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Fruits of the forest

Edward Cullinan Architects' RIBA Award-winning Downland Gridshell triumphs by combining the use of modern timber technology with energy efficiency and sustainability

Edward Cullinan describes The Downland Gridshell at the Weald & Downland Open Air Museum as 'far beyond Hi-Tech, in that it prophesies a new and ecological use of materials in the light of the knowledge gained during the last, high-technology century'. The recently opened building, already a RIBA Award winner and surely a strong candidate for this year's Stirling Prize, could be seen as a 'return to the roots' for Edward Cullinan Architects (ECA), the practice which Ted Cullinan founded in 1965. For Cullinan, however, 'it's not a matter of returning to roots - we never left them behind'.

The Gridshell, designed with engineer Buro Happold, is a highly innovative, frankly sculptural and thoroughly 'organic' structure that firmly locates ECA where it has always belonged - in the progressive regionalist camp of, for example, Murcutt, Sverre Fehn and the Patkaus, part of a romantic modern tradition which reacted against the certainties of the International Style. Cullinan's 'baggage', as he calls it, includes the work of Lubetkin and Lasdun, the Greene brothers and Wright, Rietveld and Corbusier, as well as the English Arts and Crafts sensibility in which he was nurtured as a child.

It would be all too easy, though misguided, to interpret the Gridshell as no more than a super-barn, a conscious reinterpretation of a traditional English building type using up-to-date materials and design technologies. As the museum's director, Richard Harris, points out: 'The most obvious solution to our space requirements would have been to put up a straightforward barn - but that would have been a cop-out'. The Weald & Downland Open Air Museum was launched in 1971 on a sublime site at Singleton, near Chichester, provided by that extraordinary figure, Edward James of West Dean. Since then it has rescued and re-erected a series of threatened vernacular buildings, mostly timber-framed, ranging in date from the 15th to the 19th century, from a wide area of the south-east. The museum is much more than a popular (140,000 visitors annually) tourist attraction. It is a centre for recording, research and conservation with a strong interest in fostering practical craft skills. The traditional vision of architectural history is stylistic - the Pevsner tradition of analysing facade details as part of a line of progress from Romanesque to the Modern Movement. At Singleton, the emphasis is on structure, technology and materials, so that history appears more as a continuum - the Gridshell is clearly, on one level, a response to an age-old 'functional' tradition based on the use of readily-available, locally-sourced materials.

The practical brief presented to ECA when it was selected for the project in 1995 was quite loosely defined, but specified two spaces/buildings - a climate-controlled store for its huge collection of historic artefacts (everything from Georgian planes to Victorian door handles) and a large, lofty working space in which timber frames could be conserved and reconstructed. The former would have limited public access, but the latter would form an important element in the visitor's experience of the museum and would be fully accessible. Low energy/sustainability issues should be to the fore in the scheme. The brief was developed with the help of a grant from the then newly established Heritage Lottery Fund (HLF) which subsequently provided two-thirds of the £1.8 million budget for the project. Construction work began in May 2000.

The siting of the new development was critical. For Steve Johnson, ECA's project architect, 'this wasn't to be an icon.We saw it as an industrial building, though I think the HLF - this was their first new-build project - wanted a landmark.' The site was away from the main groups of reconstructed buildings, on sloping ground, hard against a dense copse, close to the main entrance and parking area. The topography suggested a single two-storey building made sense, but the section is also the classic Cullinan juxtaposition of structure and superstructure, solid and lightweight, 'stones and sticks'. It first emerged nearly 40 years ago in Ted Cullinan's own self-built house in Camden Mews.

In this instance, the base structure housing the artefact store and a small display area is a heavy concrete structure dug into the chalk hillside, so energy efficient that a simple domestic boiler serves to maintain stable temperatures within. The heavy timber roof (locally grown ash) forms the floor for the Gridshell itself (now known as the Jerwood Gridshell Space).

Buro Happold was brought into the project at an early stage (as was project manager Boxall Sayer). The late Ted Happold had worked with Frei Otto on the timber gridshell at the Mannheim Garden Festival back in the 1970s. Built of softwood, it was constructed on the ground and raised up, using construction techniques that would be outlawed by current health and safety regulations. At its most basic, The Downland Gridshell would be a functional space enclosed against the elements with good natural light for those working there. 'You can see it as an engineer's building or a carpenter's building', says Steve Johnson. 'It's both - but it's also architecture'. Indeed, it is the luminous beauty of the Gridshell which strikes you first - then you want to know how it was achieved.

