Carlisle Citadel station is an architectural gem. Originally opened in 1847 to serve both the Lancashire and Carlisle Railway and the Caledonian Railway, it replaced several smaller stations located around the city. Being located close to the border of England and Scotland, this elegant station, even today, forms an important hub on the West Coast main line.

The station buildings have an interesting and complicated history – one that over the years has included some structural problems. Now, major refurbishment works are in progress that will rejuvenate the station’s ailing roof and, at the same time, greatly improve the platform ambience.

Constructed in a mixture of neo-Tudor and neo-Gothic styles, the station was rebuilt and enlarged in 1878-80 after the Midland Railway’s network reached the Border City via the Settle and Carlisle line. The architect was Sir William Tite, who designed many early railway stations in Britain and France, as well as the Royal Exchange in London.

The 1880 extensions to Citadel station created an elaborate building with a 400-foot frontage. It eventually served seven different railway companies, each of which had their own booking and parcels offices and passenger facilities. To complement Tite’s work, engineers Blyth and Cunningham of Edinburgh designed a seven-acre (2.83 hectare) iron and glass roof with giant screens at each end that featured ornate wooden glazing bars in a Gothic style.

The roof structure comprised 26 deep-lattice (double Warren) trusses spanning the platforms and tracks at 12.2 metre centres. Each girder had ten panels, stiffened end posts and a flat bottom tie. They supported slender cantilever half-truss hooped beams running parallel to the tracks at approximately 3.7-metre centres. The entire roof was glazed using shingled panes.

Cut backs

After neglect during World War II and afterwards, the whole roof began to fall into dilapidation so, in 1957, a decision was taken to reduce its area and repair what remained. The screens at each end were demolished, as was a large area of roof on the southwestern side of the station. At the same time, the original shingled glass panes were replaced by much larger ‘Patent Glazing’ panels. Today, the station and its roof are nonetheless impressive. Grade II* listed since 1972, the roof still sports a 50-metre clear span.

By 2014, it had become clear that intervention was again required, with the roof failing and becoming something of a liability. Investigation of the structure indicated that the steel roof trusses had sagged. This was blamed on a combination of age and the radical shortening of the existing roof span in 1957. The effects on the existing rigid glazing system and the roof drainage were severe, causing ponding of rainwater, leakage and the cracking and breakage of multiple glazing panels. Indeed, areas of the roof had been netted following falls of glass onto the station platforms.


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Access to the roof for repairs and cleaning had been restricted due to safety concerns. As a result, the glazing was in a filthy condition, which limited the light levels on the platforms below. Compounding this, the station’s lighting system was also sub-standard, resulting in light levels at the platforms being in the region of just 100 lux – equivalent to a very dark overcast day. Clearly, it was time for some serious remedial action.

Sensitive

The challenge for Network Rail’s appointed design consultant Arcadis, with support from architect Jefferson Sheard, was to provide a sensitive yet contemporary roof replacement that would preserve the original architectural aesthetic. Vital, too, was the proviso that the repair works should cause no disruption to the ongoing rail and passenger activity below.

Network Rail and its consultants have worked closely with Historic England and Carlisle City Council in order to plan the refurbishment whilst, at the same time, protecting the station’s listed building status. Separate listed building consents have been required for alterations to the roof itself, for new lighting and for the inclusion of holding-down anchors – of which more later.

Foiled

The chosen solution is centred upon the use of ETFE (ethylene tetrafluoroethylene) foil sheets and aluminium framing. Manufactured by Vector Foiltec and marketed under the Texlon® brand name, this fluorine-based co-polymer material exhibits high corrosion resistance and strength over a wide temperature range. As well as being much lighter than glass, it offers greater light transmission and is shatterproof.

Key to ETFE’s use as a roofing and glazing material, it can be stretched (by up to three times) and it will remain taught even if some variation in size occurs, such as by thermal expansion. Being related to PTFE, it also has a non-stick surface, which means that it is self-cleaning.

Although the Texlon® ETFE system has been available for over thirty-five years, its adoption for use on Network Rail structures has been relatively recent. Notable examples include Birmingham New Street station and the Manchester Victoria station concourse, as well as smaller schemes such as the footbridge at Newport and the Underground station at London Heathrow Terminal 5.

Other high profile applications within the UK include the Eden Project in Cornwall and the National Space Centre in Leicester.

A £19.5 million two-phase programme of work commenced on 30 November 2015 to reconstruct the damaged roof and to rebuild the station’s eight platforms. The first phase included repairs to the roof trusses and replacement of the glazing at a cost of £12.5 million. The task undertaken by Network Rail’s appointed main contractor Galliford Try has not been a simple one.

