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Slender steel speeds bridge deck delivery

It is going to take until the middle of 2016 before passengers travelling through Reading Station realise the full benefits of its £644million upgrade.

For Bam Nuttall project manager Mark Hepburn though, the eye is drawn by a rather more looming deadline – a 10 day rail blockade at Christmas.

He and the project team working on the £12.5million Reading Station civil improvement Package 2 will have just 50 hours of that blockade to remove the two existing box-girder decks that currently enable trains to cross over Caversham Road on the western approach to the station and slide in the new section.

As one of the most pivotal stages in the entire project, the success of the bridge slide is key to the success of the blockade.

Tight level limits between the track bed and the crown of Caversham Road as it passes under the 40 m long, 17 m span bridge meant that the designers were limited to using a steel solution for the new bridge. A concrete bridge deck capable of taking the high loading required would have proven too deep to enable the team to provide the cover required between the top of the deck and the underside of the rail.

So, the team turned to an orthotropic steel deck section solution to help.

Steel is the only answer

“There just isn’t the room for a concrete deck. We could have been looking at a 900 mm depth in concrete and we need every single centimetre we can get,” says Mr Hepburn, adding, “We could’ve lowered the road to maintain the headroom but that would have meant moving the services. The slimmer steel section allows us just enough room between the deck and the track.”

The team settled on a 650 mm deep section, as an even more slender 600 mm section developed by consultant Gifford was tripped up by Network Rail’s stringent design codes. That thinner section was deemed too thin to satisfy some of the requirements of the codes.

That 650 mm deck itself is being fabricated by Bolton based specialist Watson Steel in 3 m wide deck pieces. Three inverted ‘T’ sections located at the centreline of the deck piece and 1000 mm each side of that central ‘T’ provide the stiffness required for the 40 mm deck plate. The deck sections are then brought to Bam Nuttall’s site compound, a hundred metres or so from its final location.

Here the 17 m long, 650 mm deep sections are lifted onto their final bearings and stitched together using a series of bolted connections at 250 mm centres.  For the final 900 mm at each end of the 17 m seam, the sections are connected though a full penetration butt weld.

The sections run into a concrete trimmer beam with every second ‘T’ beam sitting on Freyssinet bearings. Even this concrete cill beam design has been fraught with difficulty. Normally the cill beam would be cast in one section with the ballast wall, but because of the complexity of the welds required at each end of the span there would not be enough room to physically carry them out if this had been the case. Instead the cill beam will be installed after the welds have been completed using a precast concrete beam, post-tensioned and grouted around Macalloy dowel bars.

Once completed, the final 1,500 tonne bridge deck will be lifted onto the giant multi-wheeled carrier boasting two twin-wheeled, double bogey transporters. Both of these feature a row of 36 bogies and are offset some 1450 mm each side of the centreline of the 17 m span during transportation from the site compound to the bridge itself.

Once in position, the deck will be lowered onto the existing abutments which have been extended and cut to level ready to cope with the new bridge. For a relatively small structure, it is quite a challenge but there are other difficult steel structures on the scheme. The widening of the Vastern Road Bridge to the east of the station will be another complex part of the same scheme that Mr Hepburn has to deal with.

The project is proving an engineering challenge but one that is being beaten thanks to the slenderness of steel.

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