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From the street, the 40-storey building's shapely form is slowly growing a skin of glazed triangular scales. The cladding emphasises the structure as it swells outwards from its slender base to a bulging waist at the 26th floor before tapering inward again to a domed pinnacle, 180 m above the pavement. The building's unique shape seems to have assured its status as an icon for the City 鈥� even before construction has finished.

Responsibility for building this icon lies with Skanska Construction, which has faced a huge challenge. The building's fat cigar shape is only made possible by the design of its steel skeleton, which forms a rigid lattice at its perimeter. The success of the project has always hung on the contractor's ability to get this structure right, but this has not been Skanska's only challenge 鈥� it has had to do all this on a tiny site in the centre of one of the most congested capitals in the world.

On that site, the building's steel skeleton is complete, ready for next week's topping out ceremony. A grid of rust-brown tubular steel columns spiral round the building from the public plaza at its base to the circular restaurant that will occupy its 40th-floor apex. Three tower cranes stand sentinel-like around the structure, transferring bundles of steel decking from a large truck to the steel erectors who clatter them into place on the building's upper levels in readiness for their blanket of concrete. "We're in the process of building the last few floors," explains Gary Clifford, Skanska's project director.

Hard on the heels of the concrete gang, on level 36, a team of fitters is busy in one of the core's four service risers. Their task is to extend the riser pipework up another level by connecting it to a E E preassembled services module that was craned in over the previous weekend when "hook-time" on the cranes was available.

Further down, Schmidlin's cladding team are hard at work on level 28. Here, Skanska's enormous goods hoist has been used to transport materials to the floor. Using a specially developed machine, the workers lift a double-glazed triangular cladding panel from its transportation stillage, thread the panel between the structural steelwork and then ease it into position on the outside of the building.

Bringing up the rear of this procession of trades is the second of Schmidlin's teams. They have just commenced installation of the glazed screen that will form the inner skin of the building's ventilated facade. The screen is significant in that it forms the boundary between the complex geometry of the building's structure and facade and the simple flatness of the office floors.

Understanding the building's geometry is, as you might have guessed, the key to its construction. "We worked hard on the geometry," says Michael Gentz, an associate at architect Foster and Partners. The building's form is defined by its floorplates, which vary in diameter throughout the height of the building. Each is divided into six rectangular "fingers" to create an orthogonal space-planning grid on the circular floors. The fingers are arranged radially around the building's central core, with a triangular area left between them to form a series of atriums. As Gentz explains, the idea is "to bring the perimeter into the offices" (see the plan view, below).

But rather than arrange the atriums vertically as empty voids, the architect has twisted each floor by 5掳 relative to the floor below so that they spiral round the building and grant each floor a series of balconies looking onto the atriums.

Structural engineer Arup designed the building's steelwork to follow the helical path of the atriums. The design uses two series of inclined tubular columns that spiral round the perimeter in opposite directions, intersecting to form a diagonal grid, or diagrid for short. By linking the intersection points of the spirals with a horizontal steel hoop at each storey, the designers have turned the diagrid into a stiff triangulated shell 鈥� a kind of steel sheath around the building (similar to the World Trade Centre towers) to provide stability for the tower and to resist wind forces.

The clever bit about the structure is that even though the building facade curves in two directions, all the structural columns are straight. The curve is achieved by bending them at the nodes where the columns intersect every second floor (you see how this works in the photo).

Although it sounds complicated, the beauty of Arup's design is that the geometry of the structure makes it considerably more buildable than it looks. "The principal elements are the same on each floor," explains Dominic Munro, an associate at Arup. Columns are the same length and diameter, the 18 nodes are the same size and are fabricated to offset the adjoining set of columns by the same angle; even the steel sections that make up the hoops that corset the tower are identical at each level.

The structure's detail design, fabrication and erection were undertaken by a joint venture between Belgian firm Victor Buyck Steel Construction and Dutch outfit Hollandia (VBH). "It was critical to get the frame in the exact location," explains Robert Obbard, managing director or Victor Buyck Steel Construction. "With a triangular grid, there's nothing you can do if it all goes wrong."

Work started on site in December 2000. The remnants of the Baltic Exchange, which was bombed in 1992, were cleared from the site and Skanska's sister company, Cementation, began work sinking 333 piles, each 750 mm in diameter, deep into the London clay. Steelwork erection began in October 2001 after the construction of a concrete pile cap and basement retaining walls, along with part of the plaza pavement.

