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Beefing up Belmont

Full closure of a major commuter route is usually to be avoided, but for a vital Stockport bridge strengthening scheme, it was the only option available.

Contractor Galliford Try is just coming to the end of its seven-year “Bridgeguard 3” framework with Network Rail to bring structures in the London North Western territory up to the standard required to cope with loading from 44t vehicles. The final bridge on the list, the Belmont Bridge in Stockport, is one of the most significant, and strengthening it has required a complete closure of the busy A6 – one of the main Manchester commuter routes.

The Belmont Bridge carries the A6 over a freight line used by 24 trains a day and often called into action as a diversion route for passenger services on the West Coast Main Line. The bridge was built in 1937 and widened in 1957, and now carries up to 25,000 vehicles a day. It is a three-span structure, with the two outer spans built in steel beam and brick jack arch construction and the centre span predominantly steel beams, with cast iron edge beams.

When the bridge was assessed for compliance with EU requirements for 44t axle loads, it was found to be significantly under strength. “When we did the assessment we found that the two side spans were rated at 7.5t and the edge girders of the centre span were rated at less than their dead load,” explains Andy Wood, Network Rail’s scheme project manager.

As an interim protection measure the local highway authority, Stockport Metropolitan Borough Council, erected concrete barriers to keep vehicles away from the edges of the centre span while a strengthening scheme was devised. This involves demolishing and replacing the entire centre span and adding a new overslab deck to the two outer spans.

The remainder of the structure – including all the brickwork to the abutments – was in reasonable shape, with the steel beams on the outer span simply requiring grit blasting and painting.

“On the outer spans, the space between the beams was just infilled with rubble and then waterproofed over, with surfacing on top. There was no deck as such,” explains Galliford Try project manager Paul Clarke. “We’ve removed the existing overburden and replaced it with a 300mm-thick reinforced concrete slab.”

The only way to carry out the work was to completely close the A6 for a period of eight weeks. “Usually it’s rail possessions that drive a job like this, but with this being such a sensitive road, it was the use of the road that dictated the programme. Network Rail and Stockport Council had a lot of discussions and felt that the best time to do it would be during the summer holiday,” explains Clarke.

In advance of the road closure – which began on 2 July – the contractor erected a temporary bridge on each side of the bridge for pedestrians, and closed the bus lanes so the gas, water and electricity services located beneath the surface could be diverted onto the temporary bridges. A fibre optic cable that serves Manchester Airport has not been diverted but supported while the work is carried out.

“We had such a tight window when the road is closed that the pressure was really on to make sure the statutory diversions and the temporary work was all completed on time,” says Clarke. “That required a lot ofnegotiations, and in some ways that’s been the hardest bit of the job.”

As soon as the road was closed, the surfacing was stripped from the entire bridge structure and the fill removed from between the beams of the outer spans. The following weekend, during a 52-hour possession of the railway line, the centre span of the bridge was demolished. One week later, under another rail possession, the new beams for the centre span were lifted into position.

The old steel and cast iron beams have been replaced by 12 new steel cill beams, sitting on the existing abutments and supporting 41 box beams. These have been made by encasing 305mm x 198mm UB section steel I-beams in concrete, to create beams that measure 340mm in depth and 510mm in width. They span 18m between the cill beams, and each weigh 30t.

Galliford Try used a 500t mobile crane to lift the new beams into position. “We couldn’t put any load on the two outer spans, so we needed a crane with a lot of reach,” explains Clarke.

The design for the beams themselves was dictated by the existing alignment. “The biggest challenge for the designers was to design a structure that maintained the headroom of the railway but didn’t alter the current road alignment,” says Clarke. “You would generally go for prestressed concrete beams, but we’ve gone for box beams to keep that construction depth to a minimum.”

Wood adds: “It’s a way of reducing the construction depth rather than having deep beams with shear studs and a composite slab. It also increases the stiffness of the beam so it’s able to carry more load with a smaller section.”

After the two rail possessions in July, the road remained closed while the contractor constructed the two deck slabs.The new overslab on the two outer spans is a chunky 300mm thick, as it replaces the fill that was providing structural strength before, and gives the extra strength required to upgrade the bridge for 44t loads

Project Details
Project: Belmont Bridge
strengthening, Stockport

Client: Network Rail

Design and build contractor: Galliford Try

Engineer: Gifford

Value: Ł4M

Conract period:
April-October 2007

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