King's Cross raises the roof

Regular commuters travelling into King's Cross are in for a pleasant shock. Instead of arriving in the usual dismal environment of the Southern concourse they will disembark in the hi-tech dome of the new Western Concourse, which opens this month.

The roofing structure is what gives the Western Concourse its identity - both in terms of its vast span and its striking form, including a huge steel funnel that tapers towards the concourse floor and supports the semi dome roof.

The steel structure rises to a height of 20 metres and spans the 150 metres length of the existing Western Range, which separates the concourse from the main train shed.

Arriving passengers enter via a gap in the restored Western Range and from here they have access to the original Victorian booking hall. There is also a mezzanine of retail outlets, which shares the curved form of the Birmingham Selfridges and is similarly studded with round disks.

Tight site

The roof design may look as though it came from the imagination of an architect, but it was the tight limitations of the historic railway site that determined design and specification.

The concrete slab of the London Underground Northern ticket hall formed the base of the site, which meant only limited space for supports. English Heritage and Camden Council did not want the roof any higher than the listed Western Range building and its Grade I listed status meant that it could not be modified to bear any loads from the concourse roof. The designers also had to incorporate the curved Great Northern Hotel to the South, which had been conveniently obliterated by Sir Norman Foster in a previous masterplan for King’s Cross.

The engineer Arup and architect John McAslan + Partners quickly realised that a large spanning half dome requiring a limited number of supports was the only form that would fit the requirements of the site and provide enough space for passengers and retail units. (It’s three times the size of the Sourthern concourse.)

Together they drew up the design for the roof which was supported by 16 tree columns and the funnel. The roof has a diagonal grid structure similar to the the Gherkin, but with beams incorporated to account for the unusual concourse shape.

The roof is covered with 1,012 glazed triangular panels at the perimeter and towards the funnel, but to provide necessary shading the rest of the roof is covered with aluminium cladding laid over acoustic panels. This gives station announcements a rare clarity and reduces ambient noise to a comfortable level.

The curvature of the Great Northern Hotel was not the same as that of the roof, so different gutter widths were used to fill the varying gap between the new and existing structures.

Picking a steel contractor

With the design finalised Vinci had to find a contractor to manufacture such a complex roof. “From previous experiences we were aware of the problems you have between cladding and structural steel work,” said Vinci project manager Simon Jenks. “We wanted one contractor to be responsible for that interface.”

Jenks plumbed for Seele, which has a track record in integrating cladding and steelwork on schemes such as the GLA building and Westfield London. “Unfortunately we were unable to find a one-stop shot in the UK,” says Jenks.

The guardians of quality on the scheme were JMP, who worked closely with Vinci and the contractors to set strict design benchmarks. “When materials came to site we knew exactly what we were getting. Seele knew what they had to deliver, and we knew where we stood on quality at each stage,” says Jenks. (See box below.)

Watching the welds

The project team kept a particularly close eye on welds. These were necessary because the roof had been designed to have no visible bolts, just lateral fixings hidden between the steel members. Seele was only able to start welding after they had completed a weld to JMP’s satisfaction and agreed to a benchmark.

Once the roof had been complete, all that was left to do was remove the temporary scaffolding supporting the 1200 tonnes of steel. It was removed in July when the roof finally became self supporting. Deflections of 120mm had been accounted for, but in the event they the structure moved only half this amount.

For the last six months workers have been preparing the retails units and installing JMP’s sci-fi designs for the counters and ticket machines. King’s Cross station may not trump Sir George Gilbert Scott’s Gothic grandeur at St Pancras, but it’s now got an unashamedly modern piece of station architecture.

Funnelling the load

The load of the 1,200 tonne steel roof is taken through the steel funnel and 16 tree columns around the perimeter of the Western Concourse. Steel members were fixed to the tree columns through steel nodes weighing 1.5 tonnes.

Each node, which look like bagpipes shorn of their pipes, was cast to the design created by JMP. The finish is the texture of the sand that was used in the casting.

JMP visited Seele’s factory in the Czech Republic to check the first castings for imperfections before giving approval for casting of all 16. This level of rigour was repeated for every design element on the site.

A lot of time was spent refining the design of the tree columns. Their final tapered design combined with the curvy nodes helped to soften the appearance. ‘We had weekly meetings with Seele to ensure the design detail wasn’t lost,” says JMP associate director Simon Goode. “The checks continued onsite to ensure the quality was maintained as the installation took place.”

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