I-5 overpasses will rise step by step.

Byline: Diane Dietz / The Register-Guard

The state wants Interstate 5 to accommodate truckloads that are 16 feet, 2 inches high. That means they'll have to raise six Lane County overpasses by as little as 4 inches and as much as 11 inches.

How the heck do you raise an overpass?

By adding height to the eight or 10 slender - 2-foot-by-2-foot - columns that hold up the bridge at each overpass.

To do that, workers burrow into each column and slide in a hydraulic jack capable of lifting 50 tons to 100 tons.

They'll seat the jacks until they're supporting the weight of the bridge, and that will allow them to chip away the concrete to expose the reinforced steel rod inside the column. Then, they'll cut the steel rod.

"It's exciting," ODOT bridge engineer Rick Hart said. "You feel a lot better when it's done."

Next, they'll simultaneously raise all the columns to the prescribed height. Then, they'll weld and/or clamp a new piece of reinforced steel to the severed ends of the old pieces in the column. Then, they'll recast the concrete around the fused rods. After it's seasoned, they'll remove the jack.

Up on top, meanwhile, the raising creates gaps on the overpass road bed, which they then fill with concrete - and then they repave the top of the whole thing.

ODOT will raise a bridge only as high as necessary, and 2 feet is about as big an increase as they've tried.

"The taller you make those skinny columns, the greater the risk of the column buckling under a seismic event," said bridge preservation manager Frank Nelson. "We worry about very tall, slender columns."