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The Oregon City Bridge, 88 years old, has never needed a major renovation … until now.

A $10.6 million project is scheduled to start Saturday. But Oregon Department of Transportation officials and others are hopeful that in two years the bridge will be ready to revitalize the neighboring communities of Oregon City and West Linn.

“For an almost 90-year-old bridge it’s in remarkably good shape,” said Susan Hanson, an ODOT spokeswoman for the project. “There is some corrosion, some damage. But actually it’s impressive how well it’s held up considering it’s never had a major renovation.”

Credit legendary Oregon bridge engineer Conde McCullough – designer of many of the state’s coastal arch bridges – for a lot of that durability. He decided to use gunite, an ancestral version of shotcrete, to protect the Oregon City Bridge’s iron interior from corrosion by essentially coating it with concrete.

But while the bridge has held up relatively well, small iron veins over time have crept through the concrete skin and into the air where they’ve corroded, leaving engineers to question the structural integrity of its interior.

Weight restrictions for commercial vehicles on the bridge were instituted about 18 months ago, and then ODOT decided to move forward with the full renovation process.

The process won’t be simple, however. The bridge is listed on the National Register of Historic Places, so it can’t be widened, changed or modified in any significant way.

And the shutdown poses a threat to Oregon City businesses that are already suffering from the downturn in the economy, she noted.

Planning for the project, ODOT officials decided to work with local businesses groups to reduce the impacts as much as possible, Hanson said.

In the past two years the agency has repeatedly met with that community and agreed to improve traffic signs for the closure so that the 13,500 drivers that use the bridge each day know how to travel to downtown. It will also put up information stressing that Main Street businesses will remain open through the process.

ODOT selected contractor based on experience and not just its bid amount, Hanson said.

“They’ve done a lot of bridge projects,” she said of the Eugene-based company.

In the process, Wildish crews will hydro-blast off the old gunite coating to reveal the iron structure underneath. Special planning is required so that no contaminants land in the Willamette River, Hanson said.

“That high-pressure water blasting pulverizes the gunite into chunks, and there will be protection around that and we’ll barge the debris away for disposal,” she said.

After the gunite is removed, crews will examine the iron structure in sections. They will repair any corrosion or replace anything that can’t be repaired.

Then the structure will be rebuilt in a manner as historically accurate as possible, Hanson said.

“We do have the original plans from when the bridge was built, but the plans don’t always say exactly what was happening,” she said.

To circumvent that problem, ODOT and the contractor have examined historical photos of the original construction in 1922. They’ve found more than 100 images that show all sorts of detail, including how railings were stacked and installed, how workers sprayed on the gunite, what the wire mesh looked like beneath the gunite coating and a host of other things, she said.

“You see a lot of things the workmen did as they constructed it, and you also see that they didn’t have safety harnesses back then,” Hanson said with a laugh.

Looking at the photos also helped engineers discover several aspects that weren’t included in the planning documents, she said.

“In the base we can see them adding steel rods in the photos, and those aren’t anywhere in the plans – there are a lot of little things like that,” Hanson said.

Workers also will remove and replace eight decorative pylons – four at each side of the bridge. The old pylons will be donated to Oregon City, West Linn and Clackamas County, Hanson said.

“They can use them in parks or as decorative features as they see fit,” she said.

After the work wraps up, workers will coat the bridge with shotcrete, and ODOT expects it will be another 90 years before another big renovation is needed, Hanson added.

To kick off the project, the Willamette Falls National Heritage Area Coalition is planning an event on the bridge from 2-4 p.m. on Saturday. Speakers, storytellers and dancers will commemorate the importance of the bridge to local communities and educate visitors about the history of the river area.