A famous art college in the former King’s Cross train sheds fuses industrial history with modern construction. Jan-Carlos Kucharek reports. Photography by John Sturrock
Forming the cultural centrepiece of developer Argent’s 27 ha mixed-use commercial scheme at London’s King’s Cross is the striking new Central Saint Martins College of Art and Design.
The £110m project consolidates all the faculties of the school into one campus.
At the end of this month 5,000 students and staff will be passing through its doors into a 39,000m2, state-of-the-art concrete and glass building containing studios, lecture theatres and seminar rooms, along with a theatre, bars, refectory and an enormous multi-use atrium running almost the full length of the building acting as the social heart of the new University of the Arts London.
But the most interesting thing about the new college is that approaching from three sides, you would be hard pressed to notice there was a new building there. This is because the whole college is flanked not only by two Victorian “east” and “west” transit sheds measuring 180m x 25m, but to the south by a beautiful 55m x 31m six-storey brick, cast iron and timber granary building.
The eastern transit shed is to become the college’s workshop spaces, while the granary building is to become the library and administrative building. Forming the campus’s outer edge, the new structure has been skillfully inserted within these existing facades, like an egg in a box.
To achieve this, the existing grade II-listed buildings, designed in 1851 by architect Lewis Cubitt, had to undergo extensive refurbishment. This would prove to be not only a significant engineering challenge, it required the contractor to jump through hoops with
the conservation lobby.
The works were carried out using a planning permission riddled with “reserved matters”, which had to be resolved in a strict seven-week period, and under a two-stage Design and Build contract — one of the least amenable contracts to use on a project full of uncertainties.
The north elevation of the granary building addresses the east-west link and the new college building. Traces of the old sheds remain and give character
The granary building and west and east elevations together barely give a clue to the modernity and size of the college behind
According to Michael Beare of AKS Ward Lister Beare, the conservation engineer for the new development, which worked on the restoration of neighbouring St Pancras, this was not made any easier by the fact that the steel trusses of the transit sheds’ roofs needed to be removed to allow the piling works for the new building to be carried out. “It gave us a lot of trouble, as the walls were effectively unsupported, sitting on corbelled foundations that ran as far as 6m down into the basement stables and at the original ground level of the site,” recalls Beare. “This meant the bespoke design of temporary propping to support the walls while new construction went ahead.”
Along their lengths, walls were out of tolerance by up to 80mm, which exceeded British Standards. Helical tie bars were inserted into the existing masonry to give additional strength to the heritage walls and were monitored for movement during construction of the new structure. The transit shed roofs were reconstructed using timber glulam beams, rather than steel, to reintroduce their original materials, as well as ensuring they conformed with the latest Part L of the Building Regulations.
Many of the blind arches that formed the original eastern transit shed were opened out to allow the sheds to become part of the new design, says Beare.
The brickwork was in bad condition and the brick bonding was not consistent through the 400mm thickness of the walls, which meant that simply removing them from beneath the arch was not possible. “It meant that not only did we have to saw through the brick, we needed additional steels inserted with a curved web and flange to support the arches themselves,” explains Beare.
All the removed bricks were re-used elsewhere on the site. Brickwork joints were all restored using original lime mortars (see box) which, due to the longer periods required to reach 28-day strength, needed to be monitored to check that walls were assuming the loadings correctly.
In addition, none of the masonry walls had damp proof courses (DPCs), as they originally relied on airflow past the surface of the brick to evaporate moisture. Installing new ones in walls that at points are 1m thick is difficult, especially as they would need to be natural slate and the use of waterproof renders would merely move the problem to elsewhere in the wall. Therefore the decision was made to ensure that internal air exchanges would continue that air flow, and particular attention was given to monitoring moisture levels in the basement.
Beare says the granary building restoration was a fascinating project due to its composite construction. Its exterior brick walls taper from 1m thick to 600mm at the top, but within is an elaborate cast iron structure of columns and beams supporting a heavy timber floor. Historically designed to take huge loads, some of the floorboards are 50mm thick, sitting on 300mm deep timber beams. “It was a sophisticated structure,” says Beare. “There were even metal strips between every floorboard, stopping dust transferring through floors. We also discovered fine threaded rods running across the building below the floors, acting as ties for the walls, preventing them from bowing outwards.”
Evidence of Second World War dogfights above King’s Cross was retained at EH’s request
At the top floor of the granary building the original hoists for the grain have been preserved
But although the building was designed to take high loads, change of use requirements meant engineers had to analyse the structure for disproportionate collapse. “It was difficult to do, as modern codes of practice are not really written to take account of the composite nature of structural materials, such as we had here, so it took time to prove that the building could meet them,” says Beare.
“In the end, we proved that the structure was so sophisticated, that each system was independent. Column heads went through floors, meaning the steel frame was an independent structure — whole sections of timber floor could be taken up without any negative effect.
It was challenging, but a fascinating result, and a great demonstration of the Victorian engineers’ abilities.”
This meant a light-filled atrium could be opened out along the length of the building, and a section of the north facade could be removed to allow for hydraulic lifts to be installed with its own framework.
In fact, the engineers liaised with both English Heritage and Camden council’s conservation department from the early stages, and was ongoing throughout the project. BAM Construction design engineer Jeff Carter recalls that the negotiation processes with both conservation teams was completely programme driven. “The challenge was the clearance of listed building conditions in the seven-week period available, as there were hundreds of them, and required a full-time team of 12 to discharge them,” he says.
