Spherical interludes.

Chemnitz, an industrial city between Leipzig and Dresden, is applying to host the European Athletics Championships in 2002. Four architects were invited by the city to design a 50 000 seat stadium. The jury's verdict was unanimously in favour of the most daring design since Gunter Behnisch's 1972 Munich Olympic Stadium. Submitted by Peter Kulka in collaboration with Ulrich Konigs, the proposal is a direct riposte to the typology of the Roman amphitheatre. Kulka's design transmutes the legacy of heavy, hierarchical containers for sporting combat, into an open orbit of overlapping spheres, rejecting the current trend of fortifying sports arenas for fear of Potential hooliganism. Harmony instead of confrontation underpins the design.

The stadium is a highly sculptural interplay between four main elements: the submerged track and grandstand below ground level, the ring of the grandstand, a forest of columns and, finally, the tensile roof. As soon as spectators enter the grounds through the main northern gate they are protected by the great undulating roof which spans most of the site. The enormous height of the roof, with the irregular pattern of columns and the landscaped grounds, make it difficult to determine the exact borderline between stadium and nature. The fluid shape of the outer roofline and grandstand keep the perimeters deliberately vague, soft and open to interpretation. Lines do not demarcate areas; at best they suggest an activity or simply a behaviour pattern. The result is a random and free play of volumes and materials, mirroring the surge of spectators around the grandstand. In effect the spectacle begins long before you reach your seat.

This highly complex swirl of forms, shapes and f unctions breaks the traditional mould of rigorously concentric stadia. The architectural design attempts to reflect the spontaneous eruption of athletic vitality, and somehow, the engineering had to develop a similar sensibility. Cecil Balmond and Robert Lang from Ove Arup & Partners created a scheme which complemented the seemingly architectural irregularity. The main focus of their attention became the tensile roof, which, according to the architect, should cover the grandstand and entrance like a translucent cloud. This naturalistic impression now had to be engineered.

The dominating feature of the roof is an irregular grid consisting of overlapping rings. To reinforce the impression of an undulating cloud, the roof structure is warped, providing additional stiffness. Each segment of the structural net can be covered or left void. These indefinite patchwork arrangements of the roof segments correspond to the irregular pattern of the forest of columns. The underlying geometry for the apparently random positioning of the roof columns is generated by two grids, one rotated above the other, so that the intersections provide nodes for the column. Each column varies in height, width and, if necessary, cantilevers provide additional support. Variations on this random, informal logic are endless, feeding on its own spontaneity with astonishing results.