Main contractor: Skanska
Client: Swiss Re
Height: 180 meters
Diameter: 50 meters
Completion date: 2002
One year construction period
10.000 tons of steel construction parts
Location: City of London
Better known as ‘the Gherkin’ due to its distinctive pickle-like shape, the 40-storey glass-and-steel structure designed by Norman Foster transformed London’s skyline. With its circular plan of varying size and spiralling light-wells, the design of 30 St. Mary Axe is an architectural marvel. The construction of the steel structure required a special approach that had never been seen before.
The building had to be constructed in a record breaking period. The construction site lies at the heart of the City’s insurance district. This posed limitations in accessibility.
A balanced diagrid structure is formed by a pattern of columns spiraling in both directions. At two-storey intervals the spirals intersect at 360 specially designed ‘nodes’. The heart of each node consists of a solid block of steel of 240mm by 140mm. This connection transfers the loads, both vertically and horizontally. The columns between the nodes are straight, with a change in direction and orientation at each node. As a result significant horizontal forces arise at these points. These forces are contained by perimeter hoops at each node level.
Due to the increasing weight of the structure, the buildings’ overall height decreases by eight centimetres. The construction expands laterally and therefore the hoops around the structure are pre-tensioned. The connection between the nodes and the radial beams had to be flexible to cope with changing forces during the construction process. These flexible hinges were locked after construction was finished.
The erection sequence progressed in two-storey bands in the following sequence:
1. Erection of steel core including access stairs and temporary bracing
2. Decking of core structure
3. Erection of diagrid columns and nodes as A-frames (pre-assembled at ground level)
4. Installation of radial beams and horizontal beams to complete the diagrid
5. Floor framing and decking
6. Floor concrete
‘The diagrid structure is a series of triangles that combine gravity and lateral support into one, making the construction stiff, efficient, and lighter than a traditional high rise steel construction.’
The diagrid steel structure provides excellent stiffness and stability for the tower, bearing a large proportion of the wind forces and gravitational loads. This reducing the loads on the foundation compared to buildings that are stiffened by a core. Thanks to this steel exo skeleton structure, the buildings interior is free of columns and is flexible in use, providing maximum lettable area.