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City slicker

Steel Swiss Re tower

A unique structure sprouting out of the city skyline is grabbing Londoners' attention. Alan Sparks scaled the new Swiss Re tower to take in the view.

'This is no ordinary job.

It has a little bit of the 'wow factor' about it, ' says Skanska project director Gary Clifford of the extraordinary edifice that is turning heads in the City of London.

Interest has boomed since the 'erotic gherkin' began to loom above its neighbours. When complete, the 41 storey Swiss Re tower will reach 179.8m tall - slightly shorter than the nearby Tower 42.

Built on the site of the bomb damaged Baltic Exchange in the heart of the City, the project has been on site since the start of 2001. The steel skeleton of the pinecone shaped building will be completed by mid October this year, on target for occupancy in 2004.

Recent tall buildings in London have generally featured concrete cores, but here the principal structure is made totally of steel. A network of column sections only taking vertical loads forms the circular core.

Steel floor beams radiate out from the core and connect to a steel exoskeleton, which acts as a perimeter tube taking all wind loads.

A diagrid makes up the exoskeleton, constructed from prefabricated 'A' frame elements that weigh up to 10.5t each.

There are 18 of these two-storey circular hollow section frames per floor - making 360 for the total building. A dome made up from prefabricated Vierendeel trussed 'orange segments' will crown the final two storeys of the diagrid.

This will be undertaken in a separate contract by Austrian specialist Waagner Biro. Here the architecturally prescribed steel members are made up of triangular sections of two plates and a cross-rod.

Central to the function of the diagrid is the connection between adjacent 'A' frame elements, the radial floor beams and the hoop tie, which resists the horizontal load. But as the floor plate changes dimension on each storey, each level of these nodes is unique. Steelwork contractor and fabricator, a Victor Buyck-Hollandia jv (VBH), plate welded all prefabricated elements in Belgium and the Netherlands before shipping over to Dartford ready for delivery to site.

Once these are in place, follow on works apply dry-lined and boarded two hour fire protection for the emergency shafts and wrap-around, fibre-based, 90 minute protection for the general structure. Architectural cladding is then fitted to the diagrid, which has tight tolerance limits. 'Just the consultation between each affected member of the design team on what tolerances were needed took four months, ' says Skanska senior project engineer, Jonathan Inman.

Eventually, the team plumped for a maximum tolerance of 5mm for all structural elements measured from the centre of the structure. To add to the conundrum facing structural engineer Arup, the structure not only shortens during construction as the dead load is applied - but also sags outwards due to the tower's curvature.

Vertically, the structure is set to drop a total of 80mm, while horizontally the skin will sag outward 25mm on each side. So the unique challenge facing the engineers and fabricators was to develop a prefabricated system that could hold the diagrid together and yet be versatile enough to allow for lateral deflection.

'The node design and geometrical challenge was make or break for the whole scheme, ' says Arup project manager John Brazier. Arup was the first to pick up the gauntlet, developing a concept system that allowed an angle section to yield in a single direction once under the applied dead load - this deformation was needed to transfer all forces into the hoop tie. Once all dead load had been applied, the angle connection could then be fully fixed to the structure.

VBH then looked to improve on Arup's suggested design.

Dozens of prototypes were produced to assess the buildability, cost efficiency and structural robustness of the design. But after much consideration, the conclusion further endorsed the Arup concept. VBH then completed the detailed design, and is today installing these nodes onto the structure's skin.

Friction bolts used in the offshore industry and conventional angle section were considered for the floorplate-skin connection, but guaranteeing the point of yield was difficult. So a made section with a curved corner was developed and found to be more practical in managing the plastic deformation.

Permanent shuttering using the Ribdeck 80 system, supplied by Richard Lees Steel Decking, allowed greater spans to be reached. This system produced an overall composite floor depth of 250mm. Pumped lightweight aggregate concrete with a retarding admixture was used for the floors, and no pre-wetting of the Lytag was needed to prevent absorption during the pressurised pumping process A single storey concrete basin sitting on 365 concrete piles bored up to 26m into the London Clay beneath supports this colossal steelwork monster. The first node for the diagrid is cast into this slab and the central core is fixed with conventional holding down bolts.

Temporary and permanent services needed apertures cut through the slabs. 'To install separately would have severely eaten into the programme, due to the fiddliness and messiness needed for each trade', says Clifford. So prefabricated frames with the services fixed to them were brought into site and craned into position, which meant each two storey fixing could be done in half a day.'

Visually striking and key to the structure's appeal are the diamond-shaped cladding elements, imported from Switzerland. These are connected to the main frame via stub brackets fixed to the hoop tie. 'To install these, manipulators use suckers to lift each element from horizontal. They are then rotated and lowered into position before a moment connector is fitted, ' adds Clifford.

Whats what

Height 179.8m

No of storeys 41 + 1 basement

Diameter at base 49.3m

Diameter at 17th floor 56.5m

Diameter at 39th floor 26.5m

Internal area 46,000m 2

Site size 0.57 ha

Structural steel 10,000t

External cladding 24,000m

Hit for six

So macho is this building that it even has a six-pack. Each floor plate has six partial segments cut out. And every storey is rotated 5infinity clockwise from the one below - producing a spiralling void throughout the building. This aims to ventilate the building naturally and maximise the penetration of natural light. 'Unfortunately, this had to be tempered by fire regulations which demanded that every sixth floor should be solid. Each block of six open floors is known as a six-pack, ' explains Skanska project director Gary Clifford.

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