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Figuring it out

Sutong - Specially designed gantries supplied by the UK's Dorman Long Technology have just begun lifting deck segments for the world's longest cable stay bridge. Adrian Greeman reports.

China's breakneck economic development continues to throw up major achievements in infrastructure, the latest a world record cable stay crossing of the giant Yangtse river about 100km inland from Shanghai. In total the Sutong bridge will be 8.2km long with a central 2,088m crossing of the main channel.

The double plane cable stay bridge at the centre will have a 1,088m-long main span and will be the world's longest, at nearly 90m longer than Japan's Tatara. The recurring 8's are no coincidence - eight is a lucky number in China.

The project will give China a new north-south trunk route from Jiayin to Nanping, part of a network being built by the ministry of communications.

Total cost for the crossing and approaches, which total 32.4km of road, is £420M.

The huge 300m-high concrete and steel towers for the bridge recently reached their full height and the main contractor, China's Second Navigation Engineering Bureau, has just begun erection work on the deck which will eventually carry a dual threelane motorway across the river.

Steel deck segments are being used for lightness and these will be lifted in from barges with specially designed gantries from Dorman Long Technology.

Eight have been supplied to allow simultaneous work outwards from both main piers, lifting a maximum of 450t for the segments nearest the piers.

'We became involved because we also did some work on the lowering of the huge 3,000t caissons used for the foundations, ' says Tom Betts, Dorman Long's principal engineer for the project.

In international tender the company's gantry mechanism for adjusting the segments' position and its computerised control system probably swung the choice their way.

The gantries each comprise an inclined steel frame structure with a variety of hydraulic jacks on board. Main DL S290 strand jacks are used for the lifting operation and there are a total of nine secondary jacks.

Three of these are used for a complicated sliding sequence which pushes the gantry forwards between lifts on travelling beams and four more specially developed screw jacks lift the frame onto or off those beams.

'Gantries are bolted down at specic points in the deck which has permanent diaphragms at 4m intervals, ' says Betts. 'They tend to want to lift at the back and topple forwards.' Remaining jacks are used during lifting to adjust the position of the jacking platform under load, moving it both longitudinally and transversely to allow the segment to be precisely positioned at the top of the lift before welding.

There is one extra jack. This is on the steel lifting beam below, which is attached at two points to the deck segment at the beginning of a lift, one for either side. By shifting the strand attachment along the beam and altering where the centre of gravity sits, the segment can also be tilted precisely.

All these jacks are controlled by a computerised system, DLP40, developed by Dorman Long, which synchronises both the jacks in each gantry and the two gantries themselves.

'And I'm pleased to say it all worked well on the tests, ' says Betts, 'because you are never quite sure with even the best design.' The control equipment and jacks were supplied from Europe and then tted into a steel frame at the site.

The nal workout for the system will be on the centre deck section which will close the gap. This is a very narrow piece which cannot use the lifting beams and instead will have four gantries, two on each side, with the strands connected directly to the four lifting points. The jacks will work together to maintain and adjust levels.

Overload tests on the north pier were carried out last week to 25% overload - 290t - by lifting the first segments off their barges and replacing them. The successful tests mean deck work is now underway andwork from the south pier should start in two weeks.

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