ON THE RIGHT FOOTING |
Two diamond-shaped towers will rise 575 ft over river. (Photo courtesy of Rob Thompson/SCDOT)
Charleston's Cooper River crossing is drawing on a flurry of advances in design and construction of cable-stayed bridges from around the globe. The project has captured the imagination of South Carolina's largest city, and the workers building it. "We have the greatest bridge in the world here," says a foreman standing on one of two 6,500-cu-yd rock islands at the base of what will be North America's longest cable-stayed span, at 1,546 ft. Above him, the legs of what will be a 575-ft-high diamond-shaped tower rise to about half their eventual height in the sweltering South Carolina heat.
The 2.5-mile-long, eight-lane crossing features a major-league roster of designers and builders who are using experience with previous cable-stayed megaprojects to achieve an affordable bridge that is aesthetically pleasing, strong enough for hurricanes, flexible enough for earthquakes and high enough to allow the next generation of ships to pass beneath.
With design just completed, the $531-million design-build contract held by Palmetto Bridge Constructorsa team of Tidewater/Skanska USA Inc. and HBG Constructors Inc./Flatiron Structures Co.is halfway through its goal of completing the project in less than four years. To do this, work on the two approaches, two interchanges and the main span are being done simultaneously, says Bobby Clair, director of engineering for special projects for the state Dept. of Transportation. "We'll have four lanes open within 44 months," he says.
At a total cost of $644 million, the Arthur Ravenel Jr. Bridge is easily the state's biggest project. To get it built, SCDOT Executive Director Elizabeth Mabry leveraged widespread support to push through an innovative financing package, including a $325-million loan from the state infrastructure bank, a $215-million Transportation Innovative Finance and Infrastructure Act loan, a $20-million contribution from the state Ports Authority and local funds. "The bridge is more than double our annual construction program," notes Clair.
The Ravenel bridge will replace two steeply graded and obsolete steel truss bridges. The two-lane, 74-year-old Grace Memorial Bridge now has a 5-ton limit and the three-lane, 35-year-old Pearman Bridge lacks a median. But they are the only connections between the north suburb of Mt. Pleasant and Charleston.
A series of public meetings helped produce the bridge's eventual design, a diamond-shaped pair of towers supporting a cable-stayed span with a design life of 100 years. Aesthetics were important. With limited funds, "We took the budget for lighting, handrails, etc. and tried to get manufacturers to rework the designs," says Donald MacDonald, the San Francisco-based project architect. The bridge's white and gray colors match regional historic buildings.
DEEP DRILL Footings feature 11 drilled shafts up to 230 ft long through rock islands. |
(Rendering courtesy of MacDonald Architects)
Three design-build teams submitted designs for either a pair of four-lane bridges or one eight-lane bridge. PBC won with the lowest cost and a single eight-lane span (ENR 6/18/01 p. 18).
Michael Abrahams, director of engineering for New York City-based Parsons Brinckerhoff, the consortium's design manager, says state and federal transportation officials were concerned with the design's 131-ft width, due to its location in a seismic and hurricane zone.
"We started a design already developed by Buckland & Taylor for the Alex Fraser Bridge in Vancouver," he says of the Vancouver-based designer working as conceptual design engineer for PB. "On the other hand, we had completed a bridge across [Boston's] Charles River, a much wider bridge. We knew we could do this."
Man-Chung Tang, chairman of San Francisco-based T.Y. Lin International, which with Omaha-based HDR Inc. is construction inspector, says the design is very similar to Korea's Seohae Grand Bridge, with almost identical main span lengths. Abrahams also notes that a design for Bangladesh's Paksey Bridge includes 1,800 meters of jointless high-level approach spans, also helpful in designing the 2.5 miles of approaches in Charleston.
Design first had to pass muster with a panel of seismic experts, mostly from California, for an 8.0 event, notes Eric Keen, HDR project manager. Plastic hinge zones are being built into towers to allow needed flexibility.
