Revised plans for the demolition of the Pearman and Grace bridges in Charleston, SC, now involve an elaborate system of strandjacks and strand wire to lower the suspended center spans of the bridges onto barges.
Demolition prime contractors, Boston-based companies Jay Cashman Inc. and Testa Corp. hired Mammoet, a Netherlands-based specialist in heavy lifting and transport projects, to provide the equipment and expertise to handle this part of the demolition.
Mammoet’s responsibilities included working with the prime contractors on the preparation work for lowering the spans, providing the equipment for the lowering of the spans, verifying that everything was in place before the spans were lowered and running the strandjacks during the lowering operations.
According to Leland Colvin, demolition project director of the South Carolina Department of Transportation (SCDOT), the new demolition strategy involved lowering the center spans of the Pearman and Grace bridges 150 ft. onto barges. The 535-ton Pearman Bridge, 380-foot span was lowered in four hours on Feb. 22. The Grace Bridge, which weighs 446 tons and measures 430 ft., was lowered in 3.5 hours on March 1. The spans were sent to a nearby steel recycling facility.
Four other sections of the old bridges, which are within the shipping channel, will be brought down using explosives. Demolition Dynamics will be in charge of blasting the steel trusses and Advanced Blasting will blast the piers and columns, which are made of concrete, timber and steel.
Time constraints were strongly considered in the demolition design.
“Because the Port of Charleston is the fourth busiest port on the eastern shore, time limitations played a major role in planning the dismantling of the bridges,” Colvin said. “To minimize the disruption in shipping in Charleston, crews had 24 hours to lower teach center span.
“Over $100 million in goods passes through the Port of Charleston daily, so the involved planning parties; therefore, the DOT, Coast Guard, contractors and the community worked together to determine the slowest day in the port, thereby minimizing the financial impact when the river was closed,” Colvin continued.
A week before the spans were lowered, the navigational channel of the Cooper River was narrowed from 600 ft. to 380 ft. while working platforms were constructed on both the east and west sides of the bridges.
These platforms held four 900-ton strandjacks and 120 high-capacity strand wires to lower the spans.
“Once the platforms, strandjacks and wires were in place, iron workers, suspended in baskets from the cranes, used cutting torches to make 24 cuts in the spans; freeing them from the cantilevers,” Colvin said.
According to Colvin, there have been no additional costs to the demolition project as a result of these changes.
Back to the Drawing Board
The new strategy was developed after the Oct. 11 test run demolition of the Pearman Bridge above the Town Creek Channel. In that case, 2,250 lbs. of explosives, meant to slice the bridge into 11 sections, failed to discharge effectively, causing the steel chunks of the bridge to become entangled, which complicated the clean up process. In addition, the retrieval time of the debris was slow because of the low visibility and the strong currents in the channel making the divers’ job even more difficult.
“The explosive method also required that an underwater survey be done to verify that all parts of the bridge were recovered from the bottom of the river,” Colvin said. “Surveying the bottom of the river was very time consuming, requiring six to seven hours. Mechanically lowering the spans eliminated the need for underwater retrieval and therefore no underwater survey was required.”
Manitowoc cranes and marine equipment were used in non-explosive dismantling
According to Colvin, equipment used in the lowering of the bridge’s center spans included two 2250 Manitowoc cranes and one 888 Manitowoc crane. All of the cranes were mounted on barges.
There were a total of seven barges at the site, along with five tugboats. Three of the tugboats were used to position the barges for receiving the lower of the spans. McAllister Towing supplied the other two tugboats for transporting the steel spans 2 mi. up the river for recycling.
Later in the demolition process, the remaining concrete piers will be exploded. The J-Cashman — a barge with a mounted Liebherr excavator with a 40-yd. bucket on a turntable, will help retrieve debris from the bottom of the channel. This equipment was specially built in Germany for Cashman. It was used first in a dredging project in Boston and this will be the second time it will be used.
Simplicity Equals Efficiency
“The new demolition strategy requires less steps, no underwater retrieval and no underwater survey,” Colvin said. “It’s basically a system of cables and jacks that will lower the center spans of the Pearman and Grace bridges across the main shipping channel onto barges.”
He said the most challenging part of the project was the close coordination required among all of the involved parties, particularly during the lowering of the spans.
Replacing the Pearman and Grace bridges is the Arthur Ravenel Jr. Bridge — the longest cable-stayed span in North America (1,546-ft. [471 m]). It has been open since July 16.
“Except for occasional 15-minute rolling delays for safety reasons, the demolitions should not disrupt traffic on the Ravenel Bridge,” stated Colvin.
The removal of both the Pearman and Grace bridges is set to be complete by February 2007. CEG