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Erotic gherkin's high tech design

Recent leaps in software sophistication and computing power are helping designers realise ever more fantastic structures. Andrew Mylius looks at the Swiss Re building.

It has been dubbed 'the erotic gherkin'. The proposed City headquarters for insurance firm Swiss Re is an erect, 180m tall torpedo. It is circular in plan, swelling outward as it rises from the ground, then tapering curvaceously inward again from its mid-point upward.

Swiss Re's tower will be a potent symbol in London's testosterone-fuelled financial quarter. Right now though, architect Foster & Partners and engineer Arup are working on the building's structure and cladding. Because of its curvature the task is complex. Computational muscle is being employed to crunch numbers and help the design team visualise potential solutions.

IT has been vital in the gestation of designs from the outset. Foster director Robin Partington describes evolution of the building's profile, which has been tweaked ceaselessly since the project began in August last year, as 'iterative'. Though in outline Swiss Re began life on paper, Microstation has been used to draw up parametric models - CAD diagrams in which numerical parameters can be changed time after time to produce new plans, sections and elevations.

Microstation allows Partington and his team to select-and-drag elements, changing the design intuitively by direct intervention on screen. It also enables numbers and values to be changed, bringing a more exact, analytical approach to bear. Alteration of a single parameter or element informs the design as a whole.

After producing a linear model, Fosters then broke the complex curve down into a series of arcs and radii. This provided Arup's engineers, led by director Adrian Campbell, with accurate figures for their own analysis and modelling. Microstation also depicts the tower's floors - every one has a different area - and the junction of each with the innovative supporting structural diagrid.

The steel diagrid is a load-bearing, self-bracing structure that envelops the tower like a huge lattice. It is composed of 36 steel 'columns' that spiral around the building at an angle of 20degrees. In plan, 18 run clockwise and 18 counter-clockwise. Over the height of the building, they intersect at every second floor, the floors themselves stiffening the diagrid against compressive and bending forces. When Partington describes the geometry as complex it is an understatement.

Microstation has automatically modified floor areas as the tower's skin has been stretched and pinched. As it mutated the software changed angles at junctions between floors and the diagrid. It also responds to the positioning of elements - such as whether diagrid columns are within or without the floor-plate - and their dimensions. All information can be recorded and interrogated. And the software allows zooming in on details or panning back to view the scheme from a distance. Designs can be viewed from any angle.

At Arup, Campbell tells a similar story. Structural engineering is being carried out using General Structural Analysis software, with Microsoft Excel and the steel fabrication X-Steel package 'bolted on'. Campbell's team is currently working on a diagrid design using fabricated tubular steel elements. This will provide a structural performance, tonnage and cost model against which to measure alternative I-beam and square section options.

GSA allows Arup to represent graphically information supplied by Foster and subject it to different forces. The structure can be pared down or beefed up to achieve optimum strength-to-weight performance. It can then be subjected to, for instance, wind or buckling loads. Forces are represented in a line model by a spectrum of colours.

It is in Excel, however, that the performance of individual elements can be interrogated. Every component of the diagrid and steel floor beams are represented on a 15,000 line spread-sheet. Excel will highlight components that fail under simulated loading allowing them to be strengthened. The new strength and weight properties will then interact with and inform the structure as a whole. You can watch the entire building undergoing automatic modification.

X-Steel, meanwhile, takes data from GSA to generate fabrication quality drawings that show in detail how the building will work. Foster and Arup have been consulting specialist steel fabricators Cleveland Bridge and Watsons on the most efficient ways of joining the diagrid members. Designs for a node have been evolved and, once fed into X-Steel, the programme has made minute alterations to its geometry. In all there are 18 nodes around the circumference of the building. Nodes at the same levels are identical. But at each of 20 levels where the opposite-spiralling columns intersect a different set of angles is required.

Campbell says detail design should be complete by the end of the year. The design team is hoping for planning approval by February next year at the latest and to be on site in spring or early summer. Construction should be complete by early 2003, says Partington.

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