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Change for the better

Bangkok's Rama VIII bridge is reaching a crucial stage but objectors are now satisfied it will fit with its surroundings. Jon Masters reports

Slipforming of the 165m high reinforced concrete pylon for Bangkok's Rama VIII asymmetric cable stayed bridge is due for completion in January 2001.

This will allow the China State Construction & Engineering Company-led consortium (CSCEC) to start critical deck construction work.

According to members of the Rama VIII consortium, work is now continuing in a more relaxed atmosphere since local objectors were satisfied by changes to the bridge's design.

Appeasement of the objectors would appear to be the final hurdle for a project which has been beset by difficulties since its inception. A consortium led by Philipp Holzmann, which had originally been awarded the Baht 1235M ($29M) design and build contract, withdrew in 1997 when the local currency collapsed.

The scheme was re-tendered in 1998 by client the Bangkok Metropolitan Authority (BMA) and won by CSCEC, second placed in the original bidding.

Lead consultant is Scott Wilson, which is looking after detailed design. Client's engineer is Mott MacDonald.

According to Scott Wilson (Thailand) director Ronnie Thomson, initial objections had concerned its location. The bridge crosses the Chao Phraya river in a historical district of Bangkok. Its new approach road from the east will pass close to a former royal palace, now owned by the Bank of Thailand.

'Objec t ions have genera l ly come from people who feel the presence of a modern bridge will spoil the vista of the old part of Bangkok from the river and from the opposite, west, riverbank, ' says Thomson.

'There have also been land acquisition issues which have now been resolved. The project now has an air of acceptance, which is making life easier for the whole team, ' he adds.

Promotion of the project by the BMA and by Thailand's King Rama IX, who initiated procurement of the new crossing in 1995, has publicised the attractive design of the new bridge, and the efforts made to ensure that its appearance complements its surroundings. The 300m main span will feature a slender deck profile and the bridge will incorporate traditional Thai architectural features, including a decorated facade and a lotus bud at the top of the pylon.

'We have also altered the position and the appearance of the eastern approach ramp to avoid encroaching on architectural features in front of the palace. This has led to the whole span being lowered by 2m, ' says Thomson.

Completion of the project was originally set for October 2000 when construction work began in August 1999.

But the programme has slipped and the unofficial opening date is now June 2001 with a final completion in September after services and cladding have been installed.

Deck construction will begin in January 2001 assuming all goes well with the remaining slipforming work. According to Thomson, the pylon is progressing smoothly due to efficient application of what is now fairly routine technology by main contractor CSCEC.

A free cantilevered method of erection has been chosen for the main span which is expected to take between three and four months. The 30 prefabricated steel deck segments - each one 10m long and weighing 30t - will be brought up river from a fabrication yard and lifted into position from crane barges.

'Erection of the main deck, particularly fixing the cables to the superstructure segments, will be a critical stage in the project, ' says Thomson. 'Part of our job is to analyse the high stresses in the cables during construction, which will be a vital process as there will be some very big forces involved before the deck reaches its seat on the far side of the river.

'The whole operation is further complicated by the need to dismantle and reassemble the cranes on the barge to get them beneath a bridge downstream of the site, and by the strong tidal currents in the river. The new bridge is in an exposed position, and although the deck erection sequence is timed for outside the monsoon season, we will be keeping a close eye on wind speeds.'

Despite these difficulties, says Thomson, importing and lifting the deck segments from the river is still a more feasible option than any other alternative. Fabrication and sequential construction from the riverbank was ruled out by restrictions imposed by lack of space and access to the site.

Support for the main span will be provided by a twin array of steel cables.

These will splay out from close vertical lines of connectors on the pylon to the outer edges of the deck. A single array of cables will connect the tower to a 75m reinforced concrete anchor block below the 175m back span, which will be supported on concrete columns.

Construction of the post-tensioned concrete back span and preparation of the rear array of cables is also on course to finish in time for the start of the main deck operation in January. Bangkok will then be a significant step closer to having one of the world's largest asymmetric cable stayed bridges and another four lanes to get traffic over the Chao Phraya river.

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