Snug as a bug in rug

If a cocoon is the perfect place for a pupa to grow before emerging as an adult insect, wouldn’t it also be a good place to protect its body from harm once dead? The reasoning may seem strange, but the Natural History Museum has seized this logic and used it as the way to stop its collection of insect and plant specimens from deteriorating. Hence the cocoon-shaped storage facility that it is building at its west London site, which will also include space for scientists to work and areas for the public. “One of our remits is to safeguard those specimens for the nation,” says Neil Greenwood, who is the Natural History Museum’s project director. “Previously these were not stored in the best conditions. The cocoon will provide an ideal storage facility.”

When you have 20 million specimens that need 3.3km of cabinets to house them, you need a very, very big cocoon. This one is 65m long, 12m wide and eight storeys high, and fills most of the rectangular building being constructed around it, the £75m Darwin Centre 2. Ed Newman-Sanders of Arup, the structural engineer on the project, likens the view from above of the enormous structure as “a submarine emerging from the water”.

The design team has treated the cocoon concept as literally as possible. Specifically, it is a silk bubble, with a series of what appear to be threads crisscrossing its surface. The overall effect is quite beautiful, ivory coloured with a subtle sheen. And as it emulates a natural object, it does not have a regular shape. “The cocoon is totally freeform, and is very difficult to define mathematically,” says Newman-Sanders. This, coupled with the fact that the cocoon was actually a structural component, meant it was not easy to build, but the design team and contractor HBG have pulled it off with the help of some very specialist contractors. How did they get on?

Rhino, the 3D modelling program, was the key to creating the form of the cocoon and enabling the contractors to build it.

CF Møller, the Danish architect that created the shape, did so using the program, then Arup used it to define the structure before passing the model over to HBG and its specialist contractors.

But how did they take this virtual model and turn it into something real? The challenge was complicated by Arup wanting to maximise the space inside the cocoon. It wanted to eliminate the internal columns the walls as a structural element, even though the samples within were very heavy and the load on the floors was about three times that of an office building.

Four structural options were considered: steel frame, precast concrete, insitu reinforced concrete and sprayed concrete. Steel frame was quickly discounted because it would have provided hiding places for the dreaded museum beetle to breed. (This is not a joke: anthrenus museorum can rapidly destroy entire collections, so the whole building has been designed without hidden nooks and crannies.) Precast was discounted as too expensive and the complex shuttering needed for reinforced concrete would have made the structure painfully slow to build.

That left sprayed concrete as the favourite because, according to Alan Grant, HBG’s construction manager, “you get the right shape the first time”. It was also cost-effective and quick. But it had one disadvantage over reinforced concrete: the building surrounding the cocoon and the floor slabs inside it had a conventional reinforced concrete structure. If the cocoon had also been built using reinforced concrete, it would have been relatively simple for one specialist contractor to build them together. Sprayed concrete, however, is a very different process and is done by a separate specialist.

Grant says integrating the two operations was a big challenge because the mess created by the sprayed concrete would not have gone down well with the reinforced concrete specialist. “The downside of sprayed concrete is it’s a messy operation,” says Grant. Our concern was you couldn’t bring the reinforced concrete frame up with the sprayed concrete. We would have ended up with a mutiny.”

The answer was to build the reinforced concrete frame first, then the cocoon, but this created the problem of how to support the slabs next to the cocoon before its construction.

Temporary steel works turned out to be very expensive, so the reinforced concrete frame contractor Getjar proposed incorporating full-sized columns to support the slabs, then removing these afterwards. This turned out to be the cheapest solution, so 13 columns supporting the slab edges were duly incorporated into the structure ready for work to start on the cocoon.

Building the cocoon structure

The £1.6m cocoon structure was built by sprayed concrete specialist Shotcrete, and according to its managing director Stuart Manning is now the first above-ground sprayed concrete structural element in the world. This is particularly impressive when you consider that all he had to help him build it was a computer model. “We were provided with the Rhino model and told that this was what we had to build,” he says. He also given a series of flat, reinforced concrete slabs with the rebar sticking out ready for incorporation into the cocoon.