The core was the first part of the structure to rise from the plaza. In the finished building, the perimeter diagrid will resist all wind forces so the building's core has to deal only with downward loads. That means it can be constructed as a simple steel structure. However, the diagrid can only cope with a lateral load after the concrete floors have been poured 鈥� and the concreting team were eight floors behind the steelwork erectors.

For construction, VBH reversed the roles of the core and the diagrid. "We had to fit temporary bracing to the core to form a stiff centre so that we could be sure of building a perfectly circular structure at its perimeter," explains Obbard. Strengthening the core allowed the lateral loads to be transferred to the completed diagrid structure eight floors below.

Construction of the core progressed four storeys ahead of the creation of the perimeter diagrid. The diagrid is formed from a series of A-shaped steel sections 鈥� Obbard calls them A-frames 鈥� which are preassembled on a jig by bolting two steel columns to a node (see the first photo). Because the site is so small and the building's portly waist overhangs most of it, few places on the plaza could be reached by the cranes. The jig had to be squeezed tight into the corner of the site, out of the shadow of the building's girth, to allow the A-frames to be craned into place.

Once in position on the diagrid, each node is tied back to the central core by a radial beam 鈥� an arrangement that leaves the floorplates column-free. Once 18 nodes are in place around the circumference, the tie-sections are added to link the nodes and form a horizontal steel hoop. This is a tricky operation. VBH designed the entire structure as a bolted assembly to eliminate the need for welding. The problem is that, for the hoop to close, the boltholes in all the tie-sections would have to line up 鈥� a task the contractor knew would be difficult. "We had to build a perfect circular structure around the exact centre of the building or nothing would fit," explains Obbard.

VBH has drawn on experience gained constructing a circular television tower in Kuala Lumpur. To provide some flexibility in the position of the nodes, it designed the radial steel members with an adjustable connection that allows the nodes to be eased in or out until the tie-section bolts drop into place. "It was the key to the structure's buildability," says Obbard.

Throughout its construction, the structure has been surveyed to ensure that it is being assembled within tolerance and that it is behaving as the engineers have calculated. A hole in the centre of each floorplate allows a theodolite laser to plumb through the middle of the building and establish the exact centre of each floor. Arup's design allows leeway for the structure to be built with its apex 40 mm from dead centre. "We were never more than 3 mm out," claims Obbard.

But even with the structure accurately assembled, the steelwork contractor's job is not finished. The building will reduce in height as the structure is loaded with the weight of the cladding and concrete floors. All buildings compress under loading, but as this tower's height decreases, its waist will increase. "The movement is a bit unusual," admits Arup's Munro. Arup has predicted that the building will shrink 125 mm and its thickness will increase 25 mm.

Arup was concerned that the building's bulging waist would be resisted by the radial beams linking the diagrid's nodes to the core 鈥� subjecting them to dangerous loadings. The solution was to give the structure enough flexibility to take up this expansion. "Arup asked for a very low resistance to expansion during construction and for the structure to be hard when set," says Obbard.

To allow the spread, VBH has used a sliding connection in conjunction with an L-shaped steel bracket to connect the radial steelwork to the nodes. The bracket will yield at a predetermined load, at which point bolts connecting the radial beam to the node will be tightened to lock the structure solid. "These bolts harden up the structure," says Obbard.

The team will have to wait until March 2003 before it is fully loaded. By this time, steelwork specialist Waagner Biro will have fitted the final part of the building's cladding 鈥� the steel and glass cap that will enclose the top two floors.

With the frame complete, the other trades are now racing upward toward the building's apex. Construction will soon have advanced enough for the fit-out contractors to start work. But Skanska's Clifford is not relaxing. "What you cannot afford on a fast-track programme is for something to go wrong," he says.

Given the unique challenge faced by the contractor, it is impressive that after 100 weeks on site, the project has slipped less than a week behind programme. This means the project team has 46 weeks to claw back the time and meet its completion date in autumn 2003. Swiss Re will not be moving in until work on the separate fit-out contract finishes in 2004. Until then, the building's prospective tenants will just have to join the others gazing skyward on the pavement of Saint Mary Axe.