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London pride Structure

aj building study

Foster and Partners' City Hall is a symbolic focus for London government as well as its head office, and a developer-led building that sets new environmental standards

For Ken Shuttleworth of Foster and Partners, 'the starting point of the project was to reduce the energy load of the building by 75 per cent'. The headquarters of the Greater London Authority, to be known as City Hall, is nothing if not environmentally responsible, a practical demonstration, the architect claims, of the potential of sustainable design in a world city where, so far, that concept has made a negligible impact.

The environmental strategy of the building, says Shuttleworth (the Foster partner in charge of the project), has determined its striking architectural form - which Ken Livingstone once memorably compared to a 'glass testicle' - and the choice of materials.

Other public bodies (and, hopefully, developers) may benefit from the practical lessons of the GLA.Yet public buildings have always had a symbolic and metaphorical agenda that extends beyond issues of practicality and economy: what does City Hall say about the GLA as an institution or about London?

For some, the choice of the site, between London Bridge and Tower Bridge, is fatally flawed. The LCC, and latterly the GLC, had its base for more than 60 years in a monumental structure constructed at public expense. Located on the South Bank, looking across to Westminster, County Hall was a potent symbol of local democracy. If Westminster shut up shop, it was said, Londoners might not care, but County Hall was the engine house of the capital.

When the Blair government resolved to establish a strategic authority for London, there was no question of a new version of County Hall being directly commissioned.

The location of the new building was determined after a 1998 developer/architect competition in which Lord Foster and his client, CIT Group, triumphed over an alternative proposal by Will Alsop to site the GLA headquarters in a revamped 1920s commercial palazzo in Bloomsbury. Foster's client offered the government what was judged the best financial deal, along with the services of a star architect who had recently completed a parliament house for the most powerful nation in Europe. The GLA and the mayor, both yet to be elected, had no say over the design of the new building, or the choice of site - which is privately owned.

The selected site is London Bridge City Phase 2, latterly known as More London. It had lain empty since clearance took place in the 1980s. In 1989, John Simpson's 'Venice on Thames' office scheme had been selected over rival proposals for extending the existing London Bridge City, which included Philip Johnson's bizarre paraphrase of the Palace of Westminster (to be executed using reflective glazing). The Simpson project was a victim of the '90s recession. A new consortium of owners, replacing the Kuwait-based St Martin's Property Corporation, played safe and brought in Foster. A masterplan for the 5.5ha site was developed during 1998, with four large office buildings, totalling nearly 200,000m 2, and a hotel, all to be designed by Foster and Partners. Inserting the GLA building into the masterplan was not difficult, since a major open space had always been planned at the riverside end of a broad pedestrian avenue cutting diagonally across the site from London Bridge Station.

The Foster masterplan represents the enlightened end of the London commercial development scene. A real attempt has been made to re-establish an organic urban grain and to make public spaces which are more than simply gaps between buildings.

Ground floors will house large areas of retailing and restaurants. Refurbished buildings along the northern edge of the site will contain cultural spaces, including a theatre - the aspiration is to create somewhere more like Covent Garden than the City. Two of the Foster office buildings are already pre-let and on site. The GLA was seen as a positive and prestigious addition to the overall development, introducing added value to the site.

Foster's reconstruction of the Berlin Reichstag (1992-99) obviously informed the design of the GLA building. Shuttleworth points out that the great debating chamber of the Reichstag could contain the whole of London's City Hall. The comparison is apt.

The budget and powers of Livingstone and his colleagues are modest when compared with those enjoyed by the mayor of Berlin.Yet the idea of transparent government which had driven the Reichstag project re-emerged at the GLA. The building, Lord Foster proclaimed, should be open and accessible to Londoners, who should be able to watch the assembly. Initially sceptical - largely on account of the procurement process - Ken Livingstone quickly warmed to the project after his election, declaring himself an enthusiast for Foster's work and perhaps sensing that the building could work to his benefit.

Working with Arup, part of the Reichstag team, Foster and Partners developed initial sketches showing a spherical form into a final scheme moulded by energy considerations but informed by the wider context.

The transparent northern facade of City Hall embraces a fine view of the Tower of London (and of various other Foster projects, incidentally, including Tower Place and SwissRe). To the south, the building's floors overhang each other, creating a system of self-shading and giving the whole structure the appearance of tilting over. As Shuttleworth says: 'It looks like a spaceship which has crashed deep into the ground.'

Early impressions of the building gave a strong impression of lightness and transparency, but 75 per cent of the completed structure is clad in solid, insulated panels.

Triple glazing, fritting and solar control blinds further reduce the element of transparency - externally City Hall may look its best after dark, when the use of vivid colour as a backdrop to the public spaces will read strongly.

Entering from the riverside through revolving doors (which can be folded back), the London elector/curious-tourist comes into a space which will soon house a giant model of central London. A level down is the cafeteria, open to all, plus a series of committee rooms. The ramp leads up to the debating chamber (where, once a month, the 25 GLA members will assemble). Supported at three points per revolