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Wood and it be nice

Good ideas can get better. A new visitor centre will demonstrate how timber gridshell technology is evolving, reports Andrew Mylius.

To make a building using your own wood gives you a good, warm feeling, ' enthuses Buro Happold senior associate Richard Harris.

On the express instruction of client the Crown Estate, a new visitor centre in Windsor Great Park, Berkshire, will be roofed with larch grown on the estate.

Crown Estate wants to minimise the environmental impact of construction by drastically reducing materials transportation costs - its home grown timber will only leave site to be sawn and worked. And in its new building the client will gain an eloquent advertisement for an important part of its work - sustainable, commercial forestry management.

'Architect Glenn Howells won a design competition for the £5M visitor centre in the park's Savill Garden last year, with a scheme inspired in major part by the acclaimed timber gridshell workshop at the Weald & Downland Museum near Chichester, West Sussex. Howells and structural engineer Haskins Robinson & Waters approached the structural brains behind Weald & Downland, Buro Happold's Harris, for technical help on the roof.

'They were also keen to work with the Green Oak Carpentry Company, which built the Weald & Downland structure, ' Harris says. 'For a project like this you need design and construction together.

'Timber gridshell construction is really a technique - a way of building a complex doubly curved shell out of a set of standardised components, ' he summarises.

The Savill Garden visitor centre roof looks rather like three peas in a pod. With a radius of 16m, its three domes have a far shallower geometry than the Weald & Downland structure, which creates geometric and structural differences, even though the basic approach to design and construction draws on tried and tested knowledge.

Where the Weald & Downland gridshell is 48m long by around 12m wide with a maximum height of 10m, Savill Garden's is an impressive 90m by 24m, with the domes reaching up 12m.

And instead of rising straight from the ground, at Savill Garden the gridshell is supported front and back on curved tubular steel beams, giving unobstructed views across the grounds.

'This one's built in the air, ' comments Buro Happold project engineer Jonathan Roynan.

Pairs of raked columns at either end of the building and to either side of its central bulge support the beam at the front of the building; shorter columns rising from a reinforced concrete wall retaining a planted embankment support the back of the roof.

The shallowness of the arch introduces significantly greater bending forces than encountered on Buro Happold's previous structure.

'This shell is working hard.

It's quite flat and non-uniform, and loads are putting the structure into bending, ' Harris says.

'Under snow and wind loading, the roof is expected to deflect by up to 200mm.

'That means we get very high compression at points where the shell meets the piers. We have to strengthen the structure locally.'

This is being done by creating transition zones, in which the steelwork is bolted to laminated veneer lumber (LVL), which in turn is bolted to the gridshell's laths.

Additional structural depth is also needed to resist bending.

Structural depth is 300mm.

Forming the roof geometry from solid timbers would be impossible, so it is being achieved by 'creating layers', says Roynon.

'You separate them out and bend them. Then you reconnect them with shear blocks.'

Accordingly, the gridshell will be composed of two layers one above the other, separated, but screwed solidly into intervening blocks of wood. The effect is to create a system of interlocking timber vierendeel girders, spanning in two directions.

While highly resistant to bending, the top and bottom chords of the trusses are subjected to large axial loads. To cope with these loads, fairly large 80mm wide by 50mm deep timber sections are being used: Larch has a flexural strength of 24kN/mm & Downland structure, which has a strength of 30kN/mm2. Weald & Downland's oak laths were a mere 35mm deep by 50mm wide.

Savill Garden's gridshell is to be set out with laths at 1m centres. To gain in-plane stiffness previous structures have been cross braced with steel wires or by timber members on the diagonal, as at the Weald & Downland. However, on this latest structure Harris set out to move gridshell technology up a peg, creating something close to a monocoque structure by using the building's plywood cladding for structural strength.

'Making the skin work structurally is an obvious thing to do - it sounds simple. But actually it is surprisingly difficult, ' he remarks. 'The difficulty is with the joints, as you're looking not just at local behaviour but globally.

You need continuity between the plywood panels.'

This is being provided by gluing 1mm thick galavanised steel 'straps' across the joints at 500mm centres, providing continuity in tension. To resist shearing in the 12mm thick, nine-ply panels, which threatens to debond them, a second skin of 3m long plywood strips will be overlaid on the diagonal.

The whole is to be weatherproofed with a 'calzip' type metal roof.

Construction of the roof, which is due to get under way in February, will be carried out using the system deployed at Weald & Downland. Falsework supplier Peri has developed extremely long telescopic props that can be adjusted to millimetric accuracy. Using these, a densely constructed scaffolding platform will be erected to the roof 's maximum height. On this, the grillage of laths will be laid out.

Then, by degrees, some of the props will be lowered, allowing the laths to sag under self weight.

By carrying out a sequence of adjustments, the roof 's final geometry will be achieved.

At that point top and bottom chords of the gridshell can be fixed rigidly together, locking it in place.

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