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The 拢7m Alnwick Garden visitor centre in Northumberland has been designed to appear almost entirely transparent, so visitors can enjoy the views and attractions of the garden all year round. The most spectacular feature of the building, which opened its doors to visitors earlier this year and has been shortlisted for the Wood awards 2006, is its barrel-vaulted diagrid roof, which displays both a refined appearance and an ingenious structure.

Angus Palmer, project leader at structural Buro Happold, says that the building was conceived as a contemporary greenhouse. He says: 鈥淭he client wanted the visitor centre to look like a modern interpretation of the conservatory. The garden is very close to Alnwick Castle, so the idea was to recreate the site where the orangery once stood.鈥�

Hopkins Architects gave the centre an organic shape to provide high-quality flexible spaces for what is said to be the North-east鈥檚 largest visitor attraction. To make the building as sustainable as possible wood was the material of choice, but a series of structural challenges forced the team to come up with an alternative solution.

Palmer says the architect was anxious not to have big, chunky roofing members that would unsettle the aesthetic balance of the pavilion. At the same time local planners wanted to keep the roof鈥檚 profile very low and for it to fit in with its surroundings.

The challenge for Buro Happold was to provide a structural solution that would support the ETFE foil roof and application loads while maintaining a slender diagrid roof framework. This was achieved by tying the column capitals that support the diagrid roof to each other, creating a cable truss which provided intermediate support to the roof. This effectively reduced the roof span and consequently the structural depth, and the aesthetic form desired by the architect could be realised.

Set in the 12-acre grounds adjoining Alnwick Castle in Northumberland the visitor centre overlooks the garden and offers horticultural enthusiasts a shelter from which to admire the many spectacular features

Visitors unaware of the gritty hub of the pavilion underground can admire the glittering, visible part of the building, making it an attraction in its own right

To accommodate the complicated geometry of the roof curve, a twist was needed either in each segment of the roofing members or in the nodes that join them together. The latter option was preferred (see diagrams). The twisted nodes are made out of steel casting for consistency, while timber sections are identical across the building. The team chose Wiltshire larch wood, as it offers minimal distortion and cracking. Other advantages include its comparitively light weight, that it does not need to be laminated and that it is free from knots.

Procurement was a challenge for the team, says Palmer, as they had to fulfil the architectural vision while keeping within reasonable costs. The project team compared the cost of using a steel contractor and a timber frame contractor, and found out that the former offered not only better prices but could also use his expertise to subcontract the timber package.

Another challenge arose from the need to comply with Part L. 鈥淏ecause of the tight environmental control of the building, we had to introduce different inflated transparent and translucent ETFE pillow types into the structure to comply with the regulations,鈥� says Palmer. The pillows, which were specified with eight different layers of glass fibre insulation thickness, help reduce the effects of solar heating and glare.

Finally, to create the most uncluttered and aesthetically pleasing building possible, all building services are located in the basement with the kitchens and back-up spaces.

Rather than the state-of-the-art greenhouse envisaged, Palmer compares the centre to an iceberg. The visitors, unaware of the gritty hub of the underground pavilion, can admire the glittering visible part of the building, making it an attraction in its own right.