Not to be outdone by the recent stunning revival of its neighbour St Pancras, this week King’s Cross station begins to unveil the restored jewels of its past and some intricate and grand new additions. Alexandra Wynne reports.
King’s Cross has for the past few decades been one of the least appealing parts of central London, despite the numerous Tube and railway lines that converge in this tightly compacted area. But that has begun to change.
This week Network Rail has unveiled a stunning £550M facelift of King’s Cross station, which, if not the grandest, it has ever undergone, is at least the grandest for decades.
The additions revealed this week have at their heart a sprawling steel diagrid roof for the new Western Concourse.
Vast but delicate
It is a vast but delicate looking structure that grows from a trunk like central funnel support before branching out to form a domed roof above the new concourse. The height differential ranges from about 19m at its peak to around 7m at its lowest. There are 16 tree columns spaced equally around the perimeter of the roof and each carries a horizontal load of about 600t.
The roof was conceptualised by architect John McAslan & Partners and designed by consultant Arup - in fact the Vinci, John McAslan, Arup team have worked on most of the packages let by Network Rail for the station in recent years.
Arup devised a geometry for the 52m span, 130m diameter roof that had to fit around a number of constraints. One of the most important aspects of the design was to recreate the hi-tech nature of the original roof that was installed in the station when it was first built in 1852, says Arup associate and project engineer Tim Worsfold.
John McAslan associate director Simon Goode adds that the roof section further recreates what was once there.
Historically, departing passengers entered via the Western Concourse and arriving passengers exited via theEastern Range to waiting horses and carriages.
“It is very difficult to accommodate even small, say 2mm, deviations if you have to weld”
Tim Worsfold, Arup
The first constraint was that the structure had to be entirely self supporting, yet fit snugly next to the Western Range building. That structure was unable to take any additional loads as it was already being bolstered to support the newly re-glazed train shed roof. The roof also had to accommodate the curved shape of the neighbouring Great Northern Hotel building, itself being refurbished by developer Argent. As a result the roof shape is irregular, “sort of semi-circular”, says Worsfold.
The second issue was the fact that work on the 8,424m2 concourse, had to take place at the same time as work below the site on London Underground’s (LUL’s) new Northern Ticket Hall.
Months of planning helped the team ensure that the ticket hall below could cope with the vast amount of scaffolding required to construct the roof, which weights approximately 1,000t. Particularly important was the need to ensure that the roof was de-propped in a carefully controlled sequence to avoid overloading the ticket hall below, mid-refurbishment.
Keeping the station open
A further complexity was the need to maintain the flow of passengers and supplies so that the station could stay in operation. King’s Cross remained open for the full construction programme, another challenge that was at least helped by the opening two years ago of the new platform zero, which helped with sequencing the works.
The process was also helped by the fact that Arup was working on the LUL project. Its underground ticket hall team even did checks on the roof team’s work.
Fabrication of the roof also presented challenges. The tree columns - also steel - are deceptively slender, aided by being elliptical in section and tapering upwards toward a node casting. From there a number of the roof’s branches are bolted to the node from the inside. However, the connections throughout the rest of the roof had to be welded.
“English Heritage wanted to the building to be seen honestly”
John Turzynski, Arup
“It is very difficult to accommodate even small, say 2mm, deviations if you have to weld,” says Worsfold.
In addition to modelling for thermal expansion of the roof, a careful job had to be made of choosing the amount of glazing. If it were entirely glazed not only would the interior become uncomfortably hot in the sunshine akin to a greenhouse, the weight would have been significant. Instead, there are 1,012 glazed triangular panels - mostly at the apex above the funnel support and at the perimeter - supplemented by 1,200 solid composite panels made partly of aluminium. This balance is intended to create the right inside temperatures and lighting without the need for too much intervention.
While the roof is the immediate show-stopper at the newly refurbished station, there has been other ambitious work going on around it. The Western Concourse scheme also included the restoration of the Western Range Building. This is a Victorian block of railway offices adjacent to the concourse, or as main contractor Vinci Construction project director Simon Jenks puts it “a semi-derelict ramshackle assortment of buildings before restoration”.
Arup director John Turzynski says it was a unique opportunity to be able to work on creating a “21st century station next to a Grade-I 19th century one”, and one that has worked very well.
The Western Range Building, along with its station façade, was designed by architect Lewis Cubitt. As it is Grade I listed, and despite a large gap caused by a Second World War bomb, the building had to be intricately restored under the guidance of English Heritage.
“English Heritage wanted the building to be seen honestly, so that when putting something new into something old, you can see it,” says Turzynski. The result is that while the bomb gap section contains new, matching stonework, the join is evident.
The Western Range was “unbelievably complicated”, adds Turzynski, in part because of the changes of level but also the varying states of disrepair.
Structurally the Western Range Building had been through a lot of modifications, with extra doorways cut in here and there over time, explains Worsfold. “The structure had been tampered with”, he says, with the result that much structural work had to be done, in particular because this building was take up to 40% of additional load coming from the newly re-glazed train shed roof whose modern plastic roof has been removed in favour of glass like the original.
A vital part of work on the Western Range Building was to create space for a new row of ticket gates - known as the southern gateline - from a building that had previously been used for plant. This involved opening it up - with the removal of a mezzanine level in the process - to create a much larger space. This was done by jacking up the existing building to allow the installation of an entirely new reinforced concrete frame.
Just north of the gateline, a further nod to the building’s history has been salvaged - again despite the corner of the space being part of the bomb gap - to reinstate the booking hall replete with original features like a 130mm thick York stone balcony.
There was a lengthy exercise of assessing whether the balcony was suitable for future use, says Worsfold. “It’s one of those classic things that if you were trying to design it to today’s codes, you’d struggle. But [the balcony] had obviously been in use for a hundred years or so.”
So while a small section around the bomb gap had to be replaced, the decision was made to keep it insitu, and create a new cantilevered balustrade to today’s standards. This was given additional support with new cast iron brackets every 2m to 3m and in the gaps between the brackets, steelwork was introduced to take bending moments on the balustrade.
Work on the refurbished Eastern Range Building - now offices - finished in 2009.
Beyond this summer’s Olympics, the project will enter the final straight as the station’s vile green canopy, which formed its southern entrance, is removed. This will make space for a new façade, public square and taxi ranks, all to be built in time for the final completion in autumn next year.