From one tiny seed...

Heatherwick Studio has taken a disused concrete silo and a grain of corn and turned them into something extraordinary. Nick Jones explains all

Heatherwick Studio has taken a disused concrete silo and a grain of corn and turned them into something extraordinary. Nick Jones explains all​

Thomas Heatherwick has always had a taste for the spectacular – the concrete hives of the Nanyang Learning Hub in Singapore (CQ 253), the rotating facades of the Bund Finance Centre in Shanghai, the giant, intertwining stills of his Bombay Sapphire gin palace in Hampshire. For every one part Santiago Calatrava, he is at least two parts Roald Dahl. Now, in Cape Town, South Africa, Heatherwick has performed his most impressive trick yet, conjuring a jaw-dropping concrete interior from a single grain of corn.

The Zeitz MOCAA is a contemporary African art gallery and hotel which has been converted from a 1920s grain silo on the city’s Victoria & Alfred Waterfront. The silo comprises two linked buildings: a tower, once the tallest building in sub-Saharan Africa, the upper floors of which now house the hotel; and a lower volume of 42 circular storage tanks. From the outside, these look much as they always did, with only some judicious blast-cleaning and the crystalline windows of the hotel and rooftop sculpture garden suggesting a change of use and fortune.

But within the scalloped perimeter wall of the storage tanks lies a space quite unlike any designed before. Eighteen of the building’s mighty concrete cylinders have been carved open, turning a tightly organised, functional grain store into an organic, sculpted atrium. This central cavity is almost literally a heart – the surgically incised tubes hang over the space like arteries dispersing visitors around the building. Heatherwick has spoken of the need to give the museum “compelling innards” to lure people in. It is hard to imagine how he could have achieved this more emphatically.

The story behind this atrium is typically Heatherwick – as much the work of a sculptor as an architect. On an early visit to the silo, which had been disused and in a state of neglect since the 1990s, the designer picked up an old grain of corn from the floor. Taking it back to his studio, he digitally scanned it and enlarged the model 10,000 times; this huge irregular ovoid would provide the form for the atrium, with all of nature’s curves and quirks adding to the space’s sense of wonder. The question was, how exactly do you excavate a precise 27m-high grain-shaped hole from a 90-year-old concrete industrial building? And perhaps more pressingly, how do you do it without the whole thing collapsing?

In order to create space for the museum, much of the lower building had to be hollowed out. “Silo projects are pretty unforgiving,” says Francis Archer, associate director at structural engineer Arup and long-time Heatherwick collaborator. “They’re basically 5m-diameter bundles of tubes.” To turn this one into modern, flexible gallery space, all but the perimeter walls of the 24 eastern-most cylinders had to be demolished and a reinforced-concrete frame and floor slabs inserted. The atrium would be scooped out of the remaining 18 cylinders.

The problem was that removing the cylinders would also remove almost all of the building’s structural integrity. The tanks were built from cheap, industrial-grade in-situ concrete, just 170mm thick and reinforced with steel hoops every metre or so. They were made rigid simply by being tightly packed together; take some of the tubes away and it would be a very different story. According to Archer, the hollowed-out building would have been in danger of breaking up even in high winds – let alone the earthquakes that South African structural codes have to protect against.

The solution was novel, and entirely untested. Arup proposed “resleeving” the remaining cylinders with an inner layer of reinforced concrete – in effect, making an internal cast of the whole building, thereby allowing the existing cylinders to become completely non-loadbearing. Here, Arup had the advantage of knowing that the contractor, WBHO, had already signed up to the project, so could discuss its feasibility, and its cost, from the outset. “It’s kind of obvious but it doesn’t always happen,” says Archer. “When you want to do something unusual, if you have a contractor there from day one who knows they’ve got the job, and who the client trusts, then designers can have confidence in proposing solutions.”

The original 1923 structure had been built using an early type of slipforming – black-and-white photos show a rickety system of timber shuttering and flimsy, unprotected working platforms being manually jacked up the outside of the cylinders. Ninety years on, the site team found themselves echoing this process with modern equipment, slipforming 270mm-thick sleeves of reinforced concrete up the inside of the remaining cylinder walls. The old and new concrete were separated by a 50mm-thick compression joint to allow for movement but connected using steel anchors at the top and bottom. The new sleeves were also linked together into a continuous structure by threading 0.5m2 reinforced-concrete connectors through the tubes. This meant the old, brittle concrete could be subtly broken up so it was no longer monolithic – from the outside, you can just about see a 40mm vertical gap between each cylinder.

For the outer cylinders, which were now essentially half-tubes forming the perimeter wall, the slipforming was a relatively straightforward process. But each of the atrium cylinders needed to be cut away below a precise point, and at a precise angle, all of which were unique and had been translated into thousands of digital coordinates.

