Just over a year after starting work on the piled foundations of an unorthodox fi nback bridge near Manchester city centre, Balfour Beatty/Bilfinger & Berger joint venture is poised to rotate it across the Manchester to Rochdale Trans-Pennine main line railway.
The odd and, in some people's eyes, less than beautiful light rail crossing is a 130m long, twin span, post-tensioned reinforced concrete box girder. What makes it rare and, to the connoisseur, wonderful, is that its structure is above the deck, not below (NCE 17 March). It is being built in the hope that the stalled Manchester Metrolink extension will be given permission.
It owes its design to site constraints - the Metrolink line has to clear Network Rail's tracks allowing headroom for future electrification. The route snakes under an existing road bridge which allows little room for a gradual height gain, yet is constrained by a maximum gradient of 6%. Keeping deck depth to a minimum - 600mm - and putting the box girder above the tram running surface, was the only way of making the structure work. For added complexity the finback bridge is on a sinuous curve and skewed across the railway to match the planned Metrolink extension's alignment.
Balfour eatty/Bilfinger & Berger opted to build the structure offline to avoid working over the railway. The bridge has been built alongside the main line tracks and will be pivoted into place on its single downstand pylon.
Although not working directly above trains, the contractor has still been working within spitting distance. 'The bridge is 3m away from the tracks at its nearest point, providing a 1m working zone. That's tight, ' says Balfour Beatty project director Jerry Corvin.
The nback bridge is heavily reinforced - 760t of its 4,000t weight is rebar. Corvin says that the job called for several different C60 concrete mixes, including self-compacting concrete to penetrate the matrix of T40 bars. These were at 125mm centres and locally displaced to make room for posttensioning cable ducts.
All that, however, is in the recent past. The next challenge is to post-tension the structure.
Subcontractor Freyssinet is installing 17 strands running inside the box girder over the crest of the fin. As they are tensioned the bridge deck will lift 60mm off its supporting falsework. Because of its double curve it will also deflect in plan by about 30mm.
Probably Corvin's most nail biting 12 hours will come late this month when his team swings the bridge 21.3° about its pier, across the railway lines, landing the nose of the main span on the eastern abutment, 91.6m from its present position.
The bridge's pier marks a point at exactly one third its length, meaning that the main span and backspan are out of balance - the main span exerts an overturning force of 1,000t.
When designing the rotation procedure Balfour Beatty/ Bilfinger & Berger considered counterbalancing the main span but the sums quickly ruled that option out: a massive 2,000t of weight would have been needed.
Instead, a hefty 1.5m wide, 40t steel skid beam will be placed across the railway. A 280t temporary steel extension bolted to the nose of the deck will track along this as the bridge swing takes place. A pair of 500t jacks between the extension and skid beam will ensure loading of the beam remains constant as the end of the bridge traces a radius across the railway lines.
The bridge will be propelled along the beam by a 900mm stroke hydraulic ram pushing against lugs jutting from the beam's top flange.
When the bridge has reached its final alignment the two 500t jacks will be extended, pushing the backspan bridge end down onto its abutment so that it can be anchored into place. The main span bridge end will sit down onto its abutment under self weight.