France's Freyssinet designed a damping system to ensure the structure can withstand 190-mile hurricane wind forces. "A lot of it came from the Oresund experience," says Lars Landen, an engineer who worked on the Denmark-Sweden link with its 490-m-long cable-stayed span. Because galvanized strands are unavailable in the U.S., each strand of the cables are coated with wax and plastic pipe coverings to prevent corrosion, he says. There are 50 to 91 strands per cable, which are about 1 to 7 in. in diameter. Pipe coverings for the 128 cables feature grooves that move away moisture that might invade the cables.
With its 186-ft vertical clearance and 1,000-ft-wide channel, the new bridge will give larger ships heading to the Port of Charleston more room. But the towers are protected by two sloped rock islands being built around the footings. The base of the two islands are constructed with 1.6 million tons of quarry stone barged from Canada.
It takes six days for the stone to travel from Canada to the site. The stone was either loaded into barges and dumped directly into the river or placed with a clamshell bucket around 425 drilled shafts. The towers are founded on the shafts, reaching to 10 ft in diameter and 230 ft deep into marl clay.
The talent pool for bridge construction extends beyond big-name contractors and designers. More than 60 disadvantaged citizens have been trained on site as welders, carpenters, surveyors and operators with another 20 to come, says Clair. Candidates must complete a two-week pre-employment process and can then work on a specific skill under the mentoring of an instructor for the next year, says Horrace Tobin, PBC training manager.
Charleston Boosts Cargo Terminals
The Port of Charleston's cargo volume is growing 10% annually, a figure that has port officials racing to keep ahead of congestion at its three terminals. The Ravenel Bridge's greater vertical clearance, a new terminal and dredging will allow more and larger ships to put into the port.
When completed, the Ravenel Bridge will offer higher and wider clearance for larger cargo and container vessels, and the potential for two-way vessel traffic. The Ports Authority Board is contributing $45 million to the project, along with other support. "The new bridge will be at least 186 ft in vertical clearance," says port Director Bernard Groseclose. The old bridges have a 150-ft vertical clearance.
Over the next three years the port authority will invest more than $150 million in terminal improvements. The port recently received permit approvals for a new terminal at the now-closed Charleston naval shipyard with 3,000 ft of berthing space and 250 acres of container storage/support area. The 1,500-acre base will be divided between the city of North Charleston and the port, with the port receiving 10,000 ft of waterfront. The state asked the Army Corps of Engineers to expedite the permitting process, says Groseclose.
The authority now is short-listing environmental consulting teams to perform studies and expects to choose a team this month. Construction could begin within a year if expedited approvals go through, and could be completed within five to seven years. The project will cost about $500 million.
A $150-million project begun in 1999 to deepen Charleston's harbor is nearing completion. It will provide the port with 47-ft-deep shipping channels. The last major contract to deepen the upper Cooper River is well under way.
Says PBC project manager Wade Watson: "We need the numbers. We need 450 craftsmen and we want local workers." In conjunction with other federal agencies, SCDOT this year is establishing a transportation education institute that will offer 20 scholarships to inner-city children for a participating technical college. Another program, just implemented, will offer college scholarships.
Speed is based on project delivery. If not for design-build, "we'd be just now signing the contract with the contractor," says Abrahams. Supplies had to be ordered on a "just-in-time" basis and constantly renegotiated; 42 large cranes, 50 barges and two tower cranes assigned as needed. But Watson and Clair say there are no major claims so far, and no major injuries. PBC would face $30,000 a day in potential penalties if late, but expects to finish well ahead of the 60-month schedule, they say.
Some 500 workers representing 50 subs are working 10-hour shifts in gen-eral. "Cable-stays get the notoriety, but they're not the hardest part of this job," says Watson. At the western end of the job, 100 metal cylinders of 30-in. pipe are set 40 ft deep to support a 3,000-ft-long, 40-ft-wide platform to set equipment rather than barging into a pristine salt marsh.
