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Moving the Sellwood Bridge: How they did it

by: DAVID F. ASHTON - No kidding! With a section of the east approach now removed, the Sellwood Bridge is DEFINITELY closed to traffic.When the Sellwood Bridge was opened 87 years ago, the designers assured the community that the steel Willamette River crossing would provide safe passage for at least 50 years. They were right.

But by the 1970s, Multnomah County officials realized the bridge would need replacement; repeated compression and crumpling of elements of the west-end approach – not yet identified as being caused by the slipping of an ancient landslide upon which the bridge had been built – made it clear that repairs would not be enough. But, one set of Commissioners after another deferred the project, “kicking the can down the road” for their successors to worry about.

As the bridge developed clear signs of strain and aging, it was still the busiest bridge in the entire State of Oregon – per lane, on weekdays – relied upon by tens of thousands to get to and from work daily.

After decades of patching and bracing the west-side ramp to the bridge, the Multnomah County Commissioners in 2006 started what would become a ten-year project to replace the bridge.

The original idea was to build half of a new bridge directly to the south of the existing Sellwood Bridge, then tear down the old bridge and build the northern half – essentially building two bridges in the same location, and then connecting them together.

But, in early 2011 Slayden/Sundt Joint Venture, the construction manager and general contractor selected for the project, pitched what at the time seemed like radical and outrageous idea. It had been proposed by a bridge-moving company called “Omega Morgan” of Hillsboro, just a dozen miles away.

The engineers suggested building a new set of “bents” (the pilings on which a bridge sits) to the north of the bridge – and then slide the 3,400 ton, 1,100-foot long continuous truss (one piece) span onto the new supports. The result, contractors estimated, would shave $5 to $10 million off the cost, and save up to one year of construction time.

In the summer of 2012, officials announced that the bridge would be indeed be moved, using this “shoo-fly” maneuver, perhaps on January 12, 2013.

At the end of 2012, five spindly-looking temporary bents were in place, their alignment veering 33 feet further north on the west end than on the east end. The new western approach, built above the former Staff Jennings location, jutted out, like a bridge to nowhere.

As the “translation” – the engineers term for “sliding over” – grew near, the date for the move was slid back a week, to January 19.

A week before the move, project spokesman Mike Pullen, and Multnomah County Senior Engineer (Bridge Section) Ed Wortman, together led a tour of site.

“The truss span will be transitioned in a ‘windshield-wiper’ fashion – 33 feet at the east end and 66 feet at the west end,” Wortman explained. “When the bridge is closed to traffic, crews will dismantle parts of the approaches on each end, leaving it free to be moved.”

Wortman pointed to a component of the west approach. “Then, they’ll take out those long, steel beams. They were ‘recycled’ from the original Burnside Bridge – which means they’re 110 years old.”

The span was supported on their original piers by two truss bearing points – making a total of ten truss bearings.

Omega Morgan crews installed “track beams” atop the five massive girders welded between the five original concrete bents and the new steel piers. The track beams contained friction-reducing Teflon “skid pads” on which 14’ long ski-shaped “skid beams’ would ride.

Where the truss-bearing point would meet the pier, workers were making sure each of the ten cradles – each supporting a truss bearing – was secured to two lifting jacks each, and was also securely attached to the “skid beam” on which the section would slide.

“Before they slide it,” Wortman said, “they’ll first lift the truss span up, between one and two inches. They’ll need to lift it a section at a time; we calculate the bridge can safely withstand an upward bend of up to 3.5 inches.”

Finally, crews installed and tested the ten horizontally-oriented 75-ton capacity hydraulic jacks that would slide the bridge north. This hydraulic jacking system was designed to push the bridge over a little – then stop and retract, while the back of the jack was moved forward – thus slowly “walking” the span, one baby-step after another.

By the evening of Friday, January 18, a day after the bridge closed for the move, the four spans of the 1,100-foot-long steel truss had been lifted into position and carefully checked.

Omega Morgan’s chief moving engineer hooked up all ten pushing jacks, double-checked the calculations that would make the west end of the truss move twice as fast as the east end, and started up the hydraulic compressor.

The bridge crept a little bit north in the darkness. Careful measurements showed that the test was a success. The Sellwood Bridge was ready for its historic journey. (See our headline story, elsewhere, for what happened next.)