08.12.17
Carlisle station: a glass act
Galliford Try is nearing completion of a major scheme for Network Rail at Carlisle station. The core of it is the replacement of glazing with ETFE, a type of clear plastic, but accessing the roof to carry out the work was a project in itself, as Paul Kirkwood reports.
Carlisle Citadel station, as it was originally named, was built in 1847-48 and further extended in 1875-76. Since then, incremental modifications have been made to reflect changing operational requirements, including a significant truncation of the train shed roof and removal of Gothic-style end screens in 1957-58. These more recent structural changes were key factors in its deterioration, which led to concerns over the safety of the glazing prompting remedial action.
The scale of the £11m project is huge. Some 110 tonnes of glass covered six platforms and the eight tracks in between, an area 160m long by 51m wide with 13 symmetrical multi-pitch valley gutters and 14 ridges. Not surprisingly, the scaffolding element of the scheme was a major undertaking. Starting in January 2016, scaffold towers and the protection deck were constructed with temporary lighting being installed as the deck progressed.
The greatest challenge of this phase of the programme was extending the scaffold deck 23m over the West Coast Main Line. Work was possible above the line only during one weekly possession between 2-8am on Sunday mornings. Cherry pickers couldn’t stretch far enough from the platforms and the position of overhead wires and supports prevented hoisting scaffold from platform level. The answer? Using a novel technique, the team assembled units of braced cassette beams then hooked them onto parallel Tirfor wires slung over the track between scaffold towers on the platforms.
As each unit was hooked and pushed out, the scaffold bridge got closer to opposing tower until it reached it. Crucially, the bottom brace between the beams provided the strength. The wires were slackened to lower the scaffold bridge onto caster wheels on rails extending from both towers in the same direction as the railway line. The bridge was then manoeuvred sideways into position and the next set of beams strung across. The wire set-up had to be re-rigged in six different locations so that the scaffold covered the whole of the railway line.
Other elements of the scaffold were designed to take account the location of cellars, old manholes, abandoned subways, inspection chambers, air shafts and other voids beneath platforms. In the absence of archive drawings, the team used ground penetrating radar to establish the integrity of the platforms and their foundations, and installed temporary bracing in places to ensure they could withstand the scaffold loads. Once the scaffold deck was in position the old glazing was removed and ancilliary and additional strengthening works were carried out.
The re-roofing could then finally begin. The material used is ETFE (ethylene tetra fluoro ethylene), a flourine-based clear, single skin plastic which, compared to glass, is lighter, offers greater light transmission, is easier to install and repair, and can cope with extremes of temperature. The roof still looks glazed due to the inclusion of new mullions. The exceptional lightness of the new roof necessitated the installation of a steel tie-down arrangement to prevent the risk of the roof being caught by an uplift of wind like a giant kite. Thirteen brackets on the roof steelworks join to steel rods coming down to the platforms, which are connected to buried weights. The system can counter estimated total uplift loads of 115 tonnes. Finally, all the wrought iron trusses were painted.
The new roof will all be in place by November, which will leave the small matter of 1,400 tonnes of scaffolding to finish dismantling. Full completion is due in February.
FOR MORE INFORMATION
W: www.gallifordtry.co.uk