ECA's aim was to build something which was 'of the soil', using local materials as far as possible. But getting the green oak, strong but supple, which the structural agenda for the frame demanded, from within Britain meant bringing it long distances by road - the south-east lacked adequate supplies. 'It was 'greener' to bring it from Normandy, ' says Johnson, 'where there are large, renewable plantations.' When the reconstructed buildings at Singleton were first built, Kent and Sussex were rich in woodlands and timber could be used freely. As a result, the woodlands largely vanished. Timber may be a renewable material, but architects have a responsibility to design with economy in mind. One of the proudest boasts of the Gridshell team is that 'this was virtually a skip-free site - nothing was wasted'.

The 48m long Gridshell is made of green oak laths, 36m long and 35 x 50mm in section. Lightweight timber frames were nothing new for ECA - it had used them in its 1990s housing at Hooke Park (where ABK had already blazed the trail with its timber workshops). But both these projects were relatively straightforward, producing cheap and highly practical, if quite roughhewn, enclosures. The elegant precision of the Gridshell has, in contrast, been aptly compared with the handbuilt frames of Second World War aircraft or classic sports cars.

As Ted Cullinan points out, Singleton is close to the sea and there is a local boatbuilding tradition - 'you could compare the building to a huge boat', he says. The boatlike structure produces an interior totally free of columns and other obstructions.

With Happold and other professionals already on board, the choice of contractors was clearly significant. Fortunately, locally based E A Chiverton could offer the skill and commitment required of the main contractor, while The Green Oak Carpentry Company was commissioned not only to construct the Gridshell but also to make an input to the later stages of the design process. To ensure stability, the structure is not a simple tunnel, like a huge garden cloche, but a complex arrangement of three humps which strike off from the curved floor. Computer modelling at Buro Happold was integral to the development of the designs, ensuring maximum economy of means, but it was a highly practical mind at Green Oak that devised the (now patented) pinless connecting device for the laths that provides both stability and the necessary degree of movement in the frame. The shell was constructed, top-down, using an internal scaffold - the Gridshell may be, in Ted Cullinan's words, 'a hippy building', but site conditions acceptable in 1960s California would not wash in 21st century Britain.

The exterior of the building uses locally grown red cedar planks rather than the fabric coverings more commonly used in structures of this kind. 'We looked at fabric and metal, ' Steve Johnson recalls, 'but there wasn't really a choice - timber seemed true to the spirit of the project and Ted was adamant from the start that we should use it'. The aluminium foil inner lining, covering a highly efficient but slimline (25mm) layer of insulation, enhances the luminosity of the interior. The clerestory glazing is of polycarbonate. ETFE was considered, but glass ruled out as incompatible with the degree of movement in the structure.

In essence, the Gridshell is a very unprecious working building (though it is equally a great place for a party - a consideration for an institution which always needs more money). The simple enclosed workshops inserted into the space and the timber stores clamped on to the interior reinforce its essentially practical nature. But it is also a building of memorable beauty, which genuinely draws on tradition and modernity to make something timeless - there is nothing hairshirt about this vision of sustainability.

ECA stresses that the Gridshell was a collaborative project, the result of teamwork, and pays tribute to the late Chris Zeuner, museum director until last year, for backing its radical proposals. But it has generated innovative and inventive architecture of international quality, confirming the dynamism of the Cullinan office and inviting a reassessment of four decades of practice.

Engineering the Downland Gridshell The environmental design of the building is simple but effective. The lower level, containing the archive store, is dug into the chalk hillside. Its retaining walls are insulated on their outer faces, creating a temperature-stabilising earth shelter.Fresh air is brought into the building through 600mm-diameter concrete pipes laid under the ground slab. In this way, the building benefits from passive cooling in the summer, or warming in the winter, from the constant temperature of the ground at an effective depth of 5m below surface level. The thermal mass of the walls is used in moderating the environment for the archive store. A small, efficient gas-fired boiler provides underfloor heating.

In winter, cold fresh air is warmed in transit under the building; while underfloor heating warms the archive store. There is heat exchange between hot extract air and inlet air; heat lost in rising from the archive store warms the floor of the workshop above.

There is a potential top-up in the workshop from three roof-mounted warm air heaters.

On a summer night, warm outside air is cooled in its passage under the building, moderating the temperature of the fresh air input; heat is lost from the walls and removed from the building by circulating the fresh air.

On a summer day, warm outside air is cooled in its passage under the building; heat from the archive store is lost into the night-cooled walls. The workshop ventilates from end to end, as with traditional workshops, by opening the large doors.

The gridshell roof The roof of the archive store forms the floor of the workshop. It is constructed with glued laminated beams and solid timber flooring. It cantilevers over the reinforced masonry walls of the archive store, providing the support to the gridshell.

