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The chances of an extreme storm system rolling through western Oregon before 2014 are less than 4 percent. But those odds were threatening enough to trigger significant changes to construction plans for the replacement of two aging Interstate-84 bridges near Troutdale.

The Oregon Department of Transportation initially contracted to use traditional bridge-building methods. But after the general contractor started working in summer 2010, a report revealed that if a flood were to occur when the project was in full swing, the amount of equipment and material the company had in the water would raise water levels enough that wreckage would increase drastically. So they came up with another plan.

鈥淭his is a situation that could have been very contentious,鈥� said Byron Perry, deputy program manager for , a joint venture of and with offices in Salem and Troutdale. 鈥淲e were able to work together to develop an approach that lets us reduce our impact as much as we practically can and still get the job done in a reasonable time.鈥�

But the new plan also increases the project’s cost by approximately $5 million and extends the schedule by a year. The budget for the entire project is now approximately $83 million, according to Jim Cox, ODOT’s bridge delivery unit manager.

Springfield-based Hamilton Construction initially planned to build a detour bridge for rerouted traffic and two temporary work bridges while the permanent east- and westbound bridges were built. But after realizing the increased flood risk, ODOT, Hamilton and Oregon Bridge Delivery Partners discussed several options and ultimately decided to use an innovative top-down construction method, which reduces impacts to water levels.

Hamilton will use a gantry crane to carry girders out to designated placements, thereby eliminating the need for one of the new work bridges in the river. The tracks that guide the crane’s base across the river extend for 840 feet 鈥� the same length as the bridges themselves. The two tracks must be aligned exactly 90 feet apart.

The gantry system has not been used on bridges in Oregon before, according to Cox. Gantry cranes are more typically used to hoist materials in shipyards and other industrial settings.

Hamilton purchased the $1.2 million gantry crane, which was delivered from Boston in nine truckloads. Three operators and two foremen went through special gantry crane training for the project, and crews are now doing trial runs in preparation for work on the eastbound bridge.

Wendell Snook, a project manager for Hamilton, hopes that the investment in training and equipment will pay off, but he says it may be too soon to tell.

鈥淲e’ll know a lot more in six months,鈥� he said. 鈥淲hether we’re buying more (gantry cranes) or looking for a buyer, we’re learning a lot and are pretty optimistic.鈥�

Even with the altered construction plans, however, the project will impact water levels in the Sandy River. The area is already susceptible to flooding because the bridges span a delta near the river’s confluence with the Columbia River. When both rivers back up, water is pushed to the delta and levels rise.

The last major flood to hit the area occurred in 1996, when riverside properties around Troutdale were damaged and one person died. The probability of a 鈥�100-year storm鈥� occurring over the four-year construction period for the Sandy River bridges, according to ODOT estimates, is 3.94 percent.

ODOT built a temporary flood barrier on the west bank of the Sandy River and is reimbursing potentially affected property owners and renters for flood insurance premiums through the end of the project, which is expected by spring 2014.

After Hamilton crews finish the eastbound bridge this summer, they will move to the westbound bridge and use the detour bridge as the new work bridge. The completed eastbound bridge will have the capacity to carry detoured traffic traveling in both directions.

And although the innovative construction method will have almost no impact on the final appearance of the bridge, Snook hopes it will encourage more creativity in the building community.

鈥淢aybe we’ve been tunnel-visioned into relying on the hoist-type machines that we always use,鈥� he said. 鈥淭his is opening up possibilities and letting the imagination start to run wild.鈥�

Snook also said that if a prospective buyer made an offer for the gantry crane, Hamilton Construction would be willing to entertain it.