Decked

Network Rail and Galliford Try have worked closely with Virgin Trains, which manages the station, to plan the work and minimise its impact on station users. The improvements have been made possible thanks to a huge scaffolding access deck which has been installed nine metres above the tracks through the station. As well as protecting the station’s platforms and running lines, it has provided safe access for the workforce. With the glass removed from the roof, this waterproof deck has also kept the station platforms dry.

Installation of the scaffolding and deck structures was, in itself, a significant feat of engineering. Covering an area equivalent to one and a half football pitches, it was estimated to weigh some 1,400 tonnes. Spanning four tracks of the West Coast main line and two bay platforms, each electrified at 25kV, it presented an installation challenge that took nine months to complete.

De-energising the OLE to allow the scaffolding installation work to be undertaken was initially restricted to just a four-hour time window each Saturday night. Although later extended to weekly six-hour slots, this restriction necessitated careful planning in order to maximise progress. For instance, cassette beams were pre-constructed and then slid across the tracks, suspended during this process from tensioned steel cables.

With the scaffolding and crash deck finally in place by November 2016, work could begin on removal of the 1950s Patent Glazing. North West Recycling Ltd, based at Kingmoor Park, Carlisle, has recycled most of the 114 tonnes of glass that was recovered from the roof. It has been used in the manufacture of window glass and beer bottles.

Once the glass was out of the way, repairs could then be undertaken on the iron and steel structure of the roof. This has included extra bracing and the addition of 4,000 metres of new steel purlins.

Repainting of the repaired roof structure was sub-contracted to Industrial Coating Services using International Paints Interseal 670HS two-pack epoxy paint as a three layer system. The cosmetic top coat, Interthane 990 Gloss, is of a rich slate grey colour, replacing the previous creamy yellow finish, which had not stood the test of time very well.

Tension

The single-ply ETFE foil has been installed in the form of 10×5 metre extruded sheets, each with a thickness of just 250 microns. In all, some 10,512 square metres of ETFE has been fitted by Vector Foiltec’s own engineers.

The installation technique includes a clever tensioning process. Each sheet is welded around its perimeter to a strip of foil folded over a ‘Keder’ rod. This perimeter assembly provides the means of structural connection between the ETFE panel and the aluminium perimeter framing.

Vertical mullions, spaced along the panels, have a concave surface onto which convex mullion caps are clamped, sandwiching the ETFE foil and thus tensioning it. At Carlisle Citadel, the 10-metre-long panels have fifteen such mullions, each of which has the effect of tensioning the foil by 3mm, giving 45mm of tensioning across its span.

With the Vector Foiltec ETFE system being just one-third the weight of conventional glazing, engineering calculations revealed that a freak gust of wind could possibly lift off the Citadel station roof! Highly unlikely as this might be, it has been necessary to mitigate the risk by attaching fourteen holding-down anchors to the roof structure.

Each anchor comprises a block of magnetite concrete, which is around 60 per cent denser than normal concrete, of up to 4.3 cubic metres (around 17 tonnes). Twelve of these anchors are buried below platform level and attached to the roof by vertical steel rods. At two locations, OLE foundations prevented burial, so the anchor blocks are discretely visible.

Citadel’s new roof system is projected to have a lifespan of at least fifty years. It’s also reckoned to be resistant to Carlisle’s troublesome seagulls. Apparently, it’s not unknown for these mischievous birds to drop stones from great heights.

Revealed

Forming phase two of the project, plans to upgrade and resurface the station platforms have been rescheduled in order to accommodate the repainting of the metalwork. Surprisingly, this repaint, which so vividly enhances the appearance of the new roof, did not originally form part of the project. Network Rail has been quite right in including it.

Dates for phase two are yet to be confirmed by Network Rail, but it seems likely that this £4.5 million second phase, which will be undertaken by Story Contracting, will commence in February 2018.

As the roof works near completion, the scaffolding has been gradually removed from the centre point of the station outwards. Just as if a giant curtain were being drawn back, the clear and bright new roof has been slowly revealed. New LED lighting completes the effect.

Chris Atkins, scheme project manager at Network Rail, said: “Passengers are really beginning to see the transformation of Carlisle station as a result of this work. The rejuvenated roof will mean a brighter, more airy and cleaner environment which will enhance the station’s beautiful features.”

This project represents a significant investment into Carlisle. It has not been without its challenges, but the result will be a greatly improved station that will provide a fitting gateway to the historic border city of Carlisle. But just watch out for those pesky seagulls!

This article was written by Stuart Marsh.


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