Wave motion.

Through its ingenious and poetic application of technology, Thomas Herzog's new exhibition hall in Hanover - site for Expo 2000 - provides a paradigm of how such very large buildings might develop humanely in the twenty-first century.

A building can be so big that it disappears in the mind of the user. Trade fair halls are extreme cases. Considered as temporary structures which only come alive during exhibitions, they have often barely attained the status of sheds. The idea that they should have their own aesthetic or engender a feeling of place has only been realised during world fairs. The Deutsche Masse, manager of Hanover's permanent trade fair grounds, started to rethink its image in 1990, with a development plan for investment up to 2005. Shortly afterwards, Hanover was chosen as the site for Expo 2000, the theme of which will be 'Man, Nature and Technology'. This will be the first time a world fair has been sited on an existing trade far site. Hanover started with an export fair in 1947 in the former production halls of an armaments factory. Fifty years later, it is the biggest permanent fair-ground in the world with 26 halls, 50 000 parking spaces and an area of 2 million square metres.

An international competition for the Expo development was won by Italian architects Arnaboldi and Cavadini, who put forward the notion of a walled city containing exhibition halls, national pavilions and parks. The concept was enlarged by Albert Speer in his Expo 2000 masterplan, which combines striking architecture with a responsive infrastructure. Speer's tactics include the creation of a sense of urban life with varied experiences between the halls, improved circulation patterns with glass arcades and canopied walkways linking the buildings. Internally, there will be an emphasis on the use of natural light and ventilation in keeping with the environmental theme, and greater responsibility in the choice of renewable raw materials to help conserve resources.

Thomas Herzog, architect of Wilkhahn's factory (AR January 1994), and the Linz Design Centre, a vast free span exhibition hall in Austria (AR May 1994), was well suited to realise this brief. Winner of Germany's 1996 ecology prize, Herzog has been in the forefront of research on solar power and energy-saving construction in partnership with EU environmental organisations and industry. His Hall 26, constructed in 39 weeks and completed last year, sets the standard for Hanover's future development.

Hall 26 is easily identified from afar with its roof profile cresting in three waves of suspended steel, glass and timber. It is essentially a tent structure. The 29m high ridges, created by three rows of steel trestle masts with pin joints, are a more elegant form of the early industrial serrated factory roof profile of north-facing skylights. Here as well, Herzog has incorporated ribbons of north-facing roof glazing for optimum daylight. The determining factors in the form of Hall 26 were the tensile roof structure, together with a requirement for energy saving lighting and climate control systems.

Overall dimensions of the hall are 220m x 115m, with six independent box pavilions (housing sanitary, catering and building services) protruding from the rectangular plan. Inside the hall these pavilions are concrete faced, but externally they are clad in wood, similar to the timber facades Herzog has used before on residential and industrial buildings. Paradoxically, for all its apparent emphasis on steel and glass, the building uses a large amount of wood. Prefabricated timber sandwich panels filled with gravel cover the roof, spanning between steel girder sections. Their undersides form the ceiling soffit internally.

Despite its size, the building is awash with light. The rhythm of the roof silhouette, the easily understood structural principal, and the transparent curtain walls create lively views at all times of the day and night. There is a physical sense of the weight of the hanging roof, but also the strength of the slender steel sections of the masts. It is rather like watching a circus juggler effortlessly balancing a twirling ball on one finger. The structure seems weightless, an impression reinforced by the optimum use of natural light internally and artificial lighting at night, which makes the building look like an ocean liner floating in the dark. Direct daylight cuts through the glazed strips at the apex of each roof ridge, but in the centre of each suspended roof section another band of glass panels iS triple glazed with retro-reflecting grids to deflect direct sun. On the ceiling soffit, six rows of polished steel panels distribute daylight within the volume. Artificial light comes from fittings mounted under the trestle masts and from suspended lighting strips under the belly of the hanging roof soffit. The convex surfaces of the ceiling diffuse both artificial and natural light.

The cost of mechanical ventilation has been halved, following computer simulation and wind tunnel trials, by using natural thermal movements. Floor outlets are impractical because of the load requirements in an exhibition space where forklift trucks or lorries deliver exhibits and heavy machinery is displayed. Fresh air has therefore been introduced at a relatively low level, 4.7m above the floor, from glass ducts in the circulation zones. At first the cool air drops to the floor, but as it is warmed by people and equipment in the hall, it rises and is expelled at the three roof ridges. For this. chimney effect, a continuous strip of adjustable flaps with roof caps on the three ridges can be individually orientated depending on wind direction to ensure no back flow into the hall. In winter pre-heated air is injected via adjustable long range nozzles from the same glass ducts.

Internally, the function of the covered space is marked out and divided by the walkways under the trestle masts, from which the glass air ducts hang. In between, the area can be colonised by all types of exhibitions, ranging from streets of individual stands to free form spaces, The basic purpose of a trade hall is to provide a mechanically serviced large volume, neutral enough for every use. They are an inheritance of the early industrial age, in which machines and trade had a higher value than the humans who serviced them, and a rethink of their design is long overdue. In Herzog's case it is interesting to see that both his Linz Design Centre exhibition hall and Hanover's Hall 26 have been compared with Paxton's Crystal Palace, the first trade fair building. Environmental responsibility may be leading us back to light and airy buildings.