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Seeing wood for the trees

Cover story: Urban renewal

Sheffield's city centre no longer relies on the steel industry to build its backbone. Instead, it is willing to try timber, as Nina Lovelace reports.

Potential visitors to Sheffield may in the past have made assumptions about the appearance of this northern city. As a region steeped in steelmaking and coal mining history, the city automatically lends itself to images of blackened stone-clad buildings and rows of terraced housing.

But visitors may be surprised to stumble across the architectural changes continuing in the city centre now.

Soaring phoenix-like above the city streets, a new 75m long 20m high glass and timber structure that will eventually house exotic palms all year round is close to completion. Light and airy, the building is the antithesis of stone cladding - and onlookers can only be impressed.

When complete, the new Winter Garden will be a key part of Sheffield's 'Heart of the City' scheme first mooted by the city council in 1995, explains council project manager Idris Hepworth.

The scheme came about because the city centre had no easy access from the station, few public spaces and was dominated by the 1970's town hall extension - unhappily resembling an egg box. As a result the town centre was failing to attract shoppers and tourists and was in decline.

'It needed a kick start, ' explains Hepworth, so work began on designing a new look to bring Sheffield's city centre into the 21st century.

A £43M masterplan was drawn up that included plans for a new art gallery providing an attractive access from the station, an open-air public space, a new hotel and enclosed 'green' space. Funding was secured from the Council, Millennium Commission and European Regional Development Fund and in 1997 construction began on the first phase - the open air Peace Gardens.

Buro Happold is designer and Interserve contractor for the scheme.

Originally the Winter Gardens was the smallest part of the overall scheme, explains Buro Happold structural engineer Richard Hennessy, 'There was an enclosed green space, but on a much smaller scale, ' he says.

Instead the Peace Gardens and Millennium Galleries, now both completed, dominated the focus of the scheme together with the demolition of the town hall extension. Construction of the Winter Gardens structure began in early 2002, over the old town hall extension footprint.

Plans for the new hotel are yet to be agreed.

As ideas changed and the design progressed however, it became apparent that the 'green space' could become something far more impressive if it could house tall palm trees and exotic plants. This way it could form a striking link between the Galleries, Peace Gardens and hotel, once it was built.

However, to keep palms and provide a transparent, light space, a large internal area free of columns and tall enough to house 20m high trees was needed.

Buro Happold and architect PRS quickly realised this could be achieved using a series of parabolic arches, explains Hennessy. 'An arch is the purest structural form you can get, as it is always in compression with very little bending, ' he says. This would remove the need for columns and provide extra height. The choice of glulam arches was a logical progression, he adds.

'Structurally, timber works well in terms of forming the shape, and for function it is ideal, ' says Hennessy. To construct the arches, the arch shape is pegged out on the ground, and strips of timber are trained into the mould and glued. 'Also it needs no covering up - it's selffinishing.'

Hennessy continues, 'With concrete, the self weight would have been enormous. How do you erect it, and how do you transport it? Plus you would have needed to make moulds good enough to make six of each type of arch.'

The effort needed to bend steel beams would also have been a considerable task, he adds.

Once the team had decided to use glulam, they needed to find a supplier - a task that proved difficult within the UK market.

'British firms approached it as a major craftsman's job, ' says Hennessy - mainly because British firms are small and are not geared up to producing 19 arches 22m wide at their base and varying 12m to 20m in height. Many said they could do it, but would have to use most of their factor capacity for the task.

'You can get a bit twitchy if 100% of a company's capacity is taken up. What if we need to hurry them up?' says Hennessy.

As a result German firm Merk won the contract instead, because it could produce the arches using only 20% of factory capacity.

The arches come in four different sizes, with the smallest positioned at either end of the structure rising up in three steps to the highest in the centre. Each arch is spaced at 3.75m centres.

Timber purlins link the arches but the main horizontal support is provided by a wide plate girder gutter linking the arches on either side of the structure 6m above ground level. The gutter additionally provides drainage and carries services.

Also at gutter height, every intermediate arch splits into two diagonal struts. These attach into the surrounding arches at their pin supports. 'By having intermediate arches it frees up a lot of space at ground level, so you can actually put doors in, ' says Hennessy.

The pin supports are in turn propped by columns at basement level, he adds. The basement has been built to allow deliveries into the hotel, galleries and surrounding shops.

'Beneath these there are simple pad foundations straight onto rock.'

The arches are designed to deflect up to 100mm in any direction at their crown due to wind or snow loading, so the glass panels covering the skeleton were designed to move as well.

As a result the structure has a movable 'skin', as the panels slightly overlap each other.

'It's like the plating on an armadillo, ' Hennessy says, adding: 'The gables are glass so there's a bit of steel, but there is no secondary steel to hold the panes up. Everything serves a function and nothing is redundant.' The glass is cleaned using ladders that can be attached to railings built along the outside of the structure.

To provide a functioning greenhouse however, the structure also needed to be fitted with a variety of services to provide power and water - without detracting from the uncluttered effect of the space. As a result, power cables to fans, circulating air within the building, are attached along the outer surface of the arch.

Other services such as water pipes for watering the plants run along the gutter or are attached to the basement roof.

Underfloor heating has also been installed into the central floor slab to ensure the greenhouse remains frost-free. 'The heating comes on automatically if there is more than 8infinityC difference between the internal and external temperatures, ' says Hennessy.

Engineers have also managed to ensure that the structure has adequate fire resistance without actually installing any fire equipment in the building whatsoever.

'We did the calculations and discovered there wasn't much fire load actually inside, as the plants are kept wet and the timber itself would only char on the surface, ' says Hennessy.

As a result the structure relies on an automatic drencher on the inside of the entrances from the Millennium Galleries and hotel to prevent a fire from spreading to the building.

Construction of the Winter Gardens is now nearing completion. The Gardens are due to open in December.

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