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Ready for lift off

KINGSTON BRIDGE; The most crucial stage in the costly repair programme of Glasgow's Kingston bridge takes place this weekend.

On Saturday morning, about 50 engineers, backed up by Scotland's largest traffic management scheme involving 200 police, will gather to oversee what is claimed to be the world's heaviest lift of a single structure. The 50,000t deck of Glasgow's problem-ridden M8 Kingston bridge should be raised 15mm clear of its bearings so repairs can be carried out. And the main desire of the engineer in overall charge of the £21M operation is that, by dawn on Monday, it will all have proved as much an anticlimax as the small distance the 10 lane bridge deck is being lifted.

'I would like it to be totally uneventful and lack any excitement,' says Ian Telford, engineering manager for Glasgow City Council, agent for client the Scottish Executive.

His feelings are understandable. As head of the team overseeing repairs of the multi span river Clyde crossing, with its horrendous 10 year structural history, Telford could benefit from less stress - both on the bridge and personally.

The lift has taken five years to plan, has cost double initial estimates, has demanded a total redesign and has ended up more than two years late.

Behind him now are decisions on how to cope with the main span's 300mm sag and its northern pier's 165mm outward tilt. He knows how to deal with the displaced rocker bearing on top of this pier and when to push back the 268m deck which slid northward with its support.

What he does not yet know is how uneventful this weekend will prove during the key operation needed to allow all these repairs to take place. Only with the deck raised just 15mm clear of its supports, and held there on jacks for the next seven months, can contractor Balfour Beatty replace the weak piers, change what was an 'inappropriate' bearing arrangement and - through its jacking subcontractor VSL - push the deck back into position.

On paper the operation looks relatively straightforward. New 2m thick concrete sides for the replacement piers have already been cast just millimetres outside their 9m tall weakened originals.

After a network of 128 computer controlled jacks on top of the new pier sections has raised the deck, a separate set of 24 thrust jacks will push it 30mm southward to correct its displacement. Old piers and bearings will be slid out sideways and the 3m wide gap they leave will be infilled with concrete to create a solid double width new support.

But what the drawings do not show is the complex interaction of loads and forces between jacks and deck - which in reality is two separate adjacent bridges linked by cross passages.

The total 224 jack system must raise and push this deck without imposing any additional stress on the weak structure. But the same network must then in effect take over the role of bridge bearings and accommodate all its live load twists and deflections, while the deck is suspended for months with 150,000 vehicles crossing it daily.

And this is the client's real doomsday fear. The lift and push take place over the next two weekends with traffic diverted.

But afterwards, if any of the 200-strong array of instruments monitoring the deck until next summer indicate that it is suffering further distress, engineers may be forced to immediately ban traffic from one of Europe's busiest motorway bridges.

A sudden unplanned closure would plunge the area's road network into gridlock within hours at an estimated cost in traffic delay alone of £500,000 a day.

To minimise the 'impossible' happening, VSL faces some of the most onerous tolerances ever placed on a lift. No jack must be more than 1mm out of level from its neighbour across the 42m wide deck or 2mm different along the full 268m lift section.

It is little wonder then that VSL agent Bruno Delalande describes his £10M jacking network as 'by far the most complex and sophisticated yet designed'.

He would, however, have said the same two years ago when the originally suggested jacking system came off the drawing board for a planned October 1997 lift. But factory and site tests then revealed a small though crucial problem.

'There were difficulties ensuring that the jacks always kept within tolerances and would fail safe before exerting eccentric loading on the deck,' Telford explains. 'So we had to go back to square one and start again.'

The jacks, then operated in lines of four either side of a pier head, include a backup network in case of failure. Only half the jacks are needed to lift the bridge and the reserve network will carry just 10% of the total load.

The concern was that, if the primary system failed or went out of tolerance, the backup jacks would not take over full loading quickly enough to prevent differential point loads damaging the deck.

Redesign options included a new jacking regime or strengthening the 8m high pier top deck diaphragm to ensure it could accommodate such loading. It took a year to decide on the former option and another to redesign the lift mechanism.

So this weekend the jacks will be linked at each pier in four groups of eight rather than eight lines of four. This means that, laterally across the deck, there are only four separate lifting points, eliminating the possibility of differential loading.

Cross passages linking the adjacent decks will be temporarily cut allowing the two halves to deflect separately.

The two-year delay has seen Balfour Beatty's £14M tender price more than double, with all the £17.5M rise attributed to the jacking problem. But major claims battles have been avoided by the client accepting from the outset total risk for the lift, so the Scottish Executive will pay up regardless of the cost.

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