...Stonecutters Bridge. Some 7,000 tonnes of parallel wire strand in 224 cables will support that bridges main span and both sets of four backspans, each stretching for 289 m.
Locked coil cables were not available in needed sizes so Stonecutters designers turned to parallel strand, says Arups Hussain. But because the enclosing ducts are bigger, allowing insertion of individual strands, high stresses were predicted in back stays in buffeting wind. "We had to reduce the wind loading and the only way was to reduce the diameter of the stay cable," he says. And that meant specifying slimmer parallel wire cables.
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| A Statement. Hong Kongs Stonecutters Bridge had an international design competition for looks. (Photo by Peter Reina for ENR) |
High Wire
"Ive done quite a lot of parallel-strand stay-cable installation, but this will be my first project with parallel wire," says Brian West, project manager with Stonecutters main contractor. The Maeda-Hitachi-Yokogawa-Hsin Chong Joint Venture formally signed its $350-million construction contract in May last year. Having to fabricate entire cables to size and lift each unit in one piece will be challenging, especially for the longer cables, says West.
With the bridge visible from Hong Kong and West Kowloon, the regions Highways Dept. wanted something special. So it launched an international design competition, awarding the prize in 2000 to a team led by U.K.-based Halcrow Group Ltd. However, the design contract was bid separately, going to Arup with Denmarks Cowi A.S.
Stonecutters 53.5-m-wide deck will comprise two parallel steel boxes with a 14.3-m air gap in between. Cross girders set at 18-m intervals link the 3.9-m-deep steel boxes. Backspans, according to the competition design, were to be monolithic with the towers for reduced maintenance. But later analysis revealed high structural torsions would arise during typhoons, says Hussain. So Arup extended the steel deck some 50 m into the 78-m-long backspans at both towers, separating it from the towers, except at horizontal buffer bearings. To transmit bending moments, the steel and concrete deck will be post-tensioned together.
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From the 298-m-tall single-leg towers, two planes of cables will fan to the anchorages on the decks edges every 18 m. For aesthetics, only the lower 60% of the circular towers was originally to be in concrete, turning into a steel tube for the rest. But that design was too flexible, causing excessive vibrations in stays, says Hussain.
To stiffen the tower, Arup extended the concrete full height, but kept the steel enclosure, acting compositely. And to ease maintenance, it specified 2-cm-thick stainless steel weighing over 1,800 tonnes.
Jump casting equipment is now being prepared to start the first tower and the contractor is procuring some of the nearly 35,000 tonnes of steelwork in a difficult market. Not only have steel prices roughly doubled since the contract was bid but West says sections of the required grade and thickness, up to 10 cm, are scarce.
As producers focus on more profitable items, "there have been times when we received rejections from the mills for certain grades and sizes," West says. "Some of our partners in the joint venture are very large steel consumers and we have been able to make some procurements by utilizing their buying leverage."
The contractor and locally based Maunsell Consultant Asia Ltd. are designing the construction process. "With a bridge of this scale, there is extensive engineering input in the construction," says Robin Sham, Maunsells transportation executive director.
Wind tunnel tests in China and the U.K. are being used to investigate responses of the freestanding tower, the growing deck cantilevers and cable stays in various construction stages, says Sham. If necessary, the contractors will develop "precautionary measures," such as temporary tuned-mass dampers in the deck to mitigate vibrations, he adds.
Maunsell now is forecasting bridge geometry at every erection stage to produce setting out data. "Verification of the effects of the erection scheme on the permanent structure is naturally a principal activity," says Sham. As erection advances, measured data will be used to refine forecasts. "It takes very tight monitoring and control," he adds.
Since starting construction early last year, the contractor this September completed the vast, 13,000-cu-m east pile cap. Partly because of the need to design around a deep fault under the west tower, foundation work there lags by about six months. The bridge is founded on piles up to 2.8 m dia reaching down 75 m deep on the east side and 110 m on the other.
The site also is gearing up to cast the four backspins on either side, some 60 m above ground on temporary steel-braced precast concrete towers. Designed by Maunsell, the tower system is being built with 2x2-m boxes, match cast in China. They are stacked over 30 boxes high and stressed together vertically across dry joints for easy removal.
Deck steelwork is due to start going up at the towers next autumn. Working with VSL Hong Kong, the joint venture has developed a system of cable-stayed girders cantilevering from tower sides to raise the first sections of deck box.
Strand jacks will raise nearly 60-m-lengths of box individual girders simultaneously on either side of each tower. The boxes will be slid toward each other to be linked with girders. And they will be slid back to be stitched to awaiting concrete backspans. The first main span lift is due in early 2007. Steel will be fabricated into panels near Beijing by China Railway Shanhaiguan Bridge Group Co. Ltd. It then will travel to Guangdong for assembly into 18-m lengths of twin boxes weighing 300 to 500 tonnes each. These will be raised by gantries from the deck and welded into place.
Working on a nine-day cycle, deck erection is due to end in summer 2008, adding 128 m the current record cable-stayed span, Japans Tatara Bridge. But Stonecutters will never hold the record if the Sutong Bridge over Chinas Yangtze River meets its 2007 completion target.
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| Bold Move. Chinas Sutong Bridge may be headed for the record books for size in 2007. (Photos courtesy of Maunsell Consultants Asia Ltd.) |
New Contender
With a 1,088-m cable-stayed main span, the bridge will form the main section of a 6 km river crossing about 100 km from Shanghai. Its A-shaped pylons will rise just over 306-m above the pile caps to support a single 35-m wide steel box girder deck with a maximum 4-m depth. Construction began in June 2003, with piling completed last year.
The bridge is designed by a consortium of China Highway Planning and Design Institute, Jiangsu Provincial Communication Planning and Design Institute and Architectural Design and Research Institute of Tongji University. Denmark's COWI is doing checking for the owner, Jiangsu Sutong Bridge Construction Commanding Dept., and is helping with construction management.
Prime contractor CHEC Second Navigation Engineering Bureau, Wuhan, has ordered eight gantries from U.K.-based Dorman Long Technology Ltd. to raise the 16-m-long deck sections, each weighing up to 450 tonnes, starting next August. Like Stonecutters Bridge, the Sutong crossing will have parallel wire cable, which is commonly used in China, notes Sham, whose team also is advising the Chinese prime contractor.