One such liaison was with the Midland stone sandstone cornices, some weighing nearly two tonnes, cantilevering over the brick parapets. These were tied to each other with giant cast iron ties set into their top faces — all in a severe state of corrosion, expanding and splitting the stone off the parapet. Spalling was also occurring due to water ingress. The ties were removed and replaced with stainless steel ones.
The decision was taken not to clean the wall. “The view was that grime was part of the building’s industrial past and that it should remain,” says Ewen Hunter, contracts manager at BAM Construction. “It even extended to damage that occurred during the Second World War, where lead from machine gun bullets had sprayed across some sections, but has been preserved untouched.”
Windows were naturally all in a bad state of repair, but being a heritage building, it was not necessary for new windows to be code compliant. However, as much effort as possible was made to ensure that they were all enhanced performance fittings. Any new glazing spanning the new internal structure and transit shed walls was designed to minimise the imposition on the listed structure, so they are held internally by spider fixings to create a seamless surface. These slide in above the old metal beams of the east-west link.
This area is probably the biggest statement of the new building, as it interfaces with the north elevation of the granary building, creating a 55m x 15m internal plaza for the college, forming the east-west link. As a means of spanning the space the architects wanted to cantilever a huge roof across from the new building to the old, so that it could merely “kiss” the granary wall.
“The architects made every effort to create a ‘soft’ joint,” says Carter. “But the spans were simply creating a structure that was far too thick, or which was deflecting too much. English Heritage would have preferred that no load be imposed on the granary wall, but this wasn’t viable.”
In the end, fabricated mild steel cranked tee beams, fixed to the roof’s primary structure, rest on a 200mm x 90mm horizontal steel parallel flange channel recessed into the granary wall’s brickwork. The glass rooflight is below this, recessed in the metal channel. Not only does this accommodate horizontal movement and give a waterproof seal, it also gives the impression of floating in the channel.
Carter explains that allowing the structure to impose on the granary wall made it far lighter than it ever could have been if cantilevered. “We managed to sell the idea that a ‘light touch’ was the more discreet option,” he says.
It’s a point of view, as you enter the glass and concrete behemoth that sits behind those fine, low Victorian dark brick walls, that you can’t help but agree with.
The main “street” of the new Central Saint Martins extends over 150m northwards between the east and west transit sheds
The new studio buildings are inserted between the existing Victorian transit shed walls and behind the granary building
Do you want lime with that?
Engineers used lime mortar in keeping with the original design
Michael Beare is not only one of the few members of the Conservation Accreditation Register for Engineers, he’s also treasurer of the Building Limes Forum, which seeks to increase awareness of the use of traditional lime mortars in new construction.
The industry has been quick to dismiss the use of lime mortar, claiming it is weak and can’t take tension, but that is purely a curing time issue, says Beare: “The main thing is that it is a cement-free mortar, with the sustainability benefits that that brings with it.”
He adds: “The fact that the free lime within it dissolves and resets means that it ‘micro-cracks’ at close centres. This is unlike cement mortar, which can take tension better, but which, as a result, will crack more significantly but at wider centres. So in a sense you want the intrinsic flexibility that lime mortar brings.”
Beare also notes that because it is more permeable than brick, salts will tend to bleed through lime mortar. With cement mortar, because it is less permeable, moisture “backs up” in the brick, and is thus more likely to result in crystallisation of the salts on the surface.
Dr Ali Arasteh, principal structural engineer at the Brick Development Association, says that lime mortar’s ability to accommodate structural movement is a great plus point, but the three months it might take to achieve the conventional 28 day strength of cement mortars does not work in its favour, given that construction contracts are so time critical. But that does not preclude its use, he says: “It’s all about the management of the project — this could be phased into the construction programme.”
He does, however, question the assumption that lime mortars contain less embodied energy. “All of this is dependent on whether the lime is locally sourced, and on things like the efficiencies of the kiln etc,” he says. “These sustainability angles need to be looked at in more detail — despite lime mortar’s obvious benefits, they should all be evaluated on a quantitative and project-by-project basis.”
Where the new meets the old
Heritage facades hide some seriously modern engineering
The new college building, designed by architect Stanton Williams, is a feat of fair-faced concrete and glass in itself, even if it is skilfully hidden behind Richard Griffith’s Architects’ heritage facades, explains BAM director of structural engineering David Carter.
The main building is a reinforced concrete frame structure at 7.6m centres, over 150m long, with ground-bearing floor slabs and pile foundations inserted into the voids between the transit sheds. The flat slab construction, designed to take loads of 10kN/m2, is stabilised by the structure’s frame action and solid reinforced concrete wall cores. The two new blocks are separated by an internal “street”, covered with an ETFE roof that allows light to pour in.
These structures are independent of the heritage walls of the transit sheds, although they are pinned back to them, giving the old walls some additional lateral restraint. The engineer, AKS Ward Lister Beare, was keen to ensure that there wouldn’t be any additional stress loadings on them.
The architects wanted the architectural expression of the structure, so the main fair-faced reinforced concrete walls of the new structure are 450mm thick and very solid.
“Given the length of the building’s walls, we needed to control cracking, so we’ve built in two movement joints and thermal breaks, even if this ended up breaking the structural integrity,” says Carter.
In terms of contract, Carter says there were “risks involved with using the D&B route, as we were dealing with a heritage structure that was being revealed and restored, but there were separate agreements in place with the client to help protect against these”.