World File Photo Highway 38 is closed at the Elk Creek tunnel this weekend, while bridge builders install a new span.

Imagine taking a full-sized, new bridge and dropping it in place over a waterway — all in a weekend’s work.

The Oregon Department of Transportation’s bridge construction contractors can do it. In fact, they are this weekend with the bridge at the west end of the Elk Creek tunnel on state Highway 38.

The bridge builders will use a new engineering technique this weekend. The work will force a temporary closure of the highway from  9 tonight through 6 a.m. Monday.

This technique, called rapid replacement, uses several steps to quickly replace an old structure with a new one. First, the bridge builders constructed the new bridge alongside the existing bridge. They built support structures for the new bridge and installed a hydraulic skid system to slide the new bridge into place once they remove the old one.

With the new bridge, support system and skid system in place, crews will shut down the highway to demolish the old structure and use the hydraulic machinery to slide the new bridge into position. Then crews will dismantle the old bridge, some of which will be recycled, and connect the new structure to the highway.

The first run of this method, the replacement of the bridge on the east side of the tunnel in May, went smoother than expected.

“Last time we were supposed to open the road at 6 a.m. on Monday, but were able to reopen at 9 p.m. on Sunday,” said Lois Cohen, the project’s public relations spokeswoman.

A conventional bridge replacement would require closing one lane 24 hours a day for about one year. This method allows traffic to move along the existing bridge until the new bridge is ready to slide into place, reducing the time construction affects motorists.

“It worked well,” she said. “Rapid replacement saved half a million motorists from travel delays.”

And it saves the state money, too.

The technique cut construction costs by about $500,000 on this portion of the project because contractors were able to avoid building a temporary bridge, Cohen said.

This bridge replacement is part of a project to replace five aging bridges on Highway 38 between Elkton and Drain. Work started in June 2007 and will continue through 2009. The five bridges scheduled for replacement were built between 1929 and 1932. Only the two bridges on either side of the Elk Creek tunnel are relying on the use of rapid replacement.