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Multnomah County engineer Ed Wortman enjoys explaining things in simple metaphors 鈥� even the process to move a 3,400-ton steel bridge down the Willamette River.

鈥淚t’s sort of like a windshield wiper on your car,鈥� he said. 鈥淚t swings through a curve, but the arm stays straight. That’s the really tricky part about this.鈥�

Next month, the 1,100-foot truss of the Sellwood Bridge will be moved from its current piers to a group of temporary piers slightly down river. The relocated span will become a detour bridge while a new bridge is being built.

Because of the positions of the temporary east and west approaches, the truss will be moved to a skewed alignment 鈥� the east end will be moved north about 33 feet while the west end will be moved north about 66 feet.

As a result, the room for error is slim, Wortman said.

Most bridges, including most of the major ones in Portland, are made up of several segments. But the Sellwood Bridge was built as a four-span continuous truss, meaning it’s made up of one long piece of steel.

In theory, the Sellwood Bridge could be cut up into segments and moved, but that would take extensive funding and time. And it would not necessarily make the relocationeasier or safer.

So the entire structure will be moved as a single piece, using transitional beams that will stretch from the current piers to the temporary ones. Ski-shaped 鈥渟kid beams,鈥� each about 14 feet long, will sit on top of the transitional beams and will slide the truss forward.

The truss will be pushed by 10 strategically placed hydraulic jacks moving in small increments. The jacks will simultaneously push the bridge forward, and then electronically adjust themselves to prepare for the next increment. This process will be repeated several times.

The best way to understand how the jacks operate, Wortman said, is to compare it to pushing a large cardboard box. You exert force to push the box a certain distance, and then you adjust your position to push it again.

This sliding strategy is commonly used in new bridge projects, Wortman said. It was used four years ago on the Sauvie Island Bridge replacement project, and it’s been used several times by Slayden Construction Group, the project’s general contractor.

Throughout the whole process, a series of safety, equipment and seismic checks will be done. The plan is to move the truss only a few inches at a time first. Eventually, the truss will be moved in three-foot or four-foot increments. The county and Slayden expect the move to take about 10 hours.

鈥淚t’s a bit of a guess of how long it will take, but the hope is to get it done in one day,鈥� Wortman said. 鈥�(There will be) probably more time being devoted to checking things than there will be actual moving.鈥�

Even with the safety checks, things could still go wrong. The truss, which was fabricated in 1924, could be severally damaged in the process by even with the simplest slip up 鈥� a hasty push or an incorrect placement on a beam, for example.

鈥�(It) requires a lot of thought, particularly for an older and spindly truss,鈥� said Chris Higgins, a civil-and-construction engineering professor at Oregon State University who is serving as a project consultant for Slayden Construction. 鈥淭he tricky part it to make sure you maintain the geometry and don’t induce stresses into the structure for which it was not intended.鈥�

Weather could also impact the move. If it’s too windy or rainy, the workers monitoring the move might be at risk. As a result, the transaction would have to be rescheduled.

Wortman said the chances of the bridge sliding off track or crumbling as a whole are slim; the jacks will prevent it from rolling. If any structural damage occurs, it would most likely be found as part of the safety checks.

鈥淚t’s like a big log across a creek 鈥� with rocks under its middle to support it,鈥� Wortman said. 鈥淚t’s not like it’s sliding down hill. Once it moves, it’s not going to take off on its own. It really can’t go too far or too fast.鈥�

The bridge will be closed from Jan. 17 through Jan. 24 for the transaction. After it is relocated, more tests will be conducted to make sure the truss was not damaged during the move.