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Oregon Tries Top-Down Construction on Bridge

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Graphic courtesy of Edelman
A customized gantry system reduced the need for in-water piles to just 12 from 100, increasing protection for salmon-filled streams.
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In-water work restrictions are so tight in the salmon-filled rivers of western Oregon that the Oregon Dept. of Transportation decided to employ a top-down bridge construction method to replace a section of Interstate 84 east of Portland. If all goes well, say ODOT officials, the method may be adopted on a more frequent basis.

Crews from Hamilton Construction Co., Springfield, Ore., have built a pair of gantry cranes in preparation for work at the confluence of the Sandy and Columbia rivers. Using twin 250,000-lb cranes, each weighing 100 tons and spanning 90 ft, construction crews will not need in-water work bridges to place a total of 49 steel girders for both the westbound and eastbound spans, says Wendell Snook, Hamilton's project manager.

The $58-million contract includes two 840-ft-long bridges, one 80 ft wide and the other 68 ft wide, to replace existing bridges in the exact same alignment. A detour bridge already has taken eastbound traffic off the I-84 bridge, allowing crews to construct the gantry system on top of a new concrete support structure, which did require an in-water work bridge to build. The new steel girders are now being placed.

With heavy runoff from nearby Mount Hood predicted and a preliminary design that would have required up to 100 in-water piles for work bridges, concerns arose that the new piles, coupled with debris coming down the Sandy River, could dramatically increase flooding risks for nearby homes and businesses. Further, with only a six-week time frame for in-water work due to endangered salmon—an interval that was later extended to 28 weeks—crews would have spent the bulk of the schedule putting in and tearing out the work bridge, says Jim Cox, ODOT's bridge delivery unit manager.

The gantry system's support structure cuts the need for piles in the river to 12 from 100 during the winter months. "It is a huge difference and less time in the water," Snook says. "Additionally, our temporary structures are on 50-foot increments. Now we have 100- to 120-ft-spans, so the opening between the piles are substantially larger."

Crews are bolting together girders on the river's bank and then using the cranes to place lengths of up to 160 ft—using two cranes to spread the weight—on the concrete support structures. The roadway will be a cast-in-place concrete deck.

Once crews finish the eastbound bridge in summer 2012, they will move to the westbound bridge, using the detour bridge as the new work bridge and the completed eastbound lanes as the detour. They plan to have everything in its final alignment by spring 2014.

Faced with the need to switch from conventional methods, Hamilton shopped around and eventually bought both custom cranes for $1.2 million from North American Industries of Boston. While the motors and hooks were standard, the size of the cranes was customized, allowing the machines to slip within inches of the detour bridge.

"We haven't put them out to the test to see if we are in love with them," Snook says. But if all goes well—and the company doesn't anticipate a steep learning curve—Snook is hopeful that Hamilton can reuse them to cut costs and possibly save time on similar projects in the future.

Cox says ODOT will be watching the project closely, hoping that a successful completion will give future project builders in the river- and creek-filled areas west of the Cascade Range "another tool in the toolbox."

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