A wood-frame roof may not sound unusual, but Buro Happold's take on it – the UK's first timber "gridshell" at a Sussex history museum – is enough to make a structural engineer whimper with fear.
A museum dedicated to recording the history of traditional rural life in Kent, Sussex and Surrey is probably the last place you would expect to find a building pioneering radical construction techniques. But the Conservation Centre, under construction at the Weald and Downland Open Air Museum near Chichester, is just that. Next summer, when the building opens to the public, it will be the first timber “gridshell” structure to be built in the UK – and one of only five ever built in the world.

The centre, which is being funded by a £1.05m Heritage Lottery Fund Grant, was designed by Edward Cullinan Architects and structural engineer Buro Happold as a national facility for the study and practice of building conservation in general, and the English timber frame tradition in particular. However, the client did not want to commission an ersatz heritage building; rather, the brief called for “an exemplary structure for modern rural buildings”. The designers interpreted this brief by marrying hi-tec form with traditional materials.

The roof has a practical as well as a symbolic purpose: the gridshell will shelter a courtyard 15 m wide and 50 m long where the carpenters will carry out timber conservation work. It will also house two enclosed workshops and a classroom where traditional crafts and construction skills will be taught. Beneath the gridshell, sunk into the chalky hillside, the centre’s basement will provide an environment-controlled archive containing the museum’s library and artefact collection.

Structure and construction

One of the advantages of the gridshell is that it needs no internal supports. “It’s an extremely strong structure,” says Cullinan’s Steve Johnson. “A gridshell is essentially a shell with holes, with the structure concentrated into strips.”

The roof was designed to have three rounded peaks with two valleys between them – Johnson uses the analogy of “a triple peanut shell” to describe its form. This is an obvious reference to the topography of the surrounding chalk downlands, but the sculptural appearance is as much a matter of structural necessity as aesthetics. “For a gridshell structure to function, its surface must always be curved,” says Johnson.

The timber grid will be constructed from a double layer of green timber laths 50 mm wide, 35 mm deep and spaced 1 m apart. As the remit required Cullinan and timber specialist Green Oak Carpentry Company to use local wood to construct the shell, the team put each of the area’s indigenous timbers – oak, ash, chestnut and sycamore – through their paces on a test rig at the University of Bath. None was up to the job.

Eventually, the designers were forced to look further afield, finishing their search in France, where green Normandy oak proved to have just the properties needed for the structure. Apart from its immense strength, the French oak has the advantage of being inherently resistant to insect attack – something that had to be carefully considered, since to avoid the introduction of toxic chemicals into the environment, the wood will not be treated.

Even though the Normandy oak seemed to have the desired properties, the lack of precedents meant that a structural analysis of its behaviour in a gridshell was always going to be difficult. In fact, that difficulty is one of the reasons that there are so few gridshell structures around, says Johnson.

The engineers from Buro Happold were fortunate in that the firm had a wealth of experience in designing tensile structures, and had even developed its own software for their structural analysis. Chris Williams, a lecturer in the Department of Architecture and Civil Engineering at the University of Bath, was able to adapt this program to make it applicable to the gridshell. Finally, to check the calculations were correct, a full-size mock-up of part of the gridshell was assembled by the carpenters and tested to destruction.

The design team’s next task was to convince the building control inspectors from Chichester council of the suitability of thin strips of timber as a structural element. The gridshell is classified as a roof space, so the integrity of the structure in fire is not an issue. However, the surface-spread of flame was. “The advantage of oak is that it has good resistance to the spread of flame,” says Johnson. “The 1 m structural grid also proved advantageous in preventing the spread of fire by spacing the timber elements far enough apart to limit the spread of flame.”

Getting it together

Assembly of the gridshell will start in November. The shell will be constructed on a scaffold table 12 m above the ground as a 30 × 52 m flat rectangle of criss-crossed green oak laths. Johnson estimates that it will take the carpenters about two weeks to assemble the checkered timber mat – not bad, given that the total length of all the timber strips placed end to end is about three-and-a-half miles.

The difficulty for the carpenters will be to transform the flat timber gridmat into the curved gridshell. “Once assembled, a gridshell is one of the lightest and most efficient forms yet devised, because it represents the building’s entire structure,” explains Johnson, “but until the gridmat’s transformation into a solid curved structure, it has no inherent strength.” Rather, in the 2D gridmat phase, the structure behaves like stiff rubber. The trick is to bend it into shape before making the shell rigid with additional stiffening timbers.

Previous gridshells have been erected by either plucking up the gridmat using a crane, or by poking the gridmat from below using poles or towers. The problem with these methods is that they lead to the formation of huge stress concentrations in the structure during the lifting process. When a timber gridshell was erected for a pavilion in Mannheim in 1973, more than 60% of the timber laths were damaged.

For the Conservation Centre, the engineers will be pioneering a new method of erection. This reverses the usual process by lowering the shell, rather than raising it. This will be done by slowly dismantling the scaffold table supporting the structure. By deciding which of the table’s scaffold members to strike, and working from the edge of the table towards the centre, the carpenters will be able to carefully control the transformation of gridmat into gridshell. Removal of the scaffolding will continue from both sides of the table until only a central spine of scaffolding remains, with the gridmat draped over it.

“The carpenters will then become sculptors,” says Johnson, as they gently push and pull the mat to mould it into its final shape, in the process transforming its square cells into the gridshell’s diamond structure. “This is the most nerve-racking part of the whole process,” he adds. “We have to ensure that the gridmat does not lock solid in the wrong position.” To reduce the likelihood of this, the carpenters have designed a clamp to hold the timber loosely in place at intersections. In all, more than 6000 clamps will be used.

The roof will be located by a series of datum marks on the workshop floor and on the gridshell itself, and will be held in position by fixings on the workshop floor’s edge. Simultaneously, parabolic glulam arches and heavy timber frames will be erected at the building’s east and west ends to form the entrance to the workshop and add lateral support to the roof.

At this stage in its construction, the roof will have some of the strength characteristics of a true gridshell, but not all. “The carpenters will then become acrobats,” says Johnson. “By clambering over the shell and using it as a scaffold, they will install a third layer of oak laths running horizontally along the length of the shell to triangulate the structure and give it its stiffness. Only when these ribs are in place and bolted down will the shell be set.”

The triangulation laths will also support the gridshell’s cladding of loose hanging wooden shingles. This will consist of three tiers of western red cedar topped by a strip of polycarbonate clerestory windows. Along the apex of the roof, tracing its undulating profile, is a ribbon of flat roof. This will be used as a safe surface for maintenance personnel to carry out repairs to the roof as well as providing a platform from which to clean the row of windows lining its flanks.

When the centre opens to the public in August, the museum will have another exhibit to add to its collection. But rather than a testament to the architecture and fabrication techniques of times long gone, this building will be a tribute to the pioneering spirit of early 21st-century construction.