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Rail | Carlisle project raises the roof

carlisle cassette beam

It took complex scaffolding construction, a seagull strategy, and a UK-first in plastic roofing to fully modernise Carlisle Citadel Station.

The station, an 8,000m² Grade II listed structure built in 1847 is an architectural gem. The roof –160m long, 51m across with 13 glazed peaks –  is its centrepiece.

But routine investigations of the structure found the steel frame supporting the glass was hogging and sagging by as much as 40mm.

Carlisle  IMG 0526

Carlisle IMG 0526

The ETFE framing system is installed.

This was partly due to a radical shortening of the roof span in the 1950s, when the canopy over the three tracks on the western side of the station was removed. This

section of  the roof had acted as part of a balanced cantilever, so its removal had some impact on the way the remaining structure behaved. “It [the remaining structure] has slightly twisted and moved,” says Peter Stubbs, project manager for principal contractor Galliford Try. “It’s perfectly fine overall, it’s not going anywhere, but it’s not as it was designed.”

It is also partly due to the weight of the glass – about 110t of glazing. Not only acting as a weight on the steel, the glass itself was also failing, developing cracks and breaking, with pieces falling on to a protection netting that was set up in recent years to protect passengers. Apparently this is due to seagull activity. “There has been anecdotal evidence that seagulls carry stones in Carlisle and drop them on the glass,” says Stubbs. The glass will be replaced with a new lightweight plastic.

Carlisle (flattened)

Carlisle (flattened)

In all, the structural steel strengthening and roof replacement accounts for an £11M investment in the station.

The platform from which this work has been taking place is a protection deck running the length of  the station. It was an impressive sight when New Civil Engineer visited in April.

Installing this scaffolding was a major challenge, primarily as it must span the West Coast Main Line (WCML) and its overhead power lines: “Because we’re above the WCML, you only get one opportunity for full possession once a week on Saturday night, and only for six hours [between 2am and 8am]. And it’s not as if you can build the span down at platform level, take down 25,000V overhead lines, lift up the scaffolding, then reinstate the electrics – all inside six hours,” says Stubbs.

Instead, scaffolding towers on the platforms were built to a height that was above that of the overhead lines. From these towers, 1.4m deep beams were launched over the railway and its electrics, guided by steel Tirfor wires tied to the existing steel roof trusses.

Carlisle ETFE installed 2

Carlisle ETFE installed 2

The ETFE when installed looks similar to glass.

With the working platform finally erected, the team could start on the roof, removing glass and replacing with transparent ethylene tetrafluoroethylene, better known as ETFE. A fluorine-based clear plastic, it has been designed for durability and strength over a wide temperature range and is 1% the weight of glass.

“Basically it’s a big plastic sheet,” says Stubbs. “But we’re replicating the glazing by putting brand new mullions back in.”

It is the first time in the UK this method of installing ETFE has been used – stretched tight in a single layer, using 5m by 10m sheets.

ETFE is not new, but elsewhere it has been deployed using a pillow system, as seen first at The Eden Project, Cornwall. In the pillow system, air is pumped constantly between a double layer, providing insulating properties. This system caught on and has been used at railway stations in Manchester, Birmingham and Newport.

Carlisle Scaffold erection

Carlisle Scaffold erection

The protection deck, almost complete.

But this type of ETFE system was embroiled in a dramatic failure at Manchester’s Victoria Station. Chief suspects in the investigation were seagulls. “What we understand has happened [in Manchester] is that seagulls have come along, they think they’re landing on water, then sit down, feel the warm air, and think ‘Ooh this is quite comfortable, we’ll sit on this for a bit longer’. Then they’ll sit and start pecking it,” says Stubbs. “If you’re constantly pecking at it in one place, it will eventually puncture. This hole allowed the air to escape, allowing the pillow to slowly deflate.”

“Rain then fills up the pillow void, and the weight of all that water ended up tearing the panel,” says Stubbs. Water plummeted to the platform below, luckily only causing minor injuries.

As the Manchester failure occurred, Galliford was beginning ETFE installation in Carlisle. Stubbs says the Manchester calamity prompted discussions among senior managers. But, he adds, Carlisle is “a totally different scenario” as the single layer skin is applied vertically or at steep angles.

Carlisle ETFE installed 2

Carlisle ETFE installed 2

The ETFE when installed looks similar to glass.

“It’s on an angle, so seagulls can’t sit on it,” Stubbs says. “It doesn’t look like water to them, because it’s on a slope.”

“There are lots of seagulls in the area. But even if they did peck a hole in this  you’d have a drip of water coming out, but no real risk of water pooling up.”

The roof has a 25 year guarantee, and German ETFE manufacturer Vector Foiletc has a contract with Network Rail to maintain it. “It can be patch repaired, and panel repaired – it’s far more easily repaired than glass. And if you fall on it, it will hold you, whereas glass… could be a nasty incident,” says Stubbs. Up to seven 50m² panels can be installed per day.

Late in the installation process, the super-light weight structure was suspected of having a crucial design flaw, according to local building inspectors. “Because we’ve taken so much weight [110t] off the roof, they’re now afraid that with an uplift of wind, it will act like a big kite,” says Stubbs.

So, a steel tie down arrangement was added; a bracket on the steelwork joins to steel rods coming down to the platform, connected to a buried weight set within the platform. These weights had to be large to counter estimated uplift loads of up to 14t.

The team opted for magnetite for the counterweights. This is a heavy iron ore normally used in the nuclear industry for shielding. Because it is 3.9t/m³ as opposed to 2.4t/m³ for concrete, the weights could be smaller in size and cause less disruption to the platforms. “So we could have a space 60% smaller and didn’t have to disturb the platforms. It [magentite] is expensive, but far cheaper than a huge excavation.”

The roof repairs are expected to finish in coming months, with the scaffolding due to leave the site in November.

 

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