Approach supports have 8-ft-deep caps, each designed specifically for a bridge segment and rising at a 4% grade. The diamond tower forms of wood and steel will be topped with prefabricated concrete panels that "fit like a glove" over the sections, notes John Young, PBC superintendent. The 26 x 30-ft, 120-ton uncoated rebar cages are each placed and filled with 700 cu yd of concrete. "We worked with local suppliers to come up with fly ash in high proportions for impermeable concrete," notes Abrahams. The mix has 40% fly ash and reaches 7,600 psi after six days.
The tower legs are hollow, allowing for maintenance elevators and access. Superstructure work has now begun with erection of steel girders. The first girders connecting the rows of pier caps are 135 ft long and 45 tons each. When done, erection of the precast concrete deck using the balanced cantilever method will follow. More than 130 concrete beams, each up to 85 tons and up to144 ft long, are also being placed for an interchange on the Charleston side of the bridge. Cable placement will begin next summer, when the towers are completed.
After the Ravenel Bridge is completed and ready for 50,000 daily travelers, demolition will begin on the two old ones, says Clair. Some of the old steel may end up as 85 acres of fishing reef from which to watch huge ships pass under the bridge.
Designers Are Busy With Demand for Cable-Stayed Bridges
The cable-stayed bridge business is busy, both in the U.S. and around the world. "Nobody has the final answer" about where bridge-building technology will eventually lead, says Michael Abrahams, engineering director for New York City-based Parsons Brinckerhoff. But there are "tremendous improvements" that are making cable-stayed bridges stronger, longer and slimmer, he says.
Monostrand cabling, where one strand from a cable can be removed for testing, isn't new but a new cradle system designed by Tallahassee-based Figg Engineering Group for Ohio's Maumee River bridge "will revolutionize the design of cable-stayed bridges for the future," claims Linda Figg, the firm's president. "It will eliminate cable-stayed anchors in the pylon and provides greater freedom in design of pylon shapes."
Cables run between the deck and the cradle system at the top of the pylon, allowing access to the stay to remove a reference strand at any point. Each strand is wrapped in its own "jacket." The bridge is slated to open in 2006.
Boston's Charles River Bridge pioneered an ungrouted cable system with internal dampers to control wind vibrations and the use of iso-tensioning, says Raymond McCabe, director of bridges for the designer, HNTB, Kansas City. "The issue before was stressing all strands at any one time, requiring a large and heavy jack," he says. "That resulted in the tower needing to be very large. Now, we're able to slim down tower elements and facilitate stressing of cables with lighter equipment."
In China, two very long cable-stayed bridges now are being built. Stonecutters Bridge in Hong Kong and the Sutong Bridge in Jiangsu Province both will have spans more than 1,000 m long, says Man-Chung Tang, chairman of T.Y. Lin International, San Francisco.
New techniques also are being applied to cables in suspension bridges. New York City's Hardesty & Hanover is applying a new analytical model for assessing fracture toughness of deteriorated wires on the Mid-Hudson suspension bridge in upstate New York.
T.Y. Lin International currently is designing San Francisco's new Bay Bridge, "by far the longest and largest self-anchored suspension bridge," Tang says. "It is especially remarkable for having a single tower in an extremely high seis-mic zone and bad soil conditions," he explains.
Elsewhere, Vancouver, B.C.-based Buckland & Taylor is involved with several cable-stayed bridges in Korea. In Greece, the Rion-Antirion Bridge will be the world's longest of the genre, with five cable-stayed spans totaling 2,250 m (ENR 1/1-1/8/01 p. 30).
Engineers in the U.S. are effectively prohibited from using galvanized strands because of lack of domestic capacity and "Buy American" laws, say designers. Still, the 230-ft-wide, triple-plane Interstate 70 bridge in St. Louis and the Desmond Bridge in Los Angeles are two examples of projects moving forward. "Cable-stayed bridges have sex appeal," says Joe LoBuono, principal with Weidlinger Associates, New York City.
The design also is being carried to pedestrian bridges. In New York City, a four-lane, 500-ft-long, $40-million cable-stayed bridge will reconnect 153rd Street across a rail yard. And a cable-stayed pedestrian swing bridge is being designed for San Diego.