Weald and Downland Gridshell

The Downland Gridshell Building (as it has come to be known) was the first major timber gridshell to be built in Britain and was commissioned by The Weald and Downland Open Air Museum in West Sussex. Completed in 2002 it was and still is quite unique.

Pioneering projects require pioneering clients. Christopher Zeuner who was the then director of the Weald and Downland Open Air Museum was such a client. Seen here with the model that was an integral part of the Museum’s successful Heritage Lottery grant.

A model like this takes quite literally hundreds of painstaking hours to complete and is a thing of great beauty.

The team comprised Ted Cullinan - Architects, Buro Happold - Structural Engineers, E A Chiverton - Main Contractors and The Green Oak Carpentry Company as Timber Structure Specialists. The Green Oak Carpentry Company were commissioned early on to assist in preparing the detailed design of the building and as carpenters were able to make an important contribution.

These wonderful drawings were the work of Steve Johnson, the project architect for Edward Cullinan Architects (now Cullinan Studios) and who is without doubt among the best and most committed architects working in timber today.

Manheim Garden Exhibition Hall completed in 1975 is the direct forerunner of the Downland Gridshell, and is the work of pioneering architect and engineer Frei Otto, known for his work with tensile structures and gridshells. Built in 1975 it took 25 years before anyone dared to attempt such a building again. Michael Dickson, previously part of Arup was involved in Manheim. He and Ted Cullinan became the visionaries responsible to the Downland Gridshell.

The 3D model generated by Buro Happold actually demonstrates that the flat form of this complex structure is a simple rectangle.

The site is steeply graded and necessitated cutting away a large quantity of chalk to build the 50 x 12 metre plan of the archive store which is part ‘earth bermed’, greatly assisting in creating a stable environment for the museum archive. Chalk was cut from the site and sold to a local farmer to create a track, ably demonstrating Chris Zeuner’s entrepreneurial skills.

Once the concrete box was complete The Green Oak Carpentry Company installed the Whitewood glulam columns and beams to form the ‘lid’ of the archive store, and the 90mm thick T&G whitewood planks to form the working floor of the workshops of the gridshell above.

The floor is down and the boundary arches assembled. These were fabricated off-site with curved laminated Siberian Larch and oak beams to form the end walls of the shell. The ribbon of ply which forms the landing point for the shell structure can be seen part installed here.

Peri ‘Rosetti’ scaffold, a versatile and light weight access scaffold system, was used here as it could be easily be installed and adapted by the carpenters with the only tool required being a hammer. Yellow ‘Peri beams’ are installed ready to take the laths of the shell in flat form.

Fresh sawn sap-free oak laths from French sawmills were used for the building and are seen here being checked and graded.

Two extending joints were used for the laths. First a factory made finger joint to form 6 metre lengths (look carefully and you can see one in the top lath) and feather scarf joints with a slope of 1:8, here ready to be glued up. It will not surprise you to know the length of the glue face of both is very similar and is c.285mm.

‘Walk the Line’. The laths were 35x50mm PAR Oak bonded fresh sawn using a PU adhesive into continuous pieces up to 50 metres in length, and as such were very fragile. 6 carpenters were required to carry a lath and had to follow a line marked on the ground to ensure the laths are not overstressed.

You can see here the first layer of the 4 layer grid laid up flat in situ. The scaffold deck is half the size of a football pitch.

Over 200 ‘Peri props’ allow adjustment of the grid above with walkways built into the scaffold to allow access to each throughout the lowering process which took a little over two weeks.

Director of The Green Oak Carpentry Company Andrew Holloway established the concept of an adjustable ‘node clamp’ to allow the laths to slide over each other as the double curvature of the shell is induced through manipulation of the scaffold. This avoided slotting the timber which would have formed a weak point causing breakages. This innovation became the subject of a UK patent.
18. Lowering commences.

Lowering commences.

Going...

Going...

Still going...

...Gone!

Laths are fixed to the ribbon of ply and later blocks will be glued in between the lath ends before the second layer of ply is fixed to clamp the shell edge in situ.

Doubled up secondary laths are inserted here to strengthen the shell at the critical weak points of the valleys. This is an example of engineering logic that is both practical and beautiful.

A moment to pause and take a bow – shell structure complete! There were many sleepless nights and moments of disquiet along the way. Christopher Zeuner died of cancer during the making of this building and alas was not able to see the results of his exemplary vision and courage.

The cladding is being installed here over a multi-foil insulation.

Locally grown Western Red Cedar is used here with gutters and capillary grooves worked into the board laps to ensure weather-tightness of the building.

More locally grown timber, this time Larch, here being used for the ‘ribbon roof’ of the structure. The clerestory structure ripples as it follows the curvature of the shell. Polycarbonate glazing is used for its flexibility as in effect each panel is twisted slightly.

This longitudinal Larch laths were laid over the cross bearers to form fixing points for the ply sheathing.

12mm thick sheets of Spruce ply are laid in a double layer to form the roof deck. Again a multi-foil insulation is used, in part due to a late decision on the part of the client to extend the use of the gridshell from carpentry workshops to educational and conference use.

Here we are using a template to determine the correct profile of the laminated oak ‘boundary arch’, thus ensuring the right curvature of the cladding and roof over-sail.

Laminating the boundary arches from 12mm thick lams of dry Oak feather scarfed into continuous lengths. Peri-props come in use here as rams ensuring adequate pressure for the glue joints, not something I imagine Peri envisaged them being used for!

The nearly completed arch sections in the archive store which made a handy dry workshop space. Each arch was formed of two sections joined with a steel flitch plate.

Roof over-sail beams being craned into position ready to take the ply sheathing. ‘Roofcrete’ a cementitious waterproofing render was used over the ply to form the weathering layer of the ribbon roof.

Boundary arch and ribbon roof assemblies complete.

In this picture you can see the how well the transition between the polycarbonate clerestory glazing and WRC cladding has been achieved. It is perhaps a controversial building liked by some and loather by others, what is undisputed is that it is a pioneering building, owing much to the unique collaboration between client, architect, engineer, and carpenter.

Employing the carpentry specialist early in the detailed design and cost planning phase helped to minimize risk to the project for several reasons: 1) it gave time to research a suitable finger jointing press for jointing the laths, 2) time to research adhesives, 3) enabled us to research the many complex timber procurement issues, 4) enabled the design of the patented nodal clamping device, and 5) helped to provide carpentry expertise for the myriad complex design decisions required for the production of detailed design drawings in order to let the main contract.

The building achieved recognition as one of the best timber structures built in Britain in the past 10 years or more; it won a RIBA regional award and was placed on the Stirling Prize shortlist, and was narrowly beaten by the Gateshead Millennium Bridge. It won numerous other awards including the highly prestigious Gold Wood Award as well as the BCIA Small Project of the Year.

The primary structure can clearly be seen in this night shot. The team achieved several firsts on this building: first timber gridshell in Britain, longest timber sections at 50+ metres, first structure to use moisture curing PU adhesive in conjunction with green oak, lightest structure with only six tons of oak used for the entire 50x15x12 metre high shell structure (not including the steel used for the nodal clamps), and lastly a team patent for the design of the nodal clamp.