Turning up the gas in King’s Cross

Three Grade II listed Victorian gasholder guide frames have been reinvented as high-end apartments in the centre of King’s Cross, London. Jack Wooler spoke to Arup’s Rob Buck and Ed Shearer about the complex refurbishment.

Rising above the canal docks and Victorian warehouses north of Kings Cross, a group of iconic Victorian gasholder guide frames stands restored and re-erected on the north bank of Regent’s Canal.

Providing 145 apartments, a gym, spa and other communal spaces, the three cylindri- cal apartment blocks offer luxury interiors, roof gardens, basement parking, and are linked by a dramatic central courtyard.

Gasholders London is part of the wider King’s Cross Central development. The single landowner at King’s Cross, the King’s Cross Central Limited Partnership, claims an unusual level of continuity and commitment, which has helped to foster a smooth development and delivery process.

As the structural and facade engineer, Arup has worked closely with Argent on its vision for King’s Cross Central since taking ownership of the land in 2000. On the Gasholders development, in collaboration with the architects WilkinsonEyre, Arup engineered the design of the complex civil, structural geotechnical and facade aspects of the project. Hoare Lea led on building services design, and the project was procured via a design and build contract with Carillion.

Heritage

The gasworks opened in the 1820s, manufacturing ‘town gas’ from coal brought in by rail. The distinctive ‘Triplet’, an inter- locking cast iron guide frame encompassing three gas holders, is the backbone of this project – it was built in 1880 driven by a surge in public demand for gas.

“The frames have for over a century defined the landscape at King’s Cross, both for the local community and for millions of Londoners and visitors who pass by year after year,” Rob Buck, associate at Arup, told Housebuilder & Developer.

In fact, four gas holder guide frames were still standing when the development started in the early 2000s, each with Grade II listing in recognition of their cultural importance, not least as surviving examples of Victorian engineering prowess. The ‘Triplet’ plus the fourth ‘No.8’ frame would be dismantled, restored and returned to a nearby site North of Regent’s Canal in 2013. The three frames of the Triplet were re-erected in the same configuration, and the fourth was turned into a public park, with a stainless steel pavilion plus a landscaped area.

“The ingenuity behind the construction of the Triplet is testament to the energy and pride that drove the industry,” said Buck, “and the dominance of the guide frames on the local skylines has made them important features at King’s Cross.” He added: “The guide frames were thus retained as the heart of the development, and the residential blocks were inspired in part by the large cylindrical storage drums that once stood within the frames.”

“The different heights of the three blocks, and the offsets and discontinuities in the facade lines allude to the movement of the old drums, which rose and fell according to the volumes of gas stored inside them.

Each frame was dismantled carefully, refurbished and re-erected in order to remain a striking feature of the landscape. Buck concluded: “Here they will be enjoyed for years by local residents and visitors to this ever more popular London destination.”

Planning

The regeneration of the former industrial sites north of King’s Cross and St. Pancras began in the early 2000s and has trans- formed what was a disused area. “It’s a complicated site,” said Buck, “with constraints from live rail assets, and a strong heritage and conservation aspect because of its industrial history.”

He continued: “It is the celebration of this heritage, blended with contemporary architecture, that gives the regeneration its unique feel and attraction.”

“The repurposing of the old Gasworks infrastructure is a thrilling starting point for much of the new public and private space.”

“As with any development of this magni- tude, especially those with such historical significance, the planning process can put up a host of barriers. In the case of Gasholders London however, “a clear and viable strategy for the refurbishment and re-use of the guide frames in the master- plan was key to success,” said Ed Shearer, structural engineer at Arup.

The frames were built before approaches to structural stability such as cross bracing were in common use. “There was no record of how they were designed, and it was not clear how they would behave once re- erected,” said Shearer.

Argent was devoted to the conservation of the heritage aspects at King’s Cross, and it was “through the close collaboration with Arup and the specialist contractor team that it was possible to map out a strategy for their restoration and re-erection.”

The height of the blocks was subject to tight planning restrictions, which, among other design constraints, helped the archi- tects arrive at the form of the buildings. According to Buck, a “key challenge” was the integration of accessible roof gardens and the atrium ‘canopy glazing’ within these restrictions.

To minimise the overall height, plant rooms were positioned in the basement to allow maximum space on rooftops, and the glazed atrium roofs have multiple pitches. “This presented challenges in weather- proofing, including opening natural ventilation and smoke clearance elements, as well as to drainage of rain water off the roof lights and their maintenance, which required that the glass be walkable.”

Geometry

“On plan,” Arup’s Shearer told Housebuilder and Developer, “WilkinsonEyre’s analogy of a timepiece has a strong relevance to both the overall form of the project and the sophisticated inter-relationship of its component parts.”

The geometry of the Triplet’s frames – three circles meeting tangentially around a common central axis, is augmented by a ‘court circle’ extruded at the axis, creating a central courtyard, where, said Shearer, “the drama of the converging frames and delicate steel walkways” is revealed. The ‘drums’ each contain a central circular atrium, around which gallery walkways provide access to the front doors of the apartments.

The circular courtyard where the frames converge in a complex geometry of ironwork is the centrepiece. To provide access from the residences between the three buildings and to the rooftop terraces, two walkway bridges were introduced at the fifth and eighth floors. “These were envisaged to be lightweight and sleek,” explained Shearer, “so as to complement rather than detract from the gasholder guide frames.”

Making use of the circular geometry, it was possible to design these walkways to be supported solely by a series of slender diagonal legs. Because these form continu- ous, uninterrupted circles, the tendency for the deck to tip over into the atrium is resis- ted by the equal and opposite action of the deck on the opposite side of the courtyard.