The Downland Gridshell is the first double-layer timber gridshell in the UK and only the fifth worldwide.This roof is a doubly curved shell made from 50mm wide x 35mm thick oak laths in four layers. It is a double hourglass shape,48m long,16m across at its widest points and 11m wide at the waists. The internal height varies between 7m and 10m.The lath spacing is 500mm in areas of high load, increasing to 1,000mm over substantial areas of the shell. Diagonal bracing of longitudinal and transverse timber rib-laths, fixed to the nodes once the shell is formed, provide shear resistance and lockin the shape.The bracing also supports the wall and roof cladding.

Shear blocks are screwed into place between the layers of the shell. In this way, parallel lines of laths act compositely; the size and position of the shear blocks is arranged to suit the forces derived from the analysis.Only at this stage could the temporary props be completely removed.

The node connector consists of three plates, the centre one having pins to locate the grid geometry of the middle lath-layers and the outer plates loosely holding the outer laths in place, allowing sliding during the formation of the shell.Two of the four bolts locating the plates are used to connect the diagonal bracing bolted in place to provide shear stiffness after forming the shell.This specially-developed connector has been patented.

Computer and physical modelling were carried out side by side throughout the design development. The difficulty of this process may explain why such an apparently attractive method of construction has been rarely used since the success of the Mannheim Gridshell in 1975. The geometry of a physical model was used to determine the boundary conditions for the computer form-finding model. It also served as a presentation model, which was of great use in persuading the funding bodies of the viability of the scheme.

Oak was selected because it is naturally durable and it is possible to omit timber treatment, because oak is the most common material used in the museum's collection of buildings and because oak is readily available from sustainable sources.Tests showed that while oak is stiffer than the other timbers tested, so needing a larger force to achieve a given curvature, it had a considerably higher bending strength, achieving a smaller bending radius prior to failure.

To overcome the problem of defects in the 35 x 50 mm oak laths, defects were cut out and the remaining short lengths of this improved graded timber were reconnected into 6m lengths using finger joints. These joints can be produced very quickly and cheaply with minimum waste. The 6m lengths were then joined on site using traditional scarf joints to produce continuous laths up to 37m long for the lattice laths and 50m long for the longitudinal rib laths. Of the 10,000 finger-joints in the structure there were only some 145 breakages during shell-forming. Repairs consisted of introducing solid blocking between laths at the point of failure.

Instead of laying out the mat at or near the floor level, as had been done on previous gridshells, a birdcage of PERI-UP scaffolding with a plan area the size of the layout mat was built at a height of 7m above the workshop floor. By laying out the mat at the height of the valleys, the lattice could be lowered into position, providing a safe access platform at all times and harnessing gravity in achieving the desired shape.

The carpenter's view The Green Oak Carpentry Company (GOCC) was invited to join the engineers and architects as carpentry design consultant 10 months before work began on site in June 2000. It was, in our view, a far-sighted decision that was to offer significant advantages to the project.

GOCC played an active role in designing some of the timber-to-timber and timber-tosteel connections with the rest of the team.

We were able to bring an in-depth understanding of the material and wideranging experience to the task. I think it likely that, in time, carpentry specialists will do much more of this sort of work. A key advantage will be to reduce the cost of timber engineering, which can be prohibitively expensive.

From the outset we were keen to establish a good understanding of the diverse properties of different timbers available to us for use in the building.Scottish-grown whitewood was used in the deck as an inexpensive, but non-durable option.Oak was used green for the gridshell where suppleness was important, but dry for lamination into the round-section catenary arches that form the awnings of the building.

Fir was used for the ribbon roof as it combined durability and suitability for small section material in the long lengths required; and locally grown Red Cedar for the cladding as it combined stability and durability.

An important innovation in the Downland building was to construct the gridshell using green oak, freshly sawn with a moisture content of more than 40 per cent.This approach was possible because of advances in adhesives tolerant of high moisture levels.The green timber's flexibility was key to the success of the project. Nearly 3,000 fractures had occurred in the Mannheim gridshell 25 years earlier. Yet the Downland Gridshell had much tighter curvature - 6m radius in places - hence the concern. Few fractures occurred, though the Downland Gridshell represents the minimum curvature possible for this building.

Timber grading was another crucial issue that required a flexible approach. It was very important that the grading rules for supply of the timber were written with timber type, source and the end use in mind.Too often they are applied without sufficient thought and simply add cost needlessly. In the end we wrote our own grading rules, applicable to all the diverse timber applications throughout the building, and agreed them with the rest of the team.

The construction industry's focus on the cheapest rather than best value has been gradually deskilling construction.This is now having a catastrophic effect on the level of carpentry expertise. Again the approach of the team was unusual - and, in the context of a Heritage Lottery Fund project, extraordinary.

The carpentry team was appointed on the basis of best price, enthusiasm and expertise.

We think the building speaks volumes for this approach to be more widely adopted.

Andrew Holloway, The Green Oak Carpentry Company

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