Building Heatherwick’s King’s Cross wonder

Heatherwick Studio’s Coal Drops Yard scheme features a spectacular, sweeping steel roof structure, which crowns this new retail centre at King’s Cross. Will Mann finds out from the architect and contractor BAM how the project is taking shape.

A slogan on the site hoardings describes Coal Drops Yard as “Victorian engineering with modern architecture”. In reality though, as BAM project manager David Packham puts it: “There’s plenty of modern engineering going into this project too.”

He is referring chiefly to the centrepiece of the 10,000 sq m retail scheme, arguably the most ambitious architecture yet on Argent’s King’s Cross railway lands development: a twisting, arcing steel roof, connecting two former coal depots with a 25m-high “kissing point” between the buildings, and in the process adding a new upper storey.

“The idea was to create a conversation between the two buildings,” explains Tamsin Green, project leader with Heatherwick Studio, which created the concept design for Coal Drops Yard. “The roofs of the two depots rise up and are pulled together, providing a focal point for the development, while beneath the new upper floor, the double-height space provides a sense of theatre, which can be used for concerts and events.”

Heatherwick’s involvement with Coal Drops Yard started in 2014, working alongside engineer Arup. BAM Construct, one of Argent’s three framework contractors at King’s Cross, was appointed to the £91.9m construction contract, and BAM Design appointed to carry out the detailed design from RIBA stage 3 onwards.

The build programme commenced in February 2016, but was preceded by a two-year pre-construction phase to carry out demolition work and survey the condition of the Victorian era depots, built in the 1850s for rail “coal drops” from the north of England.

The two-storey cast iron and brick structures were used for light industry and storage when the coal era ended, but a fire damaged one section of the eastern building and they fell into disuse in the 1990s.

On plan, the two buildings splay apart from each other, with the 150m-long eastern building separated from its 120m-long western neighbour by a 15m gap at the northern end of the site, which widens to 75m at the southern end, next to Regent’s Canal.

A viaduct between the two buildings was demolished during the preconstruction stage, creating more space for the sunken piazza area in the heart of the new development, which will sit 3m below the level of neighbouring Granary Square.

The two roofs meet at a 25m high “kissing point”

The original brick arches of the buildings are being restored and will house retail units in the completed development. Some 300,000 imperial bricks are being used across the project, 250,000 of them reclaimed from the demolition work. The interiors are being partially reconfigured, and some floors removed and reinstated as mezzanine levels to accommodate services corridors.

“When we took out these floors, this meant the walls were freestanding, so as a precaution, we installed around 50 temporary soldier shores, each one 8m high, on every elevation,” says Packham. “We also fitted 171 individual monitoring points, which were monitored weekly with Arup, though only 25-30mm movement maximum was recorded.”

Much of the original timber and cast iron structure in the depot buildings is being retained, restored and strengthened, of which more later, while a new internal steel frame has been constructed which will provide support for the roof structure.

To construct the piled foundations for the steelwork, BAM removed the existing ground floor slab, reduced the ground level by 900mm in the eastern building and 400mm in the western building during an earthworks phase, and then installed the piling mat.

Above: Trestle towers were erected to support the central node and trusses. Below: New stone flooring echoes the original cobbles

The piling itself was complicated by restricted headroom of 2.2m – excluding the burnt-out section – inside the 61 arches which will eventually become the retail units. “We worked with our subcontractor Berry Piling to identify a mini piling rig which would fit inside the arches,” explains Packham.

Some 354 mini piles, 450mm in diameter, were driven to depths up to 25m beneath the two buildings. The piles were set out in groups of six with a pile cap fitted on top of each. Additionally 1,000m of underpinning was installed across both Victorian buildings.

“Once the pile caps were fitted we constructed the floor slabs, which gave us a decent working base for the steelwork phase,” says Packham.

Including both the internal steelwork and the kissing roof, steel contractor Severfield has installed 1,350 tonnes of steel on the project.

“As the floors had been taken out, we were able to drop the internal steel beams in from above using the tower crane,” explains Packham. “The vertical columns, mostly 400mm thick, sit 600mm inside the inside face of the building, which meant the existing timber beams were not an obstruction.”

It comprises two “giraffe necks”, which face each other between the two buildings, and “ribbon” trusses which twist upwards and inwards from the depot buildings to join the tops of the giraffes in a central node at the “kissing point”. Each of the two ribbons is formed from 20 sections bolted into four trusses on site, with the purlins also fitted.

The internal floor beams were also installed at this point, and the permanent support for the existing walls was taken off the new steel members.

The internal steelwork phase, up to the eaves level of the existing buildings, was completed around January 2017. At this point, work began on the kissing roof structure.

The roof structure is approximately 75m long on the east side and 65m long on the other, and rises up in the middle to a maximum height of 25m.

Scaffolding in the temporary works incorporates an access tunnel

The giraffes, which together with the central node weigh 330 tonnes, comprise box girders 1,000mm deep, 600mm wide with 40mm flanges, spanning 50m in total and tied together with a single plated tie. The ribbons are 610mm diameter CHSs, which are connected with 508 CHSs, and 219 CHSs for braces.

To lift these sections, Severfield used a 500-tonne mobile crane. First though, 12 trestle towers had to be erected, to support loads from the giraffe and ribbon trusses and the central node ranging from 280kN up to 700kN.

A forest of scaffolding – comprising 44,000m of tube, 26,000 fittings, 4,700 3.9m-long boards and 1,720m of modular beams – completes the temporary works structure for the kissing point steelwork, cleverly designed with an access tunnel through the middle for site deliveries.

“The trusses were then oriented correctly before being lifted into position,” says Packham. “Two watchmen suspended from the tower crane in a man rider monitored this process, with the trusses adjusted remotely using air jacks to the correct orientation.

