Leipzig's luminous vault.

Leipzig is trying to regain some of its pre-war importance as a great centre of commerce and trade, and one of the chief devices in its campaign is to be a huge new exhibition and conference centre which opens this spring. The international competition for the Trade Fair was won by von Gerkan, Marg & Partner in 1992 and they asked lan Ritchie Architects to collaborate with them on the design of the central glass hall which unites all the exhibition and conference spaces.

The aim was to provide a great central circulation space, through which all visitors will pass. it has to be extremely clear in its structure and organisation to allow routes to be obvious, and it must accommodate a wide variety of f unctions from reception to relaxation, formal to informal events. The architects conceived it as a landscaped valley between the two ranks of exhibition halls and so it had to have a glazed roof as transparent as possible with obvious structure minimised. The span is 79 m; the length 243.75 m and the height 28 m to the crown (it has to enclose full-grown trees). The intention has been amply fulfilled: a smooth and airy vault seems to float over the valley, an effect reinforced by the choice of low-iron glass, which does not have the green cast of ordinary glazing.

The basic approach is simple: arched trusses of triangular cross section span the whole width of the huge space at 25 m centres. To them is attached a 3.125 m square grid of tubes which forms a curved shell. (The external diameter of the tubes is constant, but their walls vary according to local stresses.) The shell and the thin blade-like trussed arches are further related by outriggers which stabilise the shell and add increased longitudinal stiffening. The glass skin is suspended from the grid by cast steel arms, and almost all the glass panels are of standard size (3.1 x 1.55 m). in this, the vault is very different from Nicholas Grimshaw's Waterloo Station train shed (AR September 1993) which, because of site constraints, had to curve in two directions and taper as well, so every pane was different. Another difference between the two is that the Leipzig building must be more weather-tight than the London one. So almost all joints between glass sheets are in silicone and the panes are laminated with a thin pvb filling between two layers of toughened glass.

The shell and trusses are all bolted together on site. As are the end walls which, partly to allow for thermal expansion along the length of the hall, are completely independent structures, designed as a series of concentric arched trusses bearing directly onto the ground. Flexible gaskets connect the glass vault and the vertical glazed arcs at each end.

The main problems encountered in designing such a spare structure were movements due to heat and imposed loads (wind, snow and so on). Movements in the plane of the shell are expected to be up to 15 mm in each square of the grid (ie the 3.125 m squares can deform to rhomboids by this amount). Hence extremely sophisticated glass fixings were evolved that will allow the glass panels to remain roughly in the same relationship to each other while the supporting grid (and the bearing arms which are welded to it) writhe about overhead. Most of the glass fixings are articulated to give degrees of freedom where they attach to the support arms (though one attachment point in each pane is fixed). To eliminate transfer of local bending stresses to the panes, the fixings contain a spherical bearing (in effect a three-dimensional pin-joint) in the plane of the glass. Deflections in other directions to the plane of the shell are fundamentally catered for by the flexibility of the silicone joints. Similarly, the silicone's elasticity takes up the main thrust of thermal movement (which can be up to 250 mm along the whole length of the vault).

Tolerances in the steel structure are much greater than in the glass skin. To reconcile the two, fingers at the ends of the cast steel arms can be adjusted with a slot and radial movement in the plane of the shell and by screw-adjusting the length of the fingers in the plane at right angles to the glass.

The aim is to generate a modified external climate which never falls below about 8 degrees Celsius. Temperature in winter is raised by underfloor heating coils and the stone paving and trees are partly intended to remind people of the semi-external nature of the climate in the vault. in summer, the coils circulate cold waterforcooling, but the main reduction in temperature is expected to be by ventilation. The crown of the vault opens, as do glass panels at low level, allowing high ventilation rates. Protection against solar radiation is given by fritting the glazing above normal lines of view on the south side, and by local shading. In summer, treated rainwater can be sprayed over the glass to add to cooling by evaporation. This helps keep the transparent skin clean, but the main cleaning will be by robot, due to be installed this autumn.

This must be an interim report, for at the time of writing the whole complex is not yet in use, and only time will tell whether the many sophisticated design moves will all work, and particularly whether they will all work in unison. But, so far, all is well: the vault took a mere 11 months to erect and creates one of the largest and most elegant glazed spaces ever made.