Shearer explored the “purity” of the structural solution: “The slender structures of the steel bridges play counter melody to the heavier ironwork of the gasholder frames, and the cantilevered laminated glass balustrades are tastefully minimalistic in detail, ensuring that the intended slenderness is unaffected.”

The geometry of the guide frames posed a significant constraint on the coordination of the apartments and other spaces within the buildings. Shearer explained: “The design of a primary concrete frame had to respond accordingly, and the successful delivery of an efficient layout required careful integration from the start. Early decision-making was key.”

Detailed studies of the floor plates justi- fied ambitious minimum structural depths, set storey heights, and enabled early appre- hension of an “efficient, yet flexible” structural diagram. These allowed the architect to proceed with extensive massing studies to achieve the right unit mix.

“The result was three lean concrete frames, optimised for maximum internal space, on a grid accommodating a large variety of non-stacking apartment layouts and basement parking.”

Apartments are accessed via the gallery walkways around central atria within each block. Ranging the apartments around the outer perimeter maximises balcony space and views out for the residents.

“The apartments are wider on the outer perimeters and narrower towards the centres of the buildings, but the overall diameters are large enough that this effect is subtle and seems to enhance the light- ness of the living rooms and other rooms against the outer facade.”

The apartments themselves have been internally designed to the meticulous vision of Johathan Tuckey Design, and “strike a perfect balance of subtlety and modern elegance,” said the firm. Tuckey himself commented: “I’ve always been interested in how buildings can change from one thing to another, and the Gasholders is a brilliant example of what’s possible.” He continued: “I see our work as mediating between what the existing building wants and what the person who lives in it needs. The thing that’s most important though, is that these apartments are really beautiful homes.”

Facade

The curved geometry of the facade, combined with the need to install, while the gas holder guide frames were being re- erected, and the consequent constraints on access, all played into the design of the cladding system.

Buck commented: “Arup facade engineers proposed a unitised cladding system whereby facade and balconies, assembled offsite, would be lifted into place, minimising external access requirements and thus limiting the impact on the re-erection of the frames.”

Prefabrication was a necessity, as access around the facade was very limited and facade installation was to run concurrently with the frames’ reinstallation.

The curvature of the facade also presented challenges, particularly for the system of unusually high 4 metre shutters: “We had to disregard a traditional system of hung panels because it would be expensive and difficult to achieve around a curve.

“The answer was to keep the control mechanism in a straight line and use support brackets with varied lengths. These would set the shutters at an appro- priate distance away to appear to follow the curve of the building.”

To provide a smooth and robust action for the panels, it was necessary to control both the top and bottom at the same time. “We decided to fabricate this as a whole unit, including frame, motor, gearbox and four panels running on ‘trolleys.’ This avoided building a complicated system on site.” The frames were craned into place, with each tested over five cycles after installation.

The facade is manufactured with aluminium thermally-broken extrusions, with triple glazing or non-combustible insulation. Aluminium panels, perforated in a pattern to represent the original tide marks as gas holders expanded and contracted, are attached to the unitised system; they are movable where they are in front of the windows or balconies.

Sharing the same aesthetic as the balconies above, the 5 metre entrance steel canopy (cantilevering 4 metres) displays aluminium slats, with concealed lighting to the soffit and a steel feature channel around the perimeter. Built on site, this extends through the main facade.

“The canopy is held by slender steel supports and the glazed curtain walling which features bespoke steel mullions,” revealed Rob. “Although each support element was relatively lightweight, they worked in unison to provide stability to the canopy.”

Energy-efficient

Arup has gone to great lengths to ensure that the re-envisioned gas holders are as energy efficient as they are visually striking. “On typical builds, a balcony structure is bolted to the concrete slab through a thermally broken connection, and the facade system is then built around this,” said Buck.

“However,” he continued, “here the unitised curtain walling arrived from the factory as a fully weather-tight system, including brackets for the balconies.”

It was also strengthened to accept this additional load, with the larger balconies including a vertical post adjacent to the facade which reduced the support forces delivered through the curtain walling.

With the facade, the focus was on thermal performance and reducing air infiltration. All outward facing facades, including the large sliding doors on to the balconies, are triple-glazed, thermally- efficient systems.

For the inner courtyard, windows were double-glazed, as was the atrium roof. The space, although enclosed, is not fully heated. Rob commented: “The bonus for residents are lower energy bills achieved by controlling the ventilation and providing better insulation to the facade.”

Rewards

The project has been highly praised, being one of the recipients of the 2018 RIBA London Awards for architectural excellence. Few projects are able to retain this depth of heritage, especially while building apartments to modern performance standards and high-end desirability.

In the context of the wider development of King’s Cross re-envisioning an entire district of London, the efficient and successful design and build process on Gasholders London will likely serve as a model for complex refurbishment.

Shearer summed up his experience of working on the scheme: “It was an enormously rewarding project, successfully delivered.” He concludes: “There were of course challenges which the team rallied to deliver, such as the coordination of a very varied set of ambitious apartments with an unusual geometry, yet this, and other aspects, such as the re-use of the guide frames, served to make Gasholders London development stimulating new territory for both the designers and the contractor’s team. The result is all the richer for it.”

Project team:

Client: King’s Cross Central Limited Partnership
Architect: WilkinsonEyre
Main contractor: Carillion
Interiors: Jonathan Tuckey Design Structural and Facade engineering: Arup
Building services: Hoare Lea