To resleeve the cylinders before cutting would have wasted an enormous amount of concrete, so instead the coordinates were used to construct plywood formwork that would form the base of the new sleeve as well as a cutting guide for the tubes. “It was very clever shuttering,” says Archer, “often curved, and defined to the millimetre. A model was drawn in 3D of every single bit of plywood – that formwork was going to guide everything.” A gantry was erected at the top of the tubes to allow the slipforming to begin at different heights – some tubes were cut away close to ground level, others no more than 3m from the top of the building.

There was a further complication. The grain-shape was not neatly enclosed by the existing cylinders; instead it ate into both the foundations – a matrix-like structure of grain tunnels and octagonal unreinforced-concrete walls – and the eastern edge of the neighbouring tower. The tower’s original structure was a concrete-encased steel frame stabilised by concrete perimeter walls, but now the atrium was about to burst through one of these walls and two perimeter columns.

As with the grain cylinders, the tower’s structure needed a major rethink. Two reinforced-concrete cores were added to support new gallery floors to the lower levels and 500mm layers of reinforced concrete to the hotel floorplates above (these were originally 1m-thick slabs of unreinforced concrete sandwiched between steel joists). Cantilevering off the larger of the cores, two new reinforced-concrete walls were also now able to give support to the atrium, allowing the tower to be sliced open and the old perimeter columns to be taken away.

With the structural works nearing completion, attention could turn to the finishes, and specifically the cut edge of the cylinders in the atrium. This raised another challenge. Heatherwick Studio wanted a smooth, seamless finish between the old and new concrete, and the compression jointreduced and neatly concealed at the base of the pour to accommodate this. However, none of the conventional methods of concrete-cutting trialled proved up to the task of cutting the atrium’s nuanced angles into coarse, brittle, flint-based concrete. Eventually, it was decided that the only solution was to employ skilled stonecutters to painstakingly carve the concrete by hand, using diamond saws. The finished edges were then polished, emphasising the contrast between the smooth, controlled new concrete and the shiny, nougat-like black flint of the old.

The decision to cut the concrete by hand was a big call – adding more or less a year to the construction process – but then, this detail is the building in microcosm, so worth getting right. The Zeitz MOCAA both preserves the existing and transforms it into something entirely new. The idea to turn the silo into a museum – rather than, say, luxury housing – was proposed by Heatherwick because it would leave the facades untouched; there is no need to cut windows into a building that basically functions as a black box. At the same time, this is no mere facade-retention scheme, disingenuously implying an interior that has been destroyed without trace. One of the appealing things about the Zeitz MOCAA atrium is that it would never have been designed like this from scratch – it could only ever exist in this space.

Reimagining historic buildings is a fraught business, with the competing interests of heritage bodies, developers and local people all somehow to be reconciled. But Heatherwick Studio’s Zeitz MOCAA strikes a balance between past, present and future. It is unusual for an urban dockland project in that the V&A Waterfront is still an active port, with fishing vessels docked for repair from all corners of the south seas. The silo remains part of that landscape, as well as a link back to a time when Cape Town shipped grain to East Africa, India and the Far East. To step inside, however, is to enter another world entirely.

Reinventing the silo

Modernist architects always had a thing for silos. Le Corbusier called them “the first fruits of the new age”, while Walter Gropius likened them to the buildings of Ancient Egypt. It was not just that these buildings’ monumental use of exposed concrete chimed with modernists’ own aesthetic sensibilities, but that the vast tanks and vertical elevators were bold forms that perfectly expressed their noble, utilitarian function – the storing of grain in cool, dry, non-flammable conditions.

So it’s ironic that today’s architects seem to enjoy nothing more than finding new uses for silos. The pioneer was perhaps the Catalan maverick Ricardo Bofill. In 1973, he bought a disused cement factory outside Barcelona and he has spent the past four decades transforming it into his home and studio, with his 40-strong practice based in the cement silos (pictured). For Bofill, the partial destruction and reinvention of “La Fabrica” as a fantastical, overgrown gothic castle represents “the annulment of functionalism”. Instead, “any space can be allocated whatever use the architect chooses, if he or she is sufficiently skilful”.

Bofill may have felt his case was strengthened when, in 2012, C+T Architecture transformed a 1920s art deco grain store in Marseilles for an equally surprising new function – an opera house. The scalloped effect of the hollowed-out tanks lent shape to the auditorium balconies, and even acted as acoustic baffles, while the mighty concrete hoppers were refashioned as bar lighting.

The current caché of industrial structures, coupled with the inherent sustainability of adapting existing buildings, suggests that silos will continue to attract architects and developers. In Copenhagen’s North Harbour, COBE Architecture has just converted a 15-storey block of 16 concrete tubes into 38 luxury apartments, cutting windows into the monolithic exterior and draping it in a faceted steel facade. The exposed-concrete interiors have been retained, as have the varying floor heights, which means some apartments are nearly 7m high.

Elsewhere, more radical reinventions have been proposed, including a vertical crematorium in Malmö and a skydiving centre in Warsaw. It makes you wonder: where are they storing all the grain?

Architect: Heatherwick Studio
Structural engineers: Arup, Sutherland
Contractor: WBHO

Photos: Iwan Baan