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Tuesday22 July 2014

King’s Cross station, western concourse

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John McAslan & Partners’ refurbishment of King’s Cross station reveals its full Victorian glory once more

Architect John McAslan & Partners
Structural engineer Arup
Location Euston Road, London
Completion date Late 2011

Since 1972, the full splendour of King’s Cross station’s original 1852 southern facade has been hidden by a “temporary” and drab southern concourse. But in two years’ time this will finally come down, and it will be possible to fully appreciate the appearance of one of the busiest transport hubs in the UK, together with John McAslan & Partners’ complex redevelopment of the station.

The facilities housed in the seventies structure will be relocated to a new building, the western concourse, which at 8,360sq m is three times larger. Designed by McAslan’s, the building will be located to the west of the station facing Pancras Road and nestled behind the grade II listed Great Northern Hotel, currently being refurbished into a luxury boutique hotel, and the grade I listed Western Range building, which is also being restored by McAslan’s.

The roof to the station’s western concourse resembles an umbrella with half sliced off

The roof to the station’s western concourse is 150m wide x 20m tall and resembles an umbrella with one half sliced off. Its form and height have been determined by the curve of the Great Northern Hotel and the primary axis of the booking hall in the Western Range building.

Steel is being used for the primary structure, while cladding will be 70% anodised aluminium and 30% in laminated glass.

“If the whole roof had been glass, the solar gain into the concourse below would be too great,” says Simon Goode project director at John McAslan & Partners. “The aluminium cladding also provides acoustic absorption”.

The glass has been used sparingly to provide just enough daylight onto the concourse where it’s most needed.

The new concourse will house shops in a streamlined pod raised above the floor and will provide a key connection to the northern ticket hall of the underground station. It will also connect the platforms in the main train shed and reduce conflicting flows of passengers.

One of the biggest changes affecting passengers will be how they enter and exit the station. When the western concourse is complete at the end of next year, passengers will enter the station through the new building and leave through the Victorian southern facade, eventually onto a spacious hard landscaped square designed by Stanton Williams and scheduled to be complete towards the end of 2013.

Steel roof ’Tree’ supports

The roof’s primary structure is composed of 16 steel tree columns at the perimeter and a 10m-wide (at the base) central steel funnel. The columns are linked to the funnel by radial S-beams.

Each tree column has a cast-iron node that transfers the loads from the radial beams into the vertical columns. The S-beams are made from flat box sections, 150mm wide and ranging in depth from 450mm to 250mm. These are welded together on site to form 65m-long beams, which extend from the tree supports to the roof’s apex.

“The deeper beams are the main transfer lines of the roof and need to be stiffer to transfer the moments into the tree column,” explains project architect Sascha Stscherbina.

Connected to the S-beams are diagrid members, or circular hollow sections that gradually run into the funnel. The diagrid members express the spreading out of the concourse and give an aesthetic continuity while providing the roof structure with horizontal restraint.

The geometry of the secondary roof structure is the same as the primary roof structure. But, while the primary radial beams are curved, the cladding and glazed elements to the roof are faceted, hence the need to separate the two. Their physical separation also helps to lighten the structure’s appearance. Welded steel pins connect the two structures with a gap varying from 25mm to 570mm.

The secondary structure consists of powder-coated aluminium extrusions 120mm x 80mm and nodes that feature a pin with a thread at the top to allow for adjustment as the geometry changes.

Large triangular composite cladding panels (up to 7.5sq m each) are then fixed to the secondary aluminium structure by concealed toggle fixings. Working from the inside out, the composite panels comprise: a perforated metal sheet with white backliner, a layer of acoustic absorption, thermal insulation and on the outside, anodised aluminium. Once installed, the joints between the panels are sealed with silicone.

Fritted and laminated single glazed panels will be fixed to the secondary structure at the perimeter of the roof and the apex. At the perimeter, the glass cantilevers out from the aluminium gutter line by 8m providing weather protection.

Louvred Facade to existing listed building

The western concourse roof has been designed to respect the grade I listed Western Range building that it sits behind.

Its roof is kept separate from the Victorian building, and meets its facade at a low level. Its fully glazed, louvred wall allows the concourse to be flooded with daylight and to reveal the brickwork and masonry features of the original departures elevation.

Two sections of laminated single-glazed louvres – each 48m long x 15m tall – have been fixed on either side of the Western Range building. The low-iron glass louvres - each 1,500mm x 600mm - are fixed at vertical sides only and to mullions spaced at 1,500mm centres. There are no horizontal fixings, maximising visibility through to the Western Range building. The louvred facades act as a truss, with a curved radial beam acting as the top chord and a horizontal connection at first-floor level as the bottom chord, which is connected by vertical mullions and stiffened with cross bracing.

Architect John McAslan & Partners, Client Network Rail, Structural engineer Arup, Main contractor Vinci Construction UK Ltd, Subcontractor for roof package Seele

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