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SCMP Reporter

SLIPFORMING of both concrete towers of the Tsing Ma bridge has been completed, marking a key transition point in the construction of the world's longest railway suspension bridge.

Slipforming the towers involved the use of a mould which was moved progressively higher as each portion was cast.

The completion of the operation by subcontractor VSL, whose engineers carried out the work for the Mitsui-Costain part of the joint venture, opens the way for Cleveland Structural Engineering to prepare for the 1994 aerial wire-spinning process for the main suspension cables.

Bridge construction activities are concentrated on tower-top works, including a reinforced concrete plinth, or base, on which the saddles' heavy-steel parts will rest atop each tower leg.

The cast steel saddles must be hoisted to the top of the two legs on each tower, and positioned before the footbridge is installed.

 The saddles, in several parts, are being shipped to Hong Kong.

 The first pieces are scheduled to arrive and the remainder should be in Hong Kong by early February.

 Each saddle consists of a base plate grillage, a bearing system and three curved and machined castings.

 The overall weight of each of the four saddle sets is about 400 tonnes.

 Portions of the saddles are coming from England.

  Parts of the deck are being built at a facility in Shekou in China.

 Deck components are being shipped in for assembly from Japan, England, and Saudi Arabia.

 On the Ma Wan tower, where three months ago there was only a bare reclaimed island, the slipform is being dismantled at a height of 195 metres.

 According to VSL's Stuart Pearson, the slipform operation for the Ma Wan Tower ran 24 hours a day for the last 14 weeks.

 Heavy lifting consoles were being installed to lift the two top portal beams into position on the tower.

 Each 250-tonne beam is 30 metres long, eight metres high, and three metres wide.

 The beams will be raised from the ground to their 150-metre and 195-metre-high destinations in a single effort.

 Concrete pouring for the Tsing Yi tower portal beams is on schedule, and scheduled for completion by Chinese New Year.

 VSL Hong Kong Ltd is a member of the Bouygues Group, one of the largest European construction companies.

 VSL's work on the Tsing Ma bridge has involved a team of about 450 people, including 120 engineers and supervising staff.

 The company has also been awarded a contract on the southern section of the West Kowloon Expressway, sub-contracted to Aoki Corporation.

 VSL is responsible for all post-tensioning works, structural bearings and expansion joints for the southern section expressway contract.

  ''It's now in submission and planning stages, with work to start early next year,'' Mr Pearson said, estimating about 21/2 years on site.

 ''This is VSL's core business, which is highly specialised and involves technologically advanced post-tensioning work.

   ''We place the concrete and leave ducts or holes, then place the high tensile steel strand within the ducts,'' he said.

 ''We connect it to our hydraulic jacks. We stress the steel strands, and lock them at the anchorages.''  VSL has been awarded the specialist post-tensioning and structural bearings part of the works on the Western Harbour Crossing approach ramps as a subcontractor to Gammon, which is, in turn, subcontracted to the Kumagai Nishimatsu joint venture.

 ''Gammon is already on site, and VSL is gearing up to start by the new year,'' Mr Pearson said.

 Another of VSL's jobs is the specialist post-tensioning contract for the Rambler Channel bridge.

 He said the job was exciting for VSL ''because it's a technological advance for the construction industry in Hong Kong''.

 Awarded to Dragages, the Rambler Channel bridge involves an innovative contractor's alternative design for a precast segmental bridge.

 According to Mr Pearson, VSL will be supplying and installing Hong Kong's first externally pre-stressed bridge post-tensioning system.

 The design allows highway engineers to tighten the post-tensioning cables at any time during the life of the bridge.

   As a subcontractor to Dragages, VSL will start work in April on the Rambler Channel site.

   ''Essentially, we are supplying the expertise for the post-tensioning system to work with all on-site supervision and installation,'' he said.



Towers ready on schedule