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Jacking rethink delays Kingston again

STRENGTHENING WORKS to Glasgow's troubled Kingston Bridge have been delayed for a further 12 months, it was revealed this week, as engineers struggle to finalise plans for a complex deck jacking operation.

Balfour Beatty Construction is already more than 30 months behind on its original 19 month, pounds14M contract, and costs are now expected to soar by at least pounds5M. Site work has been at a standstill since January.

The 28-year-old Kingston Bridge, said to be the UK's busiest motorway crossing, is suffering from a 300mm midspan sag and a tilt in its northern pier. To redress both problems the deck, complete with full traffic flow, must be raised 15mm for six months while piers are replaced, bearings altered and all three spans post-tensioned (NCE 5 February)

But plans to raise the entire 143m long, 10 lane bridge deck this month using a network of 128 computer-controlled jacks have been put back for a year after more potential safety problems emerged in the continuing design process.

'During jacking tests we realised there was a possibility of introducing differential loading into deck diaphragms above main piers and this could lead to cracking,' said Ian Telford, engineering manager for Glasgow City Council, agent to bridge owner the Scottish Office. 'We have now had to change the proposed jacking technique and redesign much of the software that controls it.'

Lifting specialist VSL must ensure that the pounds2M jacking system can safely accommodate and counteract every conceivable loading regime on the deck while it is raised. Introducing unknown forces without 100% failsafe detection techniques could lead to immediate closure and total traffic gridlock over much of Glasgow.

'Safety is paramount and we are not going to start lifting anything until we are all sure we have covered every possible 'what if' scenario,' said Telford, who heads the 20-strong full-time jack design team.

'There would be no possibility of collapse, but any damage to the structure while it is raised could require the bridge to remain closed during emergency repairs and this would, in traffic terms, be totally unacceptable.'

David Hayward

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