Box beam bridge construction used in the building of the French Broad River Bridge in Madison County, North Carolina, has proven to be an efficient, financially encouraging and environmentally positive method of bridge construction.
Taylor & Murphy Construction Company Inc. of Asheville, NC, began the $4.7-million construction in February of two adjacent bridges over the French Broad River on SR 1001, in Marshall, NC. Box beam construction was used to build the larger of the two bridges; the other will be constructed using conventional methods.
“Typically in North Carolina, precast pieces, cord slab, concrete and metal beams are used in bridge construction,” stated John Herrin, project manager for Taylor & Murphy. “This bridge project is one of the first in North Carolina to utilize box beams.”
“Box beams are giant hollow boxes, 34-inches tall and 3-feet wide and range in length from 41 feet to 77 feet,” Herrin explained. “They are set along side each other and joined together with bolts or cable. Concrete or asphalt is then poured on top.”
According to Randy McKinney, project engineer of the North Carolina Department of Transportation (NCDOT), box beams can accommodate longer spans than cord slabs , which have a maximum length of 60 ft. Therefore, using box beams reduces the number of piers required in a channel, which lessons river blockage problems, improves aesthetics and helps reduce environmental concerns associated with stream work.
“In addition, box beams are faster to set than pouring a conventional slab,” McKinney said.
There will be 117 box beams used, 13 beams per span, for the new two-lane bridge.
According to Chris Britton, structure division manager of Taylor & Murphy, Ross Pre-stressed Concrete Inc. of Bristol, TN, manufactured the box beams.
“The box beams are shipped to the job site by truck,” Britton said. “Taylor & Murphy is handling the entire installation.”
Installation of the box beams began in early November and was expected to take 30 days.
According to McKinney, one beam is set at a time. When all 13 beams, in a span, have been set the beams will be anchored at each end with two anchor bolts per beam per end. There are four locations in the span where a cable is threaded transversely through the 13 beam assembly. These cables are tensioned to hold the beams together as one unit.
“Wearing surface — concrete — is poured over the beams to form the smooth uniform deck,” McKinney said. “The concrete will be about seven to eight inches thick in the middle of the bridge and six inches thick on each side.
Box beam bridge construction is being incorporated into more and more North Carolina projects
“It works well on straight bridges, and on one span bridges,” McKinney said. “The box beams can be placed without disturbing pristine streams. Box beams are a quicker method of construction compared to traditional methods, thus allowing for shorter periods of traffic interruptions.”
The French Broad River Bridge replacement project involves demolishing the 35-year-old, steel beam bridge and replacing it with two bridges — a 450-ft. (137 m) box beam bridge extending across the river and a 150-ft. (46 m) steel girder bridge extending from the riverbank to the island
“The old bridge consisted of two bridges that intersected in the center of the river — T-shaped, ” Herrin said. “One bridge connected the two sides of the river; an intersecting bridge went onto an island used by Madison County Recreation Center. It was obsolete, structurally unsound and in very poor condition,” McKinney said.
“The new bridges will be two lanes with sidewalks,” Herrin said. “In addition an antique, classic bridge rail will be installed to make it more aesthetically pleasing.“
Construction of the smaller of the two bridges began first.
“Construction of the bridge from the riverbank to the island began first,” Britton said. “Once complete, the T portion of the existing bridge will be removed.”
Herrin expects the island bridge will be complete in February 2006.
According to McKinney, the main subcontractors on the bridge project include Ace Steel Inc. of Mount Airy, NC, which is responsible for tying the reinforcement steel, and Lee & Sims Drilling Services Inc. of Belton, SC, which is responsible for drilling the foundation’s drill shafts. NCDOT was responsible for all design work and inspection.
According to Britton, approximately 1,100 cu. yds. (842 cu m) of concrete will be used in the construction of both bridges. In addition, .33 cu. yd. (.25 cu m) of concrete will be needed per foot of box beams.
The river’s dynamics present unique challenges to the contractors.
“What makes this project particularly challenging is due to the width and flow of the river and maintaining access to the island during the duration of construction of both bridges,” McKinney stated. “Initial site preparation involved constructing a single rock causeway, or workstation, across the river. We are limited to one rock causeway at a time to avoid damming the river and interfering with navigability and recreational activities such as rafting, kayaking, fishing and tubing.”
According to Britton, the rock causeway required 11,000 tons (10,000 t) of rock and will be relocated four times during construction.
“When the project is complete, the rocks will be hauled to a waste area,” Britton said.
Environmental concerns were emphasized when dismantling methods of the old bridge were planned.
“The old bridges are to be dismantled by cutting the structure into sections and lifting out where possible,” McKinney said. “This will minimize concrete and bridge debris falling into the river.”
According to Britton, concrete demolition equipment will be used to demolish the bridges. This equipment includes a Cat 330 excavator with a ram hoe hammer, along with an excavator with a processor for crushing the concrete. All of the equipment is owned by Taylor & Murphy.
“Concrete waste, generated from the demolished bridge, will be used to fill a low land area of a local church,” Britton said.
“The most challenging aspect of this project has been addressing all the environmental concerns and working within the constraints of the city of Marshall, the North Carolina Department of Health and Resources and other North Carolina state environmental agencies,” Herrin said.
Also, heavy rains during the summer of 2005 caused some slowdowns with the work schedule.
“Primarily this was due to the causeways, which are built at a low level to prevent damming the river, becoming submerged after a big rain. However, we have had a dry spell this fall, and construction has moved along on schedule,” Herrin said.
According to McKinney, traffic volume in the area is very low.
“Traffic will continue on the old bridge until three-fourths of the new bridge construction is complete, at which time traffic will be switched to the new bridge,” he said.
The new bridges will be wider and will meet North Carolina standards. Plus, Herrin said they will be environmentally friendly.
“Having two separate bridges so that motorist can access the island directly, along with replacing the old out-of-date bridge, will be the primary traffic improvement to the area,” McKinney said.
The estimated date of completion of both bridges is Aug. 1, 2007. CEG