Higher and wider
Huntsville bridge to provide more clearance for Tennessee River boat traffic
(09/01/2005)
By Sandra Bearden
The Tennessee River bridge near Huntsville, which will replace an obsolete relic built in 1931, will provide nearly three times the clearance of the existing bridge.
After more than 70 years in business, there's not much in the way of bridge-building that managers for Scott Bridge Co. haven't seen. Then came the Tennessee River bridge on U. S. Highway 231 at Huntsville.
"The river got so high last winter that it was necessary to put our office buildings on stilts," said Chuck Davis, vice president of engineering for the firm, based in Opelika, Ala.
Davis said that the Tennessee River experienced its third-worst flooding in history in December, cresting 17 ft. above normal level, so the steel supports had to be high enough and strong enough to withstand torrential rains and swift river currents.
River flooding pushed the expected completion date back six months to summer 2006.
David said Scott Bridge took over the project from another company that was unable to complete the job. The current contract costs $18 million.
The 56-ft.-wide bridge will be an improvement over the northbound structure now in use, which is a functionally obsolete relic completed in 1931 with a total two-lane width of 19 ft., 10 in.
Johnny L. Harris, division engineer for the Alabama Department of Transportation, said that many drivers hug the inside line and fear to pass when crossing the old bridge.
"Our average daily traffic over the northbound and (newer) southbound bridges is now about 22,600 vehicles," Harris said. "When the northbound bridge was built, traffic flow may have been 600 vehicles daily, going at much slower speeds."
Harris said plans are also under way to replace the southbound bridge, completed in 1965 and itself showing some signs of age. ALDOT will first shift traffic from the 75-year-old bridge to the new structure and then demolish the old bridge.
Eventually, a new bridge carrying northbound traffic will be built in that location and Scott's project will become southbound.
"The plan will take several years to complete, but when finished each new bridge will carry three lanes of traffic," Harris said. The new capacity will help meet future needs in fast-growing Huntsville, where the metropolitan population is now about 350,000.
Harris added that the new designs also will benefit the area's busy commercial and recreational boating traffic. Channel clearance will be nearly three times the 25-ft. clearance for the 1931 bridge.
River flooding wasn't the only problem plaguing Scott Bridge workers early in the job.
"When we assumed the job in March 2004, we began cofferdam construction," Davis said. "Plans called for building six cofferdams 52 ft. by 46 ft. by 40 ft. feet deep, using structural steel footings on rock. But in two of the six we couldn't find suitable layers of supporting rocks, so we had to drive pilings and convert the foundations from rock footings to pile footings."
Crews constructed the cofferdams with rock footings by using tremie pipes to place concrete underwater. That job was followed by pier construction.
"Piers are massive and heavily reinforced," Davis said. "Each pier is built to withstand the impact of a 1,500-kip (750-ton) river barge."
The toughest part of the project thus far has been setting the reinforcing cages inside the cofferdams.
"The cages were extremely heavy - totaling about 170 tons of steel," Davis said. "We used a 230-ton crane and a 150-ton crane, both mounted on 50- by 150-ft. barges, to set the cages in place."
In May, Scott's work crews began erecting steel girders to span the river. A three-span, continuous 975-ft. unit consists of two 300-ft. side spans and a 375-ft. main span across the navigational channel, providing a 66-ft. vertical navigational clearance. Another three-span, continuous unit is 600 ft. long. The 12-ft.-deep girders, manufactured by Carolina Steel Corp. of Montgomery, are trucked to the site and offloaded onto construction barges.
After completing erection of the steel spans, Scott employees placed metal deck forms and poured the concrete decking for the bridge. The deck will be completed this fall.
By the time Scott workers complete the job, the project will have consumed 1,500 tons of reinforcing steel, 3,700 tons of structural steel and 16,000 cu. yds. of concrete for the substructure, superstructure and deck.
Magnolia Steel of Meridian, Miss., is supplying the reinforcing steel and Carolina Steel Corp. of Montgomery is trucking in the structural steel. The concrete supplier is Alabama Concrete of Huntsville.
When the new bridge is completed, Scott will then demolish the 1931 bridge.
"We'll sell the steel to a scrapper and dispose of the concrete," Davis said. APAC-Alabama is in the process of constructing approaches to the new bridge.
the relic... to bad it can't be saved or used elsewhere
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