Shotcrete worked with Arup from an early stage in the project to determine how to approach it. The first stage was to erect scaffolding between the slabs with a series of poles sticking horizontally out into the space where the cocoon was going to be built. Using the Rhino model, a surveyor marked on the scaffolding where the reinforcement on the inner and outer side of the wall should be placed. Arup’s Newman-Sanders says they came up with the idea of using straight rather than pre-bent rebar for the reinforcement cage. Arup specified the size of the rebar and the spacing between fixing points; the idea was the rebar would naturally bend to form the correct shape.

Horizontal bands of rebar were placed first followed by the vertical bands. “That gave us a reinforcement mat that was curving exactly to the shape of the structure,” says Manning. He says forming the apertures in the walls of the cocoon to allow access for scientists and the public wasn’t easy. “The openings were most demanding,” he says. “Every opening was irregular in shape, the heads were crooked, the sides weren’t straight and the whole thing is curved.” The only way to form the apertures was to place the concrete first, then cut the rebar away afterwards. Leaving gaps for the apertures would have meant the cocoon would have lost its flowing shape.

With the rebar in place, expanded mesh was fitted to the inside face of the rebar and a 200mm thick layer of concrete sprayed on. Then a 35mm thick finishing coat was sprayed on to either side of the bulk concrete and smoothed off, ready for the next stage in the cocoon’s evolution.

The finishing touches

The next stage was to turn 3,500 square metres of raw concrete surface into something that resembled a giant silk cocoon. This was done using an ivory-coloured polished plaster finish, in which a series of channels resembling silk threads wrapping around the cocoon were incorporated. The outer surface of the cocoon was insulated to shield the interior as much as possible from external temperature variations. Because this outer layer is exposed to the sun coming through the glass facade on one side, movement of this skin was a worry. Grant says these issues were new to HBG and it wanted to minimise its risk, but the trouble was that polished plaster companies tend to be very small. “From our point of view we wanted a single point of responsibility by letting the insulation and polished plaster as a package,” he says. “We interviewed about four companies and most were very keen to do the polished plaster bit but not the insulation or setting out. Plus they couldn’t say whether the stuff would drop off.”

Only one company – Armourcoat – was prepared to take the job on as a package. Although it has done bigger contracts than this £1m job, this one was particularly challenging. “It was unique because of its complexity and because we were working from first principles,” explains Duncan MacKellar, the firm’s creative director.

The first job was to attach a 50mm thick layer of polystyrene to the concrete and coat this with a layer of reinforcing mesh embedded in resin to give a firm surface to work on. Then the surface was surveyed and marked out ready for the crisscrossing “threads”. These are actually channels cut into the insulating layer, which Armourcoat creating by fixing a series of metal tubes temporarily to the cocoon surface where the threads would be located and using a special cutting tool to make the grooves. “We got an engineers workshop to make us a twin-wheeled cutter with four roller blade wheels that would fit over the steel bar so it acted as a guide,” explains MacKellar.

Once the channels were cut, an edge bead was fitted to the sides of the grooves, then a silver-coloured plastic strip, which sits 10mm below the finished surface, was clipped in place. “We had to fit 5.6km of edge bead. The setting out of the bead was one of the hardest parts of the project for us, as it had to be perfect at the interfaces which were very hard to set up accurately,” says MacKellar.

The threads slice the surface of the cocoon up into a series of panels which vary in size from 50m2 down to fractions of a square metre. These can expand and contract with the movement taken up by the gaps between the panels. Each panel was coated up with an anti-crack coating ruled off by hand with lengths of plastic-coated MDF to give a perfect surface. Then the standard Armourcoat basecoats were applied followed by the ivory-coloured plaster finishing coat.

The end result is impressive. Anna Maria Indrio, project architect CF Møller, wanted the cocoon to look as natural as possible. “It should have a materiality that shows it is a handmade thing,” she says. And it does. The overall shape is very naturalistic, the surface is gently irregular and the texture of the plaster finish subtly varies across the surface. It will not only undoubtedly protect the museum’s valuable specimens but will attract new visitors to the Natural History Museum, like moths to a flame.