CENTRING SHEFFIELD.

A major component in an ambitious plan to bring life back to the centre of a previously run-down city, this complex is a physical link and a source of inspiration.

Sheffield was once the steel capital of the world, but its fortunes declined with those of British industry. Now, they have recovered to some extent. It is making as much steel as ever (though the new products are highly specialized, and made by far fewer people). The city has a new high-tech tram system and a combined district heat and power system which runs on town rubbish.

In the mid '90s, an ambitious plan for reviving the city centre was launched which made provision for improving and linking open spaces, introducing new uses (commercial as well as municipal) and downgrading traffic. A key element in the ambitious Heart of the City Project is the Winter Garden and Millennium Galleries. Pringle Richards Sharratt with engineers Buro Happold were appointed designers after competitive interview in 1966.

The two elements, though joined, are distinct. The Winter Garden runs roughly north-south, and is designed as a galleria, connecting two squares. Its progressively rising laminated timber parabolic arches will enclose a garden which will be open at all hours and available to everyone. Kiosks and cafes will provide daytime animation, but at night, the illuminated green path will be a key pedestrian link.

Halfway down the east side of the gardens, the galleries join the glasshouse. A long foyer gives access to the permanent collections: cutlery from Sheffield's past and the Ruskin Gallery, which will be devoted to the collections of the Guild of St George, an idealistic community founded in Sheffield by the Victorian sage as a riposte to the grinding awfulness of nineteenth-century industry. The largest gallery will be for temporary exhibitions most of which will be organized with the Tate Gallery and the Victoria and Albert Museum, the big national institutions in London whose bulging store-rooms contain far more than they can show. At the east end of the foyer, escalators and a lift connect to a lower level, where the fall in ground allows a large cafe and the education department to open at street level towards Hallam Square, part of Hallam University. A major urban road runs between cafe piazza and square, and though its traffic is to be moderated, experiences of summer cafe customers on the terrace may b e a little gasolenic.

The whole complex is informed by environmental concerns. Gallery vaults run parallel to the Winter Garden axis. These precast concrete elements (two to a vault), have been made with a pale mix incorporating micaceous sand to improve light-reflective properties. Reflected daylight enters through clerestories, bounces up into the shimmering vaults and is then diffused down onto the exhibits. As well as being a luminous device, the vaults provide thermal mass and insulation. Other thermal control elements are the galleries' hypocaust floors which are like raised office plates over a concrete slab.

Like the galleries, the Winter Garden will be a model of energy conservation. Its two-hinged parabolic laminated arches bear on tapered concrete columns through stainless-steel pins; they carry a secondary purlin structure which bears the double glass skin. There will be no supplementary heating, so the whole cathedral-like space is expected to be 5 or 6 degC above the local climate. In summer, trees and vegetation will shade the space, and automatic vents at the top of the vaults will open to make convection draw cooler air in at ground level. The floor slab will give stabilizing thermal mass.