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Staying in line

Bridges Push launching

Elegance usually comes at a price.

On the Broad Meadow Bridge project that price is the extra complications of the bridge slide. Words and pictures by Adrian Greeman.

At first sight there is not much to connect a swan filled estuary in Ireland with a rocky river valley in South Africa. But a new bridge under construction for part of the new Dublin to Belfast motorway does just that.

The link between them is an incremental push-launch method of construction and the dangers it brings of puncturing the soffit slab of the concrete box during the launch. Fourteen people died and 10 were injured when South Africa's Injaka Bridge collapsed only a quarter of the way through its launch in July 1998.

'The problem was that bridge bearings on the pier tops were not seated underneath the box webs, ' explains Mungo Stacy, engineer at specialist consultant Robert Benaim & Associates, which has devised the push launch for contractor Nuttall and its Irish joint venture partner Ascon.

'That was OK in the permanent position because they were underneath internal diaphragms which could take the point loads. But during the move the diaphragm slid along.

With just a hollow box above, the bearings pushed through the concrete, leading to a progressive collapse.' (NCE 8 August).

The simple answer would be to make sure your bearings are correctly placed for temporary and permanent conditions.

Unlike the inexperienced engineers at Injaka, most designers would be sure to do just that.

But there is more to it. The new Broad Meadow estuary bridge's twin concrete deck box girders have an unusual cross section with an elegant curving soffit, rather that the usual flat bottomed trapezoid. The latter would be much easier to slide on horizontal temporary bearings with side restraints to keep it in line.

But Broad Meadow's curved soffit allows only for a single obliquely tilted bearing on each side. The danger then is that the bridge could misalign during jacking, bringing hollow sections over the bearings.

Tackling that problem was a key factor for Benaim when it was brought in to look at alternative construction methods for the bridge. The existing design, by Arup Ireland, used balanced cantilever construction, chosen originally from a range of alternatives because of the environmental sensitivities of the estuary's inter-tidal salt marshes.

Balanced cantilever construction would mean a lot of work could be done above ground away from sensitive species.

Arup produced a five span design, with a longest span of 69m. Architect Wilkinson Eyre Associates worked with the engineer on the architectural details, choosing the curved soffit form to reduce visual intrusion.

But balanced cantilever is heavy on prestressing. Once Nuttall had won a £47.5M contract for a 6.3km length of the dual two lane motorway, it called in Benaim to reexamine the bridge and see if any savings were possible.

Benaim's solution keeps the bridge form but switches methods to a push launch. This allows the prestressing strand to be gathered together into a small number of long runs rather than the many short runs of strand needed repeatedly as balanced cantilevers grow outwards. The Benaim design uses external post-tensioning, with just 24 cables running through each deck box.

'And that produces significant savings, ' says Benaim director Mark Raiss. 'Anchorages are expensive and this way you can cut them from 1,320 to just 48. Admittedly they are bigger.' Once the proposals were examined more closely, Raiss adds, it became clear that all stressing operations could be postponed until the bridge was complete - provided there was support at the half-span positions. Temporary steel support towers would have to be erected. That means some additional temporary piling is needed in the important sea marsh, although it does not add substantially to the overall temporary works, argues Raiss.

All of which culminated in a first push early last December.

Nuttall had set up a casting yard for the 20m long deck sections on the south bank, with a curved make up jig behind the formwork; reinforcement cages are assembled here and then skidded forward into formwork for casting. Reinforcement for the inner diaphragms is made and then a second set of hanging forms on a travelling gantry is used for a second pour to make the interior webs and central top deck.

Once an eight day forming cycle was complete the challenge was to keep the deck under control and in line during the jacking sequence with only two inclined temporary bearings for guidance.

These sit underneath the internal longitudinal web positions in the box, as do the permanent bearings later in the process. The latter were to be tilted at 18infinity on either side of the box.

'That sort of angle in fact is plenty to generate enough lateral force to keep the bridge on line, ' says Stacy. 'Once you have overcome the friction with the longitudinal jacking forces there is very little needed to push the bridge sideways.'

The soffit profile of the bridge was modified slightly to create a flat surface at the bearing position for the stainless steel and PTFE slide bearings. Angle and position of these faces were modified by fractions of a degree, to allow the bearings to sit slightly asymmetrically, one 290mm higher than the other.

'Fine tuning was needed because there is a crossfall of 3infinity on each deck, ' says Stacy.

The result is that the box girder wants to turn slightly as it is moved forwards. It still needed to be watched carefully.

'Learning to control that took a bit of time, ' says James Maloney, Nuttall structures manager. The deck was stopped and surveyed every three 450mm strokes of twin 300t jacks, using small steel reference plates in the deck. Correcting any movement off line was done using 6t of kentledge variously positioned on the deck.

'The tendency to move off line did reduce as the deck increased in length and reached its full weight of 9,000t, ' Maloney reports. The first deck was successfully in place by June this year, with three weeks following for the strand threading and post-tensioning.

External cables are grouted into plastic protective sleeves.

The second deck is now about eight sections complete, around half way, and the whole project is due to complete in August next year.

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