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Like a drunken giant's meccano set, a mass of structural steel rises into the sky. The steel weaves in and out of itself, resembling a forest of branches in a pattern that appears to be completely random. Its haphazard appearance goes against the idea that one should be able to tell how a structure performs just by looking at it. This steelwork forms the structure for the walls and ceiling of a large glass atrium currently under construction as part of the 3.5 ha Federation Square development in Melbourne, Australia, which will include a cinema, a broadcasting centre, commercial buildings and a Museum of Modern Art.

The 16 m high, 16 m wide atrium will form the forecourt for the museum. It also functions as a public thoroughfare linking the city's main plaza to the river. The southern end of the atrium has been acoustically "tuned" to provide a public space for music and theatre.

Despite its apparently random nature, the design for the structure is based on a mathematical surface model called the Penfold Pinwheel Grid. This is a 2D model that describes the surface pattern of interlocking triangles commonly found in, for example, patchwork quilts. For the Federation Square project, structural engineer Atelier One has taken this 2D model and moved it into 3D by kinking the triangles so one of their corners, or nodes, is in a different plane from the other two.

Hundreds of these kinked triangles have been joined together to form a space frame to support the atrium. The space frame has an inner and outer leaf joined by struts. Two nodes in each triangle form one leaf of the structure, while the kinked part of the triangle forms the strut to bridge the cavity so that the displaced node forms the other leaf. Neil Thomas of Atelier One describes the structure: "It's rather like a flat sheet of paper that has been crumpled up and teased out again to form a surface that is now 3D because it is kinked from being creased up. So the geometric folds in the surface give the paper greater stiffness." The rationale behind this apparently irrational idea is architect LAB Architecture Studio's wish to capture the spirit of Melbourne. Despite an apparently logical, American-style street grid, there is a chaotic side to the streetscape. Networks of arcades and lanes weave in and out, and the buildings themselves do not form straight lines, but are set back from the street in a stepped and random manner. The atrium's structure reflects this logical chaos.

In line with the unusual idea, the project team have not followed a traditional hierarchy, but rather have worked together, with roles blurring into each other. Design agency Tomato was in at the start of the project and ostensibly responsible for the signage. Yet it also had input on the structural side, while the structural engineer was also involved with the graphics. Out of this collaboration came the concept of the pinwheel triangle acting as a fundamental basis for the whole scheme.

From concept to reality
Jean Francois Colin, a computer expert and mathematician at Atelier One, was given the job of turning the concept into a workable design for the atrium. He developed a software program that told the contractors where to put their pinwheel triangles. If a change had to be made, the program simply remodelled the structure to fit. Anil Hara, an earthquake and high-rise building dynamics specialist working for Atelier One, carried out the structural analysis, turning the concept into a reality.

Once the basic concept had been sorted out, 3D models were built to demonstrate how the finished building would look, and Atelier One put together some full-size samples of the pinwheel triangles to test materials and methods of production. In the end, the engineer settled on a structure formed from hollow square sections with fork-plate joints.

The designers, still not satisfied with such a bizarre and unique structure, decided to clad the atrium in glass panels that change colour every second of the day as the observer moves around. The structural space frame is clad in glass on both sides. The external layer acts as a rain shield and is made up of open jointed overlapping trapezoidal glass panels with shading devices. E E The inner layer is single-glazed and acts as the weather skin. This cladding is directly fixed to the main structure by a series of bars and brackets that support the pieces of glass at the edges. The cavity occupied by the structure acts as a naturally ventilated chimney to control the temperature of the building.

The next step was to present this solution to the firms that would put them up. The reaction was mixed. Multiplex, the managing contractor, was bemused by the complexity of the structure; the steelwork contractor, Ryband Steel, was satisfied that it could do it, although it made grumbling noises when it realised that it would have to produce about 1200 fabrication drawings; and Permasteelisa, the glazing contractor, was positively enthusiastic about the challenge presented to it.

The main difficulty, however, was with the checking engineers.

As they would be partially liable if the structure failed, they were cautious about the unconventional nature of the design. Atelier One had to spend inordinate amounts of time explaining how the structure worked before they would give it the go-ahead. In the end, it was only the reputation of Hara that swung them behind the scheme.

Currently, the steelwork is complete at the southern end of the atrium and is half-finished at the northern end. In three months, the atrium will be complete and in 10 it will open to the public – and Melbourne will have a radical new structure that proves there was method in the project team's madness.