Going for gridshell gold
Published: 12 December, 2006
The biggest timber gridshell in the UK has won prizes and popular approval. Mike Jeffree reports
When we wrote about The Crown Estates Savill Building in Windsor Great Park last year, the triple-domed, double curved, sinusoidal timber gridshell roof was just being lifted into position. The 20km of finger-jointed 80x50mm larch lathes were still being manhandled and final adjustments made as the roof was fixed to the snaking steel perimeter. But even at this stage, the buzz was that this was going to be one of the UKs most spectacular timber structures.
Officially opened in the summer, the completed building, the visitor centre for the Parks Savill Garden, has more than lived up to expectations. The public are flocking to see it and critical praise has verged on the fulsome. It also did the hat-trick at the 2006 Wood Awards, scooping the top Gold prize and structural and public access categories.
The Wood Awards judges seemed to like everything about the building; they were impressed with the structure as a whole, and the roof in particular. They liked the overall ambience internally the effect is one of light and space and were just as enthused by the detail. They couldnt find a single service box cover that wasnt aligned to the edges of the floorboards.
The building acts as a gateway and focal point for the Savill GardenMuch of the acclaim for the structure has focused on its modernity; the advanced geometry, integration of solid and engineered wood with glass and steel, up-to-the-minute environmental performance and the undulating sci-fi shape of the roof, which appears to float on the horizon. But according to architect Glenn Howells, its success is also due the fact that its old-fashioned at heart, a true craft building, with construction based on a pleasing process of collaboration. Key players in the co-operative effort were the architects, engineers Haskins Robinson Waters and Buro Happold, the Green Oak Carpentry Company and structural glazing specialist Haran, but it also involved a raft of other sub-contractors, plus the TRADA timber research centre. We bothered a lot of people! said Howells. It was all about pooling the right experience.
Materials inform designThe project also looked to the past in allowing the materials to inform building design. A mediaeval mason would have been familiar with the approach, said Howells, Its the way you get the most efficient, intelligent buildings.
A section of the multi-layer roof was first modelled, then built full-size in prototypeThe Glenn Howells team came up with the first sketches for Savill Garden in 2000 in response to a fairly open and unspecific brief. Broadly, the building had to relate to the landscape, be intelligent in terms of environmental control and have low embodied energy, said Howells. The old Savill Garden visitor centre was ageing, said Windsor Great Park deputy ranger Philip Everett. The question was do we upgrade or replace? Eventually we decided we should make the most of the opportunity create something new with a real wow factor. Other gardens have the country house at their heart. We wanted to create an equivalent focus.
The right environmental buttonsHowells was invited to pitch for the project in competition with three other practices. After opting for timber as a core material fairly early on, because it hit the right environmental buttons, it considered various structure types including lamella arches, glulam frame and a prefabricated modular approach. But the gridshell won through, with inspiration including Frei Ottos Mannheim building, and Buro Happold and Green Oak Carpentrys earlier gridshell collaboration at the Weald and Downland Museum.
The concept won The Crown Estate over on a number of grounds; because it was contemporary but not out of place, sited on a plateau overlooking the gardens and fitted in with car park improvement plans! And the gridshell roof sealed it. When we saw that, we rather leapt at it! said Everett.
The use of the larch from the Parks own Forest Stewardship Council-certified woodlands reinforces the buildings green credentials. However, it wasnt the original plan due to doubts over whether native material was strong enough. But we had this parcel of high quality larch and thought it would be wonderful to use it in our own building, said Everett. To ensure the timber was up to it, Buro Happold tested samples exhaustively. They passed, achieving C24 grade where ring width was under 4mm and C16 where it was wider.Ideally, the timber would have been processed locally, but the 400 trees earmarked, at around 70 years, were older than usual harvest age and the logs had go to a mill in Northamptonshire that could cope with the size. The sawn timber was then finger-jointed by Inwood Developments of Lewes, with offcuts, sapwood and other sections not suitable for the roof kept for other applications.
Technical challengesBoth Buro Happold and Green Oak Carpentry acknowledged that the Savill Building posed greater technical challenges than the Weald and Downland gridshell. First, at 90m long, 25m wide with an elevation of 4.5m to 8.5m, its around four times the size. The roof contours are also shallower. Weald and Downland curves to the ground, so the outward thrust is resolved in the earth, said Howells. We didnt have that option.
Timber supports would have had to be massive to take the forces exerted by the roof, so the steel ring beam was used but this posed the question of how to fasten wood to metal. The solution devised by the architects, engineers and carpenters is described by Howells as quite elaborate. The steel effectively migrates into the timber, with fingers of Kerto LVL [laminated veneer lumber] linking the two. The LVL, he added, was used because of its high compressive strength. It varies in width according to the differing roof forces and is glued, screwed and blocked to the lathe and bolted to 160 galvanised steel plates around the ring beam. Another challenge was how to resolve the inflexibility of the glass curtain walls with the natural movements of such an expanse of wooden roof. It can deflect 100mm up and 200mm down and that also exerts an outward thrust on the glass, said Howells. The solution was a bespoke mullion system devised by Haran which copes with vertical and rotational movement." Once the buildings steel perimeter was up, the roofs lower lathe and associated blocking was laid on scaffold and manipulated into shape. Two more layers of larch lattice were attached to the lower and the shell was triangulated with cross-laid layers of 12mm birch ply over the larch. This provides in-plane stiffness and acts as a diaphragm to brace the structure, with butt joints connected by steel strips, bonded above and below, to transfer tensile forces. Finally aluminium standing seam panels were fixed to support the rainscreen of 100mm slats made from Windsor Great Park oak (which is also used in the flooring). All the elements combined weigh around 30 tonnes. The Savill Building came in at just over £5m but, according to Howells, should be economical to run. We worked with Atelier 10 to predict solar penetration and the combination of shape, orientation and the height of the building, the roof overhang and inclusion of three intelligent openings in the roof will avoid excessive heat gain, he said. The building has underfloor heating, but the combination of natural heat gain and 200mm Rockwool insulation, should minimise its use.
It was obviously a great source of satisfaction for the Savill Building team that their innovative work was recognised with a trio of Wood Awards although Howells was also surprised. I knew we had a great building, but I was very impressed with the quality of other entries, notably Hopkins Alnwick visitors centre.
Everett seems equally pleased with public reaction with visitor numbers to the Savill Building on track to exceed 400,000 in the year following the opening, a rise of 50% on its predecessor.
Some traditional park users were apprehensive, said Everett. But once people walk through the doors and see the open space and grid structure, 95% are swept over. Many are specifically coming to the building to shop, eat or view exhibitions.
And the success of the structure looks set to inspire the Howells practice, for one, to do more with timber generally and the gridshell in particular. Its the right structure for assembly and performance buildings and theres scope for increasing the dimensions, said Howells. It could make a fantastic sports facility and what could be better for the London Olympics?
And the other winners were...The 2006 Wood Awards attracted a record 206 entries. The Savill Building won structural, public access categories and the top Gold Award. The following are the other category winners
Small project: suspended mezzanine in living space, London
Offsite: Roche head office, Welwyn Garden City
Is timber well placed to benefit from the opportunities presented by the London 2012 Olympics?