Capping the tip of the gherkin

Cladding specialist Schmidlin is responsible for the facade of insurance company Swiss Re's tower in the City of London – but only up to level 38. For the top two floors, it hands over to Austrian steelwork specialist Waagner Biro; this firm has the job of building the frame for the dome and cladding the building's pinnacle. It was selected because it had the expertise to design, manufacture and install the steel frame to the tolerances required.

The 22 m high dome consists of eight horizontal hoops that diminish in diameter as the building tapers to the top. A trellis of glazed triangular sections fills the gaps between the hoops. The dome has been designed to align exactly with the main cladding, rather than the structural steelwork, to ensure continuity of glazing.

The dome's structure was built up in three stages.

The first involved fabricating 18 "ladder" units at Waagner Biro's factory in Austria. The units are 3 m wide and 13.5 m high. Each has two parallel sides connected by horizontal and diagonal members, which form a row of triangles from top to bottom.

These were joined together on the 30 St Mary Axe site to make nine larger sections. Two units were placed side by side in a jig at ground level, and the gap between them filled with additional horizontal members to create a large section three windows wide.

We got our welders at the factory to weld upside down … difficult, as they were used to welding over their work

The large sections were attached to the crane and lifted in exactly the right place, so each was in the right orientation when it was positioned at the top of the building. "We wanted to lift the steelwork into its final position without having to survey it while it was suspended; this speeds the process up and makes it safer," says Richard McNaughton, senior project manager for contractor Skanska.

The second stage was to repeat the whole proces by attaching a second storey of "ladders" on top of the first. These were 1.6 m wide and 10.7 m long.

Waagner Biro designed and built a temporary support structure on the top of the building to hold the sections in place before they were welded to their neighbours. One of the challenges of the project has been quality control of the welds, which will be visible in the completed building. As they are carried out in three environments (one in the factory, one on the ground at Swiss Re and the third at the top of the building), matching quality of each was of great importance. Waagner Biro's project manager Stefan Marinitsch explains how Waagner Biro ensured that the welds would look consistent: "We got our welders at the factory to weld outside and upside down to get used to the conditions. This was difficult as they are used to welding indoors and over their work."

The order in which the steelwork was erected was also important: the first welds were in unexposed areas such as the kitchens, whereas the final welds were in more visible areas of the restaurant, by which time the welders had perfected their technique.