Precast Stay-Anchor System Marks Ohio River Crossing
Custom-designed falsework, precast tub foundations at extreme depths and the first U.S. use of a precast stay-anchor block system all pushed to fruition an initially budget-challenged bridge replacement over the Ohio River. The new 2,616-ft-long Ironton-Russell Bridge, connecting Ohio and Kentucky and scheduled to open to traffic by Thanksgiving, underwent a series of redesigns, value engineering and contracting changes to achieve an acceptable schedule and cost.
The existing cantilevered through-truss bridge, opened in 1922 as the first highway crossing over the Ohio River, currently handles about 10,000 daily vehicles, but that number is expected to increase; further, the existing crossing is restricted from carrying overweight vehicles, says Kathleen Fuller, spokeswoman for the Ohio Dept. of Transportation (ODOT). “We went through a lengthy history of getting the project to bid,” she adds. Identified for replacement in 1999, the Ironton-Russell Bridge project went through extensive studies and community outreach until 2006. But when the project went out to bid, proposals came in at $110 million or more, or about 22% over the allocated budget. Consequently, ODOT pulled together a value-engineering group that included URS, now AECOM.
“The result was recommending that ODOT redesign it as a three-span cable-stayed bridge, rather than a two-span cable-stayed bridge,” says Steven Stroh, AECOM complex bridges practice leader. Says Fuller, “The original bridge was to be two lanes with shoulders for a potential of four lanes." The originally designed 500-ft-tall single tower became two 300-ft towers, a change that resulted in a narrower superstructure.
“One of the key aspects of the [original] design was that the Coast Guard had required the pier on the east bank to be a certain distance—only 50 feet—from shore,” adds Stroh. “That meant, if you put a tower there, there would not be enough room for a big-enough side span before you get into the curvature on that side of the river.” The design team brought in ODOT, the Federal Highway Administration, the Coast Guard, and Paducah, Ky.-based Seamen’s Church Institute, which used a simulator to describe navigational situations with the theoretical bridge tower in various locations. Ultimately, the agencies "allowed us to move that tower 150 feet out into the river, so we could do the three-span bridge,” says Stroh.
The new structure has a 900-ft cable-stayed center span—ODOT’s longest ever—and two 370-ft cable-stayed side spans. Brayman Construction Corp. submitted an $81.2-million bid that left about $15 million on the table; the company began construction in 2012, says Thomas Hesmond, Brayman superintendent. Use of the precast anchor blocks, specialized falsework and deep tub foundations came about due to Brayman’s partnering with VSL and Finley Engineering Group, adds Hesmond. “Finley had done some falsework-related stuff before, and VSL had seen partially precast blocks before. It all came together,” he says.
The substructure consists of six piers and two abutments, along with the two towers in the river. Drilled shaft foundations range in size up to 96-in. diameters. Crews worked from barge-mounted equipment in water averaging 45 ft deep to install precast cofferdams for the tower foundations. Tying the precast tubs with sheet followers into the drilled shafts elimininated need for a conventional cofferedam in the wet. While precast tubs are not unusual, the depths were—in fact, about 30 ft into the below the river's normal pool, says Stephen Muck, chairman and CEO of Brayman. “The river flows are so significant that the bottom of the river is scoured clean. Pinning sheet piles in 45 ft of water and trying to build a conventional cofferdam with no soil overburden was not a good option.”
While the main structure consists of 22,500 cu yd of cast-in-place, reinforced concrete and 5.8 million lb of rebar, the contractor used precast elements in the back spans, cofferdams, stay-anchor blocks and the floor beams on the side spans, which provided access out of the water to build the main span. “The biggest learning curve was for Brayman Precast, one of our companies, in casting the anchor blocks,” says Muck. “The learning curve was primarily related to the density of the rebar and the post-tensioning ducts. The tolerances were super-tight.”
The falsework supporting the back spans also required a learning curve, but “it only took one or two segments to get up to speed,” says Hesmond. The modular falsework system was used on both approaches and allowed for one, rather than two, form travelers to build the bridge deck. Crews used the precast-concrete stay-anchor block, secured to the traveler, to anchor the stay cables before placing concrete in the form traveler and installing 30-ft-long, 42-ft-wide segments. Project officials estimate the system helped to reduce the bridge-deck construction cycle to five days from a 10-day duration, reducing the overall construction schedule by 150 working days.
That reduction proved valuable, as weather-related delays and excessive water elevations, leading to dispute resolution processes, added about a year to the schedule and about $5 million in change orders but no claims, says Dave Bame, ODOT construction engineer. The bridge will open to traffic this November, with demolition and total completion scheduled for summer 2017.