“Each lift only took about two hours maximum, but with considerable preparation at ground level. None of the components had to be taken back to ground once we’d oriented them and lifted them up.

“The connections with the roof support structure are on a 25 bolt ‘clock face’, which were plotted at Severfield’s factory to a tolerance of just 2mm.”

With the steelwork in position, in June the temporary propping towers were removed. “The first deflection of the structure was comfortably within the predicted tolerances of 35mm,” says Packham.

In August, BAM installed the suspended upper floor slab, which is supported by the arcing roof structure and also joins on to the internal steel structure. The floor slab is 15m long at the kissing point, and this space will be part of the anchor tenant’s retail store.

While the steelwork was erected, the timber roof structure of the existing buildings was undergoing extensive restoration. The structure comprises “king post” trusses, a central portal frame and wrought iron strengthening plates at the node points. Each truss was surveyed to assess any decay and the overall condition.

“The structure had to be strengthened because of the weight of the new slates,” explains Packham. “Where required, new rafters were bolted to each side of the existing rafters and all trusses had a new steel plate added on each side, bolted to the truss at the rafter and collar joint. Decayed trusses were replaced where possible by reusing existing elements from those trusses not being retained.”

In the eastern depot, only three of the 30 trusses had to be removed, with 20 retained in their original position,
and the remainder moved to new positions. In the western building, only five of the 32 rafters were retained, these were also strengthened and the wrought iron bottom chords of each truss were replaced.

The roof build-up comprised 18mm thick plywood, with purlins at 350mm centres, plus insulation.

“Because of the deflection of the structure, we have used furrings so that the line of the slates follows the curve of the roof as intended by the design,” explains Packham.

Some 80,000 Welsh slates are being used, of which 25,000 had to be hand cut by contractor Attleys for the curved sections. The roof will be finished with a copper edge on the external sides of the buildings.

As BAM progresses towards the contract’s completion in the summer, the site workforce will rise to a peak of 220. The next big activity for Packham’s team is installation of the silicon-bonded glazing on the upper storey.

“Each of these glass units are bespoke as they slope up towards the kissing point,” he explains. “The largest is 8m tall by 2m wide.”

The building services plant is being housed in a shed behind the eastern building, and in a plant room in the centre of the western building, though the heating and hot water is served through underground utility pipes from Argent’s central energy centre on the King’s Cross development.

BAM is due to complete its contract in August, with fit-out of the retail units following, ahead of the new shopping centre’s opening in time for Christmas.

How Heatherwick worked with the team

Heatherwick Studio is famous for its projects all round the world, but Coal Drops Yard is less than a mile from its London office in King’s Cross.

The practice began work on the project in 2014 and led the design up until the end of RIBA stage 3.

“Detail designer BAM Design worked with us from early in stage 3, and then we worked alongside them through stage 4 – we didn’t want a handover, we wanted a more transitional approach,” explains Tamsin Green, Heatherwick Studio’s project leader on Coal Drops Yard. But the practice’s involvement will continue through to completion.

“We are the concept architect but we take an interest in the detail, in how the materials are made and crafted, which you can’t really do at stage 3, so we like to be involved in the later stages too,” she explains.

Heatherwick has a 9.30am walk around the site with BAM and Argent, plus a weekly workshop, a fortnightly design meeting, and frequent ad hoc meetings.

The practice also works closely with key subcontractors. “We visited Severfield’s factory twice to see the fabrication of the ribbon trusses, and also because the central node in the kissing point is the one part of the roof structure where the steelwork will be visible,” says Green. “We wanted to check the quality of welding because it would be exposed.”

Heatherwick will also put forward names for key packages, such as facade contractor Frener & Reifer. “That said, BAM has an established supply chain at King’s Cross – for example the masonry contractor Pyramid,” Green adds.

There have been “adjustments” to the design after stage 3, as Green puts it, due to a “better understanding of the existing buildings” once demolition was completed. “We’d had to make assumptions about tolerances at earlier design stages, so in some cases adjustments were made where new elements were meeting the existing fabric.” There have been 170 variations across the project.

The mock up and sample schedule of materials was initially 126 items – rationalised to 31. The desired specification has not always been possible due to unavailability. The roof slates, for instance, were to be 300mm wide, but as the quarry seam was not deep enough, were revised to 250mm.

Granite paving was specified for the public space between the buildings – to recapture the atmosphere of the original, cobbled yard – at stage 4. However, the Stirling stone from Scotland, which comes in red and grey, also did not come out of the quarry as expected.

“We wanted a percentage of red stones, to help warm up the tone of the bricks,” says Green, explaining that most of the material coming in was grey, so “we have had to leave holes in the paving for the moment”, to be filled with red stone later on.

“Generally, the standard of the work is very high,” she says, adding that the project team welcomes working together, and the weekly site visits and design meetings help identify and resolve issues quickly.

Using BIM to link old and new

BIM has been key to developing and implementing Heatherwick’s design.

“At the end of stage two, a point cloud survey of the buildings was created, and this formed the basis of our BIM model,” explains Tamsin Green. “However, at that point much of the demolition had not been completed, so it was difficult to envisage how the old and new elements of the design would come together.”

Coal Drops Yard is the first project where Heatherwick Studio has used a BIM environment, and Green says it was difficult to create the complex curves of the roof structure using BIM.

“We primarily design in Rhino, but when working with more detailed drawings and coordinating with other disciplines, the BIM environment becomes really useful, as it has on the project,” explains Green.

“BAM has encouraged use of Autodesk’s 360 Glue, which is cloud-based and allows all the project team to login and look at the BIM model, and that has been very helpful.”