Steel Shots: A New Arch for a New Age

The new Sellwood Bridge features a 1,275-ft-long three-span steel deck arch, which has a span arrangement of 385-ft, 425-ft, 465-ft, with two arch ribs per span. The progression of span lengths generally follows the rise of the bridge in grade from west to east. (Photo: Nick Garibbo/Nick’s Photo Design) 

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 for the Burnside Bridge. The result was a unique and efficient four-span continuous steel truss costing a mere $541,000. At 32 ft wide, the bridge was extremely narrow: two lanes, no shoulders or median and one 4-ft-wide 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.

For more about the project, see the article “New Arch for a New Age” in our October issue (available now!).