Bridge installed over the Cedar Valley lines at Rochdale

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Engineers indicated an inauspicious start to the possession to install 405 tonnes of metalwork over the Calder Valley lines at Rochdale. The site engineers had already struggled with Siberian temperatures in the run-up to the lift and the bridge had an unusual five-sided shape to complicate matters. What else was waiting to upset the 18 months of planning for this new Metrolink bridge?

Geometry lesson

The New Year countdown held a special significance for engineers at Rochdale this year. Network Rail had granted a 72-hour possession, starting at 23:50 on 31st December, to install a 52m long bridge – one of the largest of the new structures on the Manchester Metrolink extensions. This bridge has a rather special shape, being an irregular pentagon in plan. An additional centre pier on one side supports the point of the pentagon. Thus, one side of the bridge consists of a single span of 52m whereas the other has two spans of 33m.

Trying to define the skew of the structure is tricky as there is not a single obvious feature to measure. For instance, the long main girder is approximately 70 degrees to the tracks below. However, the east abutment is also skewed to the tracks, giving a skew of 60 degrees between girder and abutment. On the two-span side the skew is lower at 52 degrees. Yet the west abutment is square to the tram and heavy rail tracks. So it’s probably safest just to say that the skew is high.

The reason for this shape is to give a grade-separated crossing within the constraints of a site that never originally accommodated one. The location is just east of Rochdale Station on the Calder Valley Manchester to Leeds line, at the former junction with the Oldham Loop. Previously, trains from Oldham ran into the bay platform at Rochdale or joined the main lines to run on to Manchester.

The Oldham Loop is being converted to operate trams as part of a new Metrolink extension. They will remain separate from the heavy rail network, initially terminating at a tramstop beside Rochdale Station. The confirmation of further funding in August 2011 means that ultimately the trams will continue on the streets into the heart of Rochdale town centre, around 1km further north, thus making full use of their ability to integrate with road traffic into the urban environment.

For the 700m between the station and the old junction to the Loop, the tram will make use of the alignment of the long-abandoned Down Goods line on the north side of the formation and will share the unused width of five Network Rail underbridges, similar to the arrangement outside Manchester Victoria. The section between Rochdale Station and the next stop at Newbold, including the new bridge, will be single-line running for the tram.

At the crossing the tram needs to rise within its own footprint while still running parallel to the Network Rail lines, then turn sharply to cross and join the Loop line to the south. These constraints are overcome by harnessing the tram’s ability to climb steep gradients and negotiate tight curves. The chosen approach uses a 4% gradient giving a ramp length of 180m. The curve is 150m radius with 30m transitions, giving a design speed of 25mph.

The centre pier picks up one side of the bridge where the tram alignment starts to transition back to the parallel-running straight. This was considered preferable by the bridge’s designers, Jacobs, to the alternative of a portal-type structure which would have had large unused areas and create a tunnel effect.

Other characteristics of the bridge were set by considerations of maintenance and future-proofing, both for the Greater Manchester Passenger Transport Executive – the bridge owner – and for Network Rail considerations. Thus, a clearance of 5.1m over the running rails was provided to allow for potential future electrification by Network Rail, weathering steel was chosen to eliminate future painting, and the centre pier was set away from the running rails by 4.5m to avoid any requirement to design for collision loading. In addition, the east approach was complicated by the need to retain a connection from the tram system to the heavy rail network, allowing for future access by works trains.

Rochdale bridge at factory
The trial erection of the structure at Mabey Bridge's factory

Kit of parts

The fabricator, Mabey Bridge, has turned the construction of steel bridges into a reliable, well-controlled process. Prior to the possession, the usual good practice was followed with all the metalwork on site by the week before Christmas in carefully assigned positions. The main girders were delivered to the site in sections and spliced together.

The bridge is of half-through form with 3.25m deep main girders, with 21 deck panels spanning transversely between them. Mabey followed its usual procedures to produce a computer model of all the metal elements which was then used to control the cutting of the plates. The entire structure was trial-erected at the firm’s Chepstow plant at the end of November.

The deck panels include shear plates at the ends of the transverse beams, to allow the deck panels to slot directly onto matching plates on the main beams during erection. The final weld-up of these critical connectors occurred during the trial erection with the plates bolted in place, in order to ensure accurate alignment and negate the effects of heat distortion during the welding. The stud-end connections were angled to assist with ease of erection.

Mabey was contracted to fabricate and erect the bridge. The planning included a series of hold points at which the erection could be aborted with the structure in a safe configuration, although obviously it was hoped that these would not be needed. The main concern beforehand was high winds during the lift.

Rochdale bridge beam
The 52m beam was lifted by a 1,000-tonne crane before members of the workforce guided it into place

Let it snow, let it snow, let it snow

The principal contractor for the tram project is MPact-Thales, a consortium of Laing O’Rourke, VolkerRail and Thales. With the steel fabrication in hand in South Wales, MPT’s concerns were limited to providing something for the steelwork to sit on and provide access for the cranes.

MPT planned to give its employees a Christmas break with work stood down until the New Year possession. However, the final few concrete pours nearly gave the turkey a lucky escape – with overnight temperatures dropping to -12C, even the north-west’s most advanced concrete batching plant froze up.

Likewise the final batch of deck plates was thought to have left the fabricator’s yard when reports came in that traffic was at a standstill on the M5 motorway due to the snow. All this goes to show the importance of a bit of float in the programme to cater for the unexpected.

Site access was constrained and was along the trackbed to the west abutment. Fencing was established early on at 2m from the Network Rail running rails to allow both abutments and the pier to be constructed in the segregated protection of green zones. This required the semaphore signal cable to be shifted beyond the fenceline.

The Ainscough cranes had to sit in the yard of an adjacent distribution depot. For the longest main beam – a 110-tonne lift at 22m radius – a 1,000-tonne crane with superlift was needed. Unfortunately as the crane would block the doors to the depot, it was not possible to bring it into position until depot operations ceased at 16:00 on 31st December – hence the electrical problems during rigging the crane were only discovered after the possession had been taken, rather than beforehand.

Lifted spirits

The crane fault was resolved and erection commenced. Fortunately the temperatures had increased to (just) positive figures and there was little wind. In addition to the thousand-tonner, a 200-tonne crane and another of 55 tonnes were on hand at opposite sides of the tracks to lift in the first of the deck panels. Four MEWPs (Mobile Elevating Work Platforms) were also required to provide access for bolting the connections.

The erection proceeded swiftly and smoothly, undoubtedly helped by the presence of a hot food van on site doling out free bacon butties at all times of the day and night. All main girders were in place by 13:00 on 1st January and the deck panels followed at around two per hour to complete shortly after 00:00 on 2nd January with all bolts hand-tight. Once all steelwork was in place, final tightening of the bolts followed.

Further possessions over subsequent weekends have completed the butt welds between deck panel units and allowed the installation of precast panels for the western approach. In addition, over the coming summer Network Rail will be resignalling the Rochdale area to immunise against the DC power drawn by the tram and to remove Rochdale’s signal box which will not have adequate sighting once the tram begins operation.

Rochdale bridge elevating platforms
Engineers worked from elevating platforms to bolt the panels together

Handback

The final ‘over and out’ text message says it all – apart from the sense of relief for works satisfactorily completed. Additional work was also carried out, including the removal of falsework used during the abutment pours that would otherwise have needed an additional possession. MPT’s senior engineer Rob Salmon said “It was a real team effort and everyone worked together to make this a successful lift.”

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