Fluffy is an adjective that Eden's operations manager George Elworthy accepts with grace. Though clearly a man of action - he takes much of the credit for turning Eden from a fantastic idea into reality - Elworthy comes across as a gentle, tree-hugging kind of guy.
And when he laid out his brief for a new education centre last year his engineer, Scott Wilson project manager Richard Hoyes, quickly entered a state of 'desperation and despair'.
'When I arrived all the Eden people started talking about what they wanted it to 'feel' like, ' Hoyes explains, 'while I wanted to talk about structures. I thought they were incredibly fluffy.'
For anyone in the dark, Eden is a horticultural marvel consisting of two enormous hot houses or 'biomes' placed within a disused china clay quarry near St Austell, Cornwall. The biomes house slices of steaming tropical rainforest and Mediterranean riviera, while the clay pit has been converted into a lush patchwork of wild and domesticated planting.
Many of the 1.4M tourists who head to Eden each year do so as much for the vast scale and intricate engineering of the biomes' ETFE 'bubble wrapped' geodesic space frames as for the plants.
Visitors generally can't help but gush about the place.
Elworthy retorts: 'Tim Smit [Eden's founder] charged me with coming up with a different way of teaching - to get away from classrooms and blackboards. We wanted to have a building where learning could be done through art, sculpture and performance, discussion groups, workshops and talkshops. And we wanted a building that captured and conveyed the ideas that Eden's all about - growth, plant life, ecosystems, sustainability.'
'I couldn't see how we were ever going to get a set of buildable ideas, ' says Hoyes. 'But actually, before I knew it we had detail drawings and a building schedule.'
Design and construction of the £13M education centre is being carried out by the same team that so successfully delivered the biomes. Joint venture contractor Alfred McAlpine and Sir Robert McAlpine is leading the project under a guaranteed maximum price design and build contract, employing structural engineer SKM Anthony Hunts and architect Nicholas Grimshaw & Partners.
Sculptor Peter Randle-Page has been advising on design, and is just about to start hacking at a 100t lump of local granite to form the new building's nucleus.
Remarkably, the structure now springing from the ground a stone's throw from the warm temperate biome manages to satisfy both Hoyes and Elworthy's desires.
Its structure derives from study of plant growth. Leaves on many plants grow from the main stem in a spiral. Typically, the angle between the leaf spurs is a constant 137.5infinity, close to that of the Golden Angle, a geometric ideal worked out by 12th century mathematician Leonardo Fibonacci.
This principle of 'Fibonacci phyllotaxis' has been used to set out the education centre's three leafed plan.
Fibonacci also codified other patterns of growth. Distinctive spirals can be traced in flower heads, pine cones, cauliflowers and cabbages, and snail shells.
These typically conform to the sequence 0, 1, 1, 2, 3, 5, 8, 13, 21, 34. . . in which each number is the sum of the previous two.
Structural elements of the education centre's roof are accordingly set out in Fibonacci spirals. There are two sets of spirals radiating in different directions and creating a complex overlapping pattern.
The roof was to spread from a trunk-like structure at the building's centre like the canopy of an engineered tree. That it should be constructed in timber was virtually a foregone conclusion.
'This is absolutely great, ' Elworthy enthuses. 'We've got nature's two major growth forms set out in our buildings. The biomes are cellular, like a honeycomb, and Fibonacci The education centre is part of a £33M scheme to equip Eden for its 1.4M visitor a year onslaught - it was designed for 650,000. Work includes creation of new carparks and road improvements, construction of staff buildings to replace portacabins, a new gateway and ticketing building at the lip of the pit, and a lift to get the infirm or disabled up and down the 55infinity slopes more easily. Funding is from the Millennium Commission, South West Regional Development Agency and the Government Office for the South West.
phyllotaxis for the education centre, which is vegetative.'
There was a hitch, however.
The roof is a three-dimensional curved shell structure and needs to be buttressed around its perimeter - it will be restrained by external concrete buttresses at its tips and by internal load bearing walls at its re-entrant points. But 'the first model based on the Fibonacci series was symmetrical [there were equal numbers of clockwise and anti-clockwise spirals]', recalls SKM Anthony Hunts technical director Alan Jones. 'It wasn't particularly structurally efficient.
'From its centre to the tips of the roof is 35m to 40m. Because the beams form a lattice you'd have severe compression. The beams also curved very severely and there was a good deal of secondary bending.
Jones says that even though it is omnipresent in nature the Fibonacci spiral does not translate well to manmade shell structures. 'If you look at nature this geometry occurs in solid objects, not thin shells.'
To get around the problems of bending and compression, Jones looked at bracing the lattice with structural diaphragms and at reducing its scale - at putting in more structure. But the breakthrough came when RandlePage pointed out that in nature Fibonacci spirals are always asymmetrical, with unequal numbers of clockwise and anticlockwise spirals.
Applied to the roof, this produced a structure with beams which radiate clockwise from the hub being shorter and straighter than those radiating anticlockwise.
The straighter beams hit the perimeter at a relatively direct angle and provide reasonable arching action. They are restrained against bending by the longer, more curved beams.
Deflections are expected to be further reduced by joint details designed by supplier of the glued laminate (glulam) timber roof structure, Swiss firm Haring. Steel inserts in the ends of the glulam beams enable them to be rigidly joined to one another with steel dowels, much like a conventional bolted butt joint.
One of Elworthy's prime objectives was to keep waste and energy consumption during construction to an absolute minimum. Glulam fits the bill perfectly, Hoyes claims: The timber is from renewable sources and compared to steel requires little energy input during fabrication.
The McAlpine JV, meanwhile, has set out to limit wet construction processes and the wastage they often incur. Except for the education centre's reinforced concrete raft foundation, all of the concrete structures have been precast off site, even including the building's three storey lift shaft.
With Eden's own design team the contractor has also been sourcing alternatives to conventional insulation and finishing materials - strawboard instead of gypsum plasterboard, and hemp acoustic and thermal insulation instead of mineral-based products. Elworthy has imposed a blanket ban on PVC.
'I see building as a social process, not just a technical one.
That means using materials and construction processes that cause minimum environmental or health and safety impact, ' he says.
'The building has to stand for Eden principles, and I'm immovable on certain points. I may appear fluffy on the outside but I have a hard centre.'
Unusually, beams making up the education centre roof are curved in both vertical and horizontal planes. Glulam structures normally curve in the horizontal plane only.
'We did look at faceting in the vertical plane, which is a fairly conventional glulam technique for delivering a threedimensionally curved shell, ' says SKM Anthony Hunts technical director Alan Jones.
'But it didn't look natural. It didn't fulfil the brief that the thing needed to look organic.
The bottom line is it that looked like an engineered structure.
'The building went through a rigorous value engineering process [£3M was shaved off the total construction bill]. The double curve on the beams was a major cost, but was never in jeopardy.'
It will be clad in copper and photovoltaic cells, which will provide an estimated 10% of the building's energy.