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Boxing day

Northern Ireland - While investment is on the way, strengthening the longest bridge in Ireland could not wait.

The Foyle Bridge near Derry is the longest bridge in Ireland.

Snaking 866m across Lough Foyle, the box girder structure's 234m central span stands 30m above the River Foyle to allow vessels to pass underneath. The bridge was opened in 1984, and carries 36,000 vehicles a day.

The bridge stands on the furthest reaches of the European Union, but fulfilling the requirements of EU standards has meant that major work to the bridge is necessary just 20 years after opening.

An assessment in 1998 by the Northern Ireland Roads Service revealed that strengthening was needed to cope with increased traffic volumes and to enable it carry 40t vehicles as demanded by the EU.

The bridge deck consists of seven prestressed concrete approach spans and three steel spans bridging the river. The steelwork comprises two box girder structures with the 11.4m wide cantilevering decks running in parallel only 100mm apart. It is this steelwork that is in need of strengthening. This is now being carried out under a £10M project.

The fabric of the bridge steelwork resonates with Northern Ireland's industrial heritage: each box girder was made at the Harland & Wolff shipyard in Belfast in the early 1980s to a design by consultant Freeman Fox.

Enough steel to build a medium sized cross channel ferry went into the massive spans (NCE 1 July 1982), 9m high at the concrete support piers to 3.5m high at midspan.

The legacy of the original construction has influenced the work today.

'The main span was dropped in and the entire point of the construction from a design point of view was to ensure that there was no dead load moment at the midspan. With the much greater live load from increased traffic loading this design means that there is some moment capacity at midspan, as there is little moment capacity at the supports. So there is overstress - effectively the increased traffic loading is breaking the back of the bridge, ' says Hyder project supervisor David Yau.

Desktop design studies yielded more than physical inspections. 'The structure is too massive to see signs of distress.

Concrete cracks whereas steel just becomes plastic and starts moving, ' says Yau.

'The problem here is that the bridge is moving anyway in its normal working condition, so it is difficult to identify distress.

But we have found cracked welds.' The chosen solution is a mirror image of conventional prestressing via tendons, which precompresses the structural element. 'We've gone for reducing the compressive forces in the bottom flange over the main piers, ' says Yau. 'This involves putting tension into the steel from precompressed struts.' Buildability and safety were major considerations in the design. Inside, the bridge box resembles an aircraft hangar.

Installing struts at the top would have required working at height, with major safety considerations.

But the thickness of the steel at the girders' top flanges was more than a consideration - a surprisingly thin 13mm in places for a structure of such massive size, suggesting considerable confidence at the time of design in construction and material quality.

The thinness no doubt yielded considerable materials savings but demanded fabrication and construction of the highest quality, leaving little room for error. 'We've needed to strengthen these top plates considerably, while in the bottom by comparison the steel sections have been over 60mm thick, ' says Yau.

But thickening the upper plates is only small beer compared to the major element of the strengthening works being carried out by Northern Irish contractor Farrans Construction.

At each pier diaphragm a concrete anchor block is cast insitu. Pairs of 508m diameter steel tubes with a 50mm wall thickness extend 70m away from the anchor block into the deck boxes, where their outer ends are picked up by fabricated steel anchorages. These tubes are made up of sections up to 5m long, connected by fabricated steel 'goalposts' which also support them above the lower flange and restrain buckling forces.

These tubes are lowered in through slots drilled into the bridge from the carriageway above. These are hidden beneath tarpaulin clad 'sheds'.

Disruption to traffic is limited to lane closures around these, keeping the bridge open and motorists largely oblivious to the large-scale works under way.

Once lowered and assembled, the tubes are precompressed by flatjacks inserted 30m away from the pier diaphragm - these push in both directions to achieve up to 1,000t of precompression.

Massive tapered steel wedges, 1,400mm long and tapering down from 200mm thick, are inserted before the jacks are released.

Forces generated by jacking are transferred permanently to the bottom flanges of the bridge, reducing dead load stresses and giving the extra resistance needed against increased traffic loads.

The strengthening work is also providing an opportunity to revisit the paintwork on the bridge, as well as resurface the carriageway. Completion is due this summer.

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