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Bridge project off to a flying start

BRIDGES

Construction of a massive airport 40km west of Seoul is the reason behind another major Korean bridge now taking shape. Both rail and road will be carried on the 4.42km double deck Youngjong Grand Bridge, largely a heavy Warren Truss structure with a suspension bridge as its focus.

The Warren Truss superstructure will carry six lanes of traffic on the 41m wide upper level, while 11m below on the 35m wide lower deck, there will be two lanes on either side of the twin railway tracks. A clearance of 35m will allow ships to pass under the bridge.

The 500m suspension bridge will be the world's first two-storey, self anchored bridge carrying both road and railway. Furthermore, unlike a conventional suspension bridge its cables come almost to single points at the tops of the diamond shaped, box girder steel towers, giving a complex three dimensional geometry with the hangers at varying inclinations. This section is set between 750m of truss span and 1,140m of steel box girder span on one side and 1,500m of truss span and 480m of box girder span on the other.

Client for the bridge is New Seoul International Airport Freeway Company, and contractors include Samsung, Hanjin, Kumho, Kolon and Dong Ah. Design is by Japan's Chodai and Yooshin of Korea. Tony Gee & Partners is working for Hanjin on aspects such as the erection scheme and temporary works.

Leading contractor with a 29% share is Samsung which is responsible for the two towers and the piers. Originally, the plan for the tower foundations was to install a cofferdam, dewater and then drive a series of 3m diameter piles for each tower. This was abandoned when driving the sheet piles proved too difficult.

'In the end we chose to use an unmanned pneumatic caisson which we had seen operating in Japan,' says site director Man Geun Yoon. Two of these 47m by 18m precast concrete jackets were towed out to sea by a floating crane and lowered on to the sea bed.

Once in position, compressed air at a higher pressure than that of the ground water was forced into the working chamber at the bottom of the caisson. Operators using remotely operated mechanical shovels then dug down some 20m to reach load bearing rock.

Each caisson was sunk in stages as the main body was formed. The 33m high body is cast from 27Mpa concrete and treated to protect against salt- water corrosion. Doka supplied the formwork to form the 1.7m thick external walls and the internal webs. The Top 50 large area forms were handled by some of the six Potain tower cranes supplied to Samsung and Hanjin.

Doka also supplied formwork for the piers whose foundations were constructed in sheet piled cofferdams. To provide support for the sheet pile walls against tidal pressure, steel struts had been inserted. This prevented the use of Doka's MF240 climbing formwork. Instead, special brackets had to be made and the steel supports were cut out as the formwork climbed. The majority of the concrete was supplied from Korea's largest floating batching plant

Deck erection is being carried out from the water using temporary trestles. Three sections are loaded on to a barge and positioned between the piers. Four 400t connecting sections are then placed by floating crane to form three span module. The barge rises with the tide and once the high point is reached the floating crane lifts the module and places it on the piers, supported by the trestles. Two of the modules are in place and the third of the three-span sections is being assembled.

Hanjin is responsible for 11 spans in groups of three, two and a half, one and a half and one module. This means Hanjin has to interface with other contractors in three locations.

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