The challenge of the Millennium Building lay in the organisation of the structural elements to respond to the angles and projections of the three triangular wings, which step back at the upper levels to allow for sun penetration to the external garden and 'lido' spaces.
The concrete frame is based on an orthogonal 7.2m grid with alternate rows of columns along the north-south axis displaced by 3.6m. The resulting diagonal column layout allows substantial standardisation and repetition of floor plate construction and allows the angled perimeter to be built without variation to the basic structural format. The layout also meets the requirements for changes in slab level to accommodate the areas of external hard and soft landscaping and lido spaces at the various levels.
The floor plates are constant-thickness ribbed concrete slabs spanning spine beams, thus allowing for versatility in the positioning of slab penetration for services. The structure of the heavily serviced western edge is changed to a flat slab at the lower levels to provide a robust bracing to the contiguous piled basement wall and to link the concrete stair cores that provide the lateral resistance of the building. A diagonal expansion joint is positioned on the edge of the central wing to minimise the complexity of the waterproofing and finishes detailing.
The foundations follow the successful principles established for the Stage 1 development of the No 1 Court to the north and consist of single large-diameter bored piles at the column positions. The curved roof structure is a steel framework supported on cantilevering exposed concrete columns. Roof-overhangs at all edges are supported by cantilevering primary beams.
The architecture of the building demanded that some structural elements, such as the west facade cantilevering staircase, the link bridges into Centre Court and the central exposed concrete scissor stair, were designed, detailed and constructed with considerable care.
An unusual feature of the design was that the construction of the southern wing had to be delayed until the final year of the contract to allow the facilities under the southern stand of the original No 1 Court to remain operational for the 1999 Championships. The stand was demolished and the wing constructed, serviced and finished between July 1999 and the opening of the completed building for the Millennium Championships.