Feature

Behind the Scenes at the Exhibition

For a design museum, it is ironic that so much of the groundbreaking design of Kengo Kuma’s V&A Dundee – from the formwork to the reinforcement – will never be seen. Tony Whitehead finds out what lies behind this extraordinary structure.

“Take two decks of cards and place them on a table a few inches apart. Then twist them so they join together.” This is how one engineer describes the new V&A Museum in Dundee – surely one of the most remarkably shaped buildings to have been constructed from concrete. None of its 21 elevations are vertical. Many of them are curved in two dimensions. Concrete walls up to 18m high incline outwards at a variety of unlikely angles and onto these are attached no fewer than 2,429 precast-concrete elements weighing up to three tonnes each. Designed by renowned Japanese architect Kengo Kuma, this extraordinary, £80 million, 8,000m2 building looks (from some angles) like a ship about to sail off into the Firth of Tay, on whose banks it stands. But such is the nature of the building that every perspective suggests something different: sails, a cowry shell or the stratified rock formations that were Kuma’s original inspiration.

To the construction-minded visitor, however, one pressing question immediately presents itself: how does it stand up? “The overall structure works as one,” explains project architect Maurizio Mucciola at PiM studio. “The external walls are tied back via the slabs and the roof to the two central cores – so the building was not stable until the roof structure was complete. It meant much of the supporting exterior formwork and falsework had to stay in place for more than a year until the roof was finished.” The design presented such unique technical and construction challenges that in its early stages a variety of structural solutions, including using steel for the upper storeys, or building it entirely from precast blocks, were considered. However, says Mucciola, “In the end, in-situ concrete gave us the most flexible solution, combining technical ability with the freedom to create the shapes we wanted.”

The V&A Dundee opened its doors in September. Its three floors contain a restaurant, extensive storage facilities, an auditorium, classrooms and a library as well as several galleries providing 1,650m² of exhibition space. It is statement architecture in every way – a new point of focus for the city and one designed to reconnect Dundee to its riverside heritage. As such, its shape is designed to attract and fascinate, but the story of how it was created is no less intriguing. Mucciola explains that the outside of the building is formed from mainly 300mm-thick, heavily reinforced, high-strength in-situ concrete, though in some places, where the spiralling tangle of stresses was particularly high, the thickness has been increased to 400mm. “But we wanted the facade to be as thin as practically possible, not only to save weight and concrete, but also because it is punctuated with some 60 or so windows. If the external wall was any thicker (on top of the insulation and render on the interior side of the concrete), very little light would find its way through to the gallery spaces of the interior.”

Achieving the required wall thickness while accommodating the complex stresses and loadings created by the building’s twisting shape was a key challenge for the principal structural engineer, Arup associate Graeme Moncur. “The early designs had 600mm-thick walls,” he says, “but by extensive modelling and analysis we were able to reduce this to mostly 300mm of concrete.” This was achieved first by identifying more exactly what stresses lay where. Then, says Moncur, “We were able to reduce the wall strength needed by slightly modifying the design so that in places the height of some walls was reduced a little, or the incline made slightly less steep, all the while remaining true to the architect‘s original vision.” In conjunction with the concrete contractor and supplier, Arup developed a high-strength 70kN mix that included 27% fly ash.

“A cement substitute was desirable because this is a high-strength mix with quite a high cement content so GGBS [ground granulated blast-furnace slag] would normally have been used, both to reduce the CO2 footprint, and also to reduce the heat of hydration,” says Moncur. “However, the facade concrete contains pigments to make it very dark – almost black – so as GGBS makes concrete paler we opted for fly ash instead.” The concrete also contained small amounts of silica fume, micro silica and limestone fines to help strengthen the mix: “The micro silica helps to infill pores and densify the concrete, which is good for durability given that the building is situated on a brackish, tidal river estuary and will have to resist salts in the air.”

