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