It was Portland’ s first“ fixed span” bridge across the Willamette and the first to not be designed for streetcars.
The NBI rating of 2 for the old bridge reflected a number of critical issues ranging from movement of an ancient landslide on the west bank of the Willamette to general deterioration of the 90-year old concrete approach structures.
The County began the NEPA( National Environmental Policy Act) process in 2006, and an engineering team of CH2M and T. Y. Lin International( TYLI) was retained to perform the engineering studies and develop alternatives for a new crossing. The evaluations included rehabilitation and replacement options for the main bridge, a dozen structure types for the main crossing and various alignments and project configurations. The recommendation was replacement on the same alignment, and through an active and meaningful public outreach process, the Community Advisory Committee’ s( CAC) preferred alternative – a steel deck arch – was approved by the County Board of Commissioners.
12 Steel Construction Vol. 40 No. 6 2016