Finally, adds Moncur, the mix contained plasticisers to increase flowability. “This was vital because of the amount of dense reinforcement, and the many odd angles and inclines where air might become trapped.”At one point the team considered using spiral reinforcement in order to create tunnels through which vibrating pokers could be inserted, but were unable to source spirals long enough. Fortunately, this proved unnecessary: “In the end, the mix’s good flowability meant we had very few problems with air.” Having hit on a mix that was dark enough, strong enough, salt-resistant and flowable enough to negotiate the reinforcement, the team then had to set about creating foundations that would be able to take the unusual loads presented by the V&A’s irregular shape. “It’s not like a normal building,“ says Moncur. “You have inclined forces coming down, so you have to resolve horizontal forces through the beams and slab and into the piles.” In practice, this meant using 849m3 of concrete to sink 198 concrete piles 11m down to bedrock.

A further 2,053m3 of concrete and 364 tonnes of reinforcement was deployed to create pile caps and a grillage of ground beams, typically 1m square, on top of which was laid the 300mm-deep ground slab. Moncur explains that since the V&A is effectively two buildings, each tied back to its own core, each exerts its own inclined forces: “So the ground slab is in compression in some places and in tension in others, and has to act to stop the buildings pushing into each other.” These unusual forces were more active during construction, before the roofs were completed, so for additional stability a temporary concrete slab was also laid around the outside of the building to stabilise the ground and also to provide a firm basis for the huge amounts of formwork and falsework involved in creating the difficult exterior facades.

Malcolm Boyd, construction manager with BAM Construct UK, says: “Because of the nature of the walls, each piece of shuttering was unique and had to be precisely positioned in order. If the wrong shutter arrived at the wrong time, we couldn’t just move on to the next one. Correct sequencing was vital.” Twenty designers were working on the project for formwork supplier Peri, and such was the size of the job that it was manufactured at five locations across Europe, in Scotland, England, Ireland and Germany. In all Peri delivered 1,200 bespoke shutters at a rate of around 1,000m2 per month. The facility in Germany was used to provide the majority of single-use timber carcasses, which were cut with CNC machinery. These were then sent to the firm’s UK depot in Rugby for the addition of plywood skins and steel formwork before delivery to site.

9km of irregular cladding

V&A Dundee is clad with 2,429 precast “planks” up to 4.4m long and ranging in weight from 0.9 to 2.8 tonnes. Each lies horizontally against the in-situ concrete walls of the facade, but because these walls are curved and inclined, the back face of every element had to be specifically angled in order to ensure a good fit. Over a period of 12 months, Techrete produced nearly 9km of these precast elements.

To do so efficiently, it worked with a steel mould manufacturer to develop a means of producing elements with variable facets. Essentially this comprised a reusable steel mould that could be tilted. The face of the element was arranged so as to be level and horizontal, at the top of the mould, with the required angle created by tilting the mould below. Sean Callow, Techrete’s production planner, says: “We could have spent a small fortune getting a mould for every angle – so we had to find an adjustable solution. Initially we looked at maybe having a hydraulic or motor-driven mould. But this would also have been expensive.

By working on the mould’s centre of gravity and the point at which it rotated, we managed to get it nicely balanced – so that two men could adjust the mould and lock it into position by hand.” The length of each element could be varied by moving the moulds’ adjustable stop-end panels. Some of the moulds were slightly larger to create deeper elements – again in order to accommodate the curving walls. “Altogether we had eight adjustable moulds and a couple of more standard ones,” says Callow.

The mix, a reconstituted granite, was designed to reflect the ruggedness of the local Scottish landscape. An exposed finish was created by applying a retarder to the moulds during the production stage. This prevents the cement setting hard on the surfaces of the units, allowing a thin layer to be easily removed with a light power wash to expose the aggregates.

Each element is fixed to the building using cast-in stainless-steel hooks, which fit into brackets located in channels cast into the facade. Techrete used building information modelling (BIM) to design all of the component parts and also developed a GPS system to place all the channels and fixings to assist site installation.

Photos: Ross Fraser McLean

Construction on site required a team of 30 joiners, working in squads of four or five. “The outward-leaning shutter would go on first, ”explains Boyd. “To locate the shutters precisely, we used electronic distance measurers (EDMs) to scan the shutter, take readings and draw up a heatmap on screen. Positioning was vital as we were fitting unique shapes one with another. In addition, we were effectively building two buildings that had to join precisely – so we were working to tolerances of about 1-2mm. At times it was like building a bridge.”

He adds that the formwork featured a controlled- permeability liner to reduce moisture on the surface of the concrete, a technique that results in a denser, more hard-wearing exterior finish. The shuttering also came with pre-applied forms for the cast-in channels that would be used to fix the precast cladding elements. Positioning the reinforcement in these curved forms was something of a challenge – the starter bars for the most curved sections of walls emerged from the foundations like a twisted fan. The reinforcement was then positioned using the external shutter as a template. This also involved reverse-positioning the reinforcement with vertical reinforcement on the exterior side(to follow the line of the shutter) and horizontal bars (normally on the outside) towards the interior face. “Bar diameters ranged up to 30mm, but in sharply curved areas these would be difficult to work,” says Moncur.

“Some were specially shaped, but in places thicker bars were replaced by a pair of thinner ones (say, two 16mm) to enable them to be more easily bent into shape on site.” With the extraordinary formwork and reinforcement taking shape, the first pours for the facades could commence – typically in 4m x 10m sections. In all, some 2,300m3 of concrete and 1,300 tonnes of reinforcement were used for the exterior walls alone. As the walls rose, so the first- and second-floor metal deck and concrete composite slabs were also constructed. Along with a number of shear walls, these were vital for tying the inclined walls back to the cores. Male-female connections had to be specially designed to cope with the forces involved and ensure the facades remained firmly connected. “At roof level, the walls are held back to the cores via steel roof trusses, each exerting about 4,000kN in tension, the equivalent to 16 fully-loaded 6m3 concrete trucks,” says Moncur. “The cast-in steel plate connections for these had to be very precisely positioned and weighed 1.1 tonnes each.”

With roofs and structure complete, Careys could commence dismantling the external formwork which had supported the walls. “The loads spiral down,” says Boyd, “so it was important to de-prop in a balanced way – simultaneously north and south, and then again east and west.” As the formwork came down, and the diaphragm ties of the building’s floors and roof took the full strain of the inclined walls for the first time, Boyd admits that it was an anxious time. “We were worried about cracks,” he admits, “but I’m pleased to say there weren’t any.” Now it is complete, with its stunning precast cladding (see box), the construction team at times seem almost bewildered by what they have managed to create. As Moncur says: “Arup worked on the Sydney Opera House and the Taichung Opera House – so we’ve done some pretty unusual buildings. But nobody has ever done anything like this before.” “It certainly extended the grey matter,“ agrees Boyd. “The design really made you think hard every day. It might sound like a nightmare to build, but actually it was a great experience. “I’ll never get the opportunity to do anything like this again. It’s been a privilege. And it looks fantastic.

Access: climbing the cliff

Rather as an overhanging rock face presents mountaineers with their most testing climbs, so constructing the outward leaning walls of V&A Dundee provided a series of unique challenges for the construction team. “Access was always going to be tricky,” says BAM construction manager, Malcolm Boyd. “We developed a number of ways to maintain access safely. For example, there was so much formwork and falsework we turned it to our advantage, building in steps and handrails as we went and strengthening the formwork tops to create platforms from which we could work on the next set of pours.” Attaching the precast elements was also much harder than if the walls had been vertical. “Obviously a crane rope hangs vertically,” says Boyd, “so for the steepest inclined walls – those nearest the ground – it was impossible to use a crane as the planks would hang too far from the wall.

"The solution was to use fork-lifts for the lower sections, though even this approach needed refining: “The precast planks had to be angled exactly to get the hooks to fit into the brackets,” says Boyd. “To do this required fitting special tilting tables to the forklifts. Once modified we could slot them in nicely.” For the higher, less inclined levels, the cladding was attached with the help of mobile cranes and a lifting beam: “We would have the precast on one side of the beam and ballast on the other. By varying the pick up point of the beam we could tilt the plank into place on the facade.”

Once in position, the element would be connected manually, using where possible the access platforms built into the supporting falsework. At the peak of construction, the team were fitting some 22 of the precast elements per day.

Photo: V&A Dundee, Drawings: KKAA

Photos: V&A Dundee, Drawings: KKAA

PROJECT TEAM

Architect:  Kengo Kuma
Delivery architect: PiM.studio
Main contractor: BAM Construct UK
Structural engineer: Arup
Concrete structure: Careys
Formwork: PERI UK
Precast facade supplier: Techrete