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Stepping up the pace Canary Wharf's latest prestige office building is rising rapidly, says Lisa Russell. Photographs by David Jones.

The most spectacular feature of Canary Wharf's new Citibank headquarters building is something that isn't really there at all. Cut outs in the floor are opening up a full height stepped atrium, that will bring light to the 2,500 workers throughout the 17 storey building.

The block is taking shape between the new Jubilee Line station and just to the east of the tower, One Canada Square. It will house Citibank's corporate front office. The original developments at Canary Wharf were speculative, but most of the new construction is for specific occupiers. Elsewhere on the wharf, Credit Suisse is gaining an extension, while the Canary Riverside complex will mix leisure, hotel and residential.

Citibank's atrium will link the main building to a tower that houses lifts, meeting rooms and so on. Slim bridges cross at every level from one tower to the other, giving some dramatic views. The atrium steps in, letting in light from top to bottom.

Various ideas for the building's design and structure were discussed in the course of 1996 and ground breaking took place in February 1997. Construction has been rapid: topping out of the steel frame is due in May.

Fast track construction meant sequencing the construction packages. 'Everything was staggered. We tendered for the piles first and then the substructure and then the superstructure,' recalls Ove Arup & Partners associate Roger Ridsdill Smith, who with Deb Thomas and Nigel Annereau, has been responsible for the design. Arup, led by director John Brazier, is structural engineer while the architect is Foster & Partners.

The building is in a reclaimed dock; essentially the team was presented with a hole surrounded by sheet piles.

The three basement levels, plus the lower ground and ground floors, are all concrete and there is a slipformed core. The remaining superstructure is composite steel. The main west block will stand 17 storeys, while the east tower will be 23 storeys.

Keller installed 130, 1.5m diameter bored, cast insitu piles. Every pile was base grouted, to ensure perfect transfer of loads, explains Canary Wharf Contractors structural design manager Bill Holloway. With the frame nearing full height, floor deck construction and fire protection are following up behind. Fittings are in place for the Schmidlin cladding system, which has to be proof against blasts. London Docklands still bears the scars of the 1995 bombing.

Achieving the construction in fast time has meant careful appraisal of the construction sequence. The steel springs off the promenade, or lower ground slab, rather than the ground floor slab, to clear the plaza area from the critical path, allowing finalisation of its layout.

The slipformed cores were built from the concrete basement slab. For logistical and craneage reasons, the towers were on the critical path. 'We have used the slipformed towers as tower cranes,' adds Holloway.

Designing for the unusual layout meant that stability had to be carefully checked. The central cores of the main block are very eccentric. 'Stability in the east-west direction wasn't a problem, but in the north-south direction, we had the east tower as an eccentric mass, joined by quite slim bridges,' says Ridsdill Smith. There is a line of bracing up the easternmost facade of the east tower but analysis was needed for the relative dynamics of tall thin east tower and the shorter fatter west block. 'What we found was that the bridges are stiff enough to make the buildings act as one,' says Ridsdill Smith. 'Cathedral wall' crosses of steel bracing add to the the east-west stability from the cores; while the north-south stability is provided by the cores and the east tower bracing.

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