A Tokyo art gallery perched atop a skyscraper needed a ground-level entrance building to lure visitors in. The architect's response – a giant glass, elliptical Japanese lantern – demanded some inspirational structural engineering.
It is hard to stumble across the new Mori Arts Centre in Tokyo because it is located 53 storeys up, on top of an office block. To help visitors find their way to it, Mori – the Japanese development giant responsible for the building and eponymous gallery – has decided to build a spectacular entrance at ground level.

The challenge for the architect was to solve the complex problem of visitors arriving from all directions and all levels. The solution is an entrance building that looks a bit like a truncated lava lamp, funnelling visitors up to the gallery as they arrive from four different levels. A spiral staircase winds around the entry structure's mushroom-shaped core, which also incorporates lifts, up through the first four levels to a fifth, which is connected by a bridge to the first floor of the tall office block. From here, visitors take dedicated lifts up to the art gallery at the top of the office building.

The architect wanted the external envelope to be as transparent as possible, with the cladding frame barely visible – a brief that posed something of a challenge to the structural engineer. "The most difficult aspect for us was to try and create a transparent structure to support the external skin of the building to the shape and form the architect wanted," says Tim McFarlane, partner at engineer Dewhurst McFarlane, designers of the structure and the cladding system.

This tall order evolved because Mori chief executive Minoru Mori wanted the new building to double up as both entrance and sculpture. Architect Gluckman Mayner decided that the entry structure should have an axis reflecting the line of the main building, in order to help guide visitors, so an elliptical form was chosen. Project architect Sam Brown describes how the finishing touch came about: "The curve came from Mr Mori himself; he wanted it to be more Japanese, so the metaphor of a garden lamp emerged." The external envelope had to be transparent so that visitors could enjoy the adjacent garden.

Another engineer had already designed a cladding system before Dewhurst McFarlane was appointed, but this failed to satisfy Gluckman Mayner. "The architect felt the transparency of the glass was compromised by the supporting structure, so he approached us because of our expertise in glass," remembers McFarlane diplomatically. The original concept for the structure used heavy, steel trusses braced with cross-members. The external glazing was flush all the way around and jointed with silicone. "It looked like a commercial building," McFarlane says, rather more scathingly.

"It's about as complicated a geometry as we have ever met on a project," says McFarlane. Initially, he considered using a cable net to replace the steel sections, as this would have been visually less intrusive. "The problem is the structure curves out; a cable naturally wants to lie in a straight line," says McFarlane. The initially apparent solutions to this problem all took up too much space inside the building, so McFarlane examined the 3D geometry in more detail to see if a more elegant solution could be found.

He finally hit on the idea of using steel rings encircling the whole structure, to act as compression members holding the steel cables in the form of a curve.

The cladding structure works in a similar way to a paper lampshade. These have wooden hoops to hold the paper out in a curve, while the paper holds the hoops in place. In the Mori Arts Centre entrance building, the role of the paper is taken by a diagonally arranged series of cables stretched between the outer rim of the mushroom-shaped core and an anchor beam at ground level. A series of 12 steel hoops push the cables out to form the elliptical shape of the structure. The diagonal arrangement of cables is to lock the hoops in place and to stop them spinning around. The 22 mm-thick hoops, which are set approximately 1.6 m apart, vary in diameter to define the shape of the curve from the ground to the top of the building. The bottom ring is 20 m × 16 m, while the top ring is 16 m × 13 m.

Glass is attached to the steel rings and acts as a rain screen. The laminated glass shingles are simple, flat sheets and are all the same shape and size for each one of the 12 layers. Even the specially designed connectors fixing glass to steel ring are a construction first, as the glass abuts the rings at a large range of different angles, and the connectors have to support two sheets of glass at once because the shingles overlap. They also have to accommodate out-of-tolerance holes in the rings and the glass.

Such a unique structure had to be thoroughly tested before construction started. According to McFarlane, one great advantage of working with the Japanese is that they are prepared to spend money on testing such ground-breaking structures, thereby making them possible. The rings were initially tested for buckling. "We applied up to three times the design load and it worked beautifully," says McFarlane.

The idea was also tested for buildability. "We then spent a considerable amount of money building a full-sized mock-up to understand how the geometry would go together, to see whether there would be any buildability problems and to see how it would look on site."

The results were all positive and work on site is in progress. The project should be finished in October, in time to stop visitors getting lost when the galleries are completed next year.