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Round peg in a square hole

construction study

Swiss Re's emerging 40-storey headquarters in the heart of the City of London - designed by Foster and Partners - plans to radically transform London's skyline with its cigar-shaped structure, and is a clear argument in favour of tall buildings

Re-insurance company Swiss Re is currently located at 71-77 Leadenhall Street in Aldgate, on the site of one of the first great religious buildings to be dissolved by Henry VIII. The Augustinian Holy Trinity Priory, built in 1108, was once home to the all-powerful medieval kings until it was seized and its wealth incorporated. Swiss Re's new offices are now being built on the site of the dissolution of the Baltic Exchange building; a triumph of insurance over global trading.

Maybe the symbolism is intended to be apposite.

St Mary Axe, a small side street off Leadenhall Street, is to be home to Swiss Re's new 40-storey headquarters. Located in the heart of the city of London's commercial district, it shows the importance of this area to the client's identity - resisting the temptation to relocate to Docklands, and opting for the coherence of a historic financial area.

After the 1992 bombing of the Baltic Exchange, and its subsequent demolition, scheme proposals for office space on the site included the maximisation of the ground floor area - building right up to the boundary, keeping the height down and providing a central atrium.However, the developer found lettings difficult, for a variety of reasons, and with the backing of the Corporation of London, Foster and Partners was drafted in to reassess the potential of the site - the dialectical relationship between commercial need and architectural form - opting for the practical utilisation of only a portion of the site.

Building a round building on a square site has resulted in 'unintentional' public spaces, which will be landscaped and form 'organic' areas of respite from the street.

Pedestrian flows, especially in this area of London, says architect Paul Scott, follow alleyways rather than roads. The building shape will encourage people to lose themselves walking around the base of the structure, 'revelling in its aerodynamic form'. Halfway through the construction phase and all of the space has been let.

The shape has been arrived at after months of exploration into the most 'efficient structure for the site'. It is an appropriate form; determined by wind tests (tested in model form and using computational flow dynamics), approved by the local authority (which even requested that it be taller to improve the proportions), and aiding light penetration at ground level. The architect says that 'conceptually the project develops ideas first explored in the Climatroffice design with Buckminster Fuller in the early 1970s, but only with advanced computer technologies has it become realisable. Through parametric modelling, the curved surfaces have been rationalised into segmented flat panes.

Effectively, the building has been broken down into a kit of parts which repeat throughout the scheme, thus rending a fantastically complex idea into its simplest components. Conversely, the appearance of rigidity - of a pattern-book of repeating elements - belies the non-standardised nature of much of the structure and fabric.

The floor plate comprises 'fingers', which radiate from the central access core and provide parallel edged floor spaces based on a 1.5m seating grid. The resulting triangular 'dead' areas have been cut away to improve air flows and natural lighting. Successive floors have been set back to provide notional balconies (floors with no soffit above) and which create, as the floor plates rotate, the spiral of triangulated ventilation voids.

These are demarcated with tinted glass and have openable lights (operated by a central BMS for ventilation and for smoke release) so that the 'twisting' air flow taps in to the building's pressure differential rather than just relying on the stack effect.

Around the perimeter of each floor is an air plenum, opening to external air through a louvre blade at the horizontal glazing joints. This plenum is over-designed by 50 per cent in case future uses for the building (additional conference room partitioning, for example), demand additional ventilation.

Open floor areas comply with fire separation and compartmentation requirements and are generally divided into six storey groupings - full floor to full soffit - with connecting ventilation voids. Standing on the mid-floor of these six-storey compartments affords fantastic views out through a vast area of glazing. At the ground floor entrance, the visitor passes through an unglazed, four-storey portion of the grid and, looking up, the triangulated cut away floor has been glazed to provide views directly into the heart of the building.

The structure is based on an external structural A-frame; the core is the loadbearing element and the frame provides stiffness.

The plan is based on a 10 degree radial grid, a 20 degree superstructure grid and a 5 degree cladding module. Scott says that each tubular steel member of the A-frame has been designed with zero tolerances, to connect to radial beams 'like spokes on a bike'. As all joints have bolted connections, the structural accuracy relies on the node points and uses the bolts as locating pins - tightening up to 'pull the building into line'.

Because of the confined nature of the site, and the high profile of neighbouring buildings, the works is confined to two steel deliveries per day. Steel arriving from the Low Countries is stockpiled in Dartford ('to avoid strikes or ferry problems') and is delivered to site just in time. At the time of going to press, construction was progressing at two storeys per fortnight, including one week to get the external A-frame in place.

Windows are offered up from inside (alternating storey-height diamond shaped and half-storey triangular ones). These are attached to cladding rails - hooked on the top and bottom. Even though they have been designed to accommodate fluctuations of 75mm, they have, on average, been laid to tolerances of 5mm throughout. The simplicity of the design belies the need for the geometry of these windows to alter from floor to floor; as the building bows outwards, so the facets have to alter.

Unfortunately, the slender tubular frame which emphasises the feat of engineering efficiency has had to be clad in fire-resistant insulation, which then requires the additional disguise of aluminium panels. It is a shame that this has bulked out the frame, eating into the clear areas of glazing and thus reducing light and views. The other shame is that, to encourage the natural air flows, a secondary glazing wall has been constructed around the perimeter, and the vertical lines of this glazed partitioning clash with the careful geometry of the diamond external cladding beyond.

The steel frame has been taken down to the pile cap, 6m below ground, determined by the size of service equipment and loading bay requirements. 'Fortunately, ' says Scott, 'the floor level came just above the water table, otherwise the costs would have been prohibitive.' A 100mm-diameter hole, cut through the ventilation duct in the basement, actually extends through the whole building, to allow a theodolite laser to pass up through the building for accurate setting out for the day.

Currently the site - a stone's throw from Lord Rogers' Lloyd's Building - is dominated by Tower 42 (formerly the NatWest Tower) at 183m, and the Commercial Union building at 120m. Foster's proposal, at 180m, will complete the triumvirate but effectively this one building will do most to radically alter the London skyline; refusing to conform to the orthogonal orthodoxies of modern commercial structures. Viewed up close, with the straight lines of the adjoining buildings catching the eye, it had sometimes simply looked cylindrical. But as it has progressed and begun to attract the eye in its own right, the cigar-shape has become more apparent.Now its elevational distinctiveness is striking, and a worthy argument in favour of tall buildings done well.

CREDITS

CLIENT (AND PROPOSED OWNER/OCCUPIER) Swiss Reinsurance Company (Swiss Re)

ARCHITECT Foster and Partners: Norman Foster, Ken Shuttleworth, Robin Partington, Simon Reed, Francis Aish, Gamma Basra, Geoff Bee, Ian Bogle, Joel Davenport, Beb Dobbin, Michael Gentz, Rob Harrison, Chris Kallan, Jürgen Kuppers, Paul Leadbeatter, Stuart Milne, Sumaiya Mazari, Beverly Nasmith, Jacob Nørlov, Tim O'Rourke, Ben Puddy, Jason Parker, Sebastian Schoell, Narinder Sagoo, Michael Sehmsdorf, Paul Scott, Neil Vandersteen, John Walden, Tim WalpoleWalsh, Hugh Whitehead, Richard Wotton, Helen Yabsley

PROJECT MANAGEMENT RWG Associates

COST CONSULTANT Gardiner and Theobald

PLANNING CONSULTANT Montagu Evans

PLANNING SUPERVISOR Osprey Management

LEGAL ADVISER Linklaters & Alliance

URBAN DESIGN AND CONSERVATION CONSULTANT The Richard Coleman Consultancy

STRUCTURAL ENGINEER Arup M&E

ENGINEER Hilson Moran Partnership

FIRE ENGINEER Arup Fire

LIFT ENGINEER Van Deusen & Associates

CLADDING CONSULTANT Emmer Pfenninger

SHELL AND CORE MAIN CONTRACTOR Skanska Construction UK

SUBCONTRACTORS demolition Keltbray; monitoring and surveys Skanska Technology; sub and superstructure PC Harrington Contractors; facade access Reef UK; steel frame Victor Buyck - Hollandia Joint Venture; metal decking Richard Lees Steel Decking; fire protection Dragonchain;

services Skanska Rashleigh Weatherfoil; curtain walling Schmidlin; top of building Waagner Biro; drylining Lightweight Plastering & Drylining; WC fit-out Swift Horsmann; lifts Kone; cleaning access equipment Streetcrane Express; blockwork Bansal Building (London); stone paving Stirling Stone (Management); pre-cast concrete Mallings Products; temporary electricsWoodlands Site Electrical Services; tower cranes Select Plant Hire Company; hoists Universal Builders Supply; logistics Clipfine;

scaffolding, temporary gantry Bow Scaffolding (Southern); lighting Speirs and Major;

acoustics Sandy Brown Associates

WEBLINKS

Swiss Reinsurance Company www. swissre. com

Foster and Partners www. fosterandpartners. com

Montagu Evans www. montagu-evans. co. uk

Knight Frank www. knightfrank. co. uk

Arup www. arup. com

Hilson Moran Partnership www. hilsonmoran. co. uk

Van Deusen & Associates www. vdassoc. com

Linklaters & Alliance www. linklaters. com

The Richard Coleman Consultancy www. citydesigner. com

Skanska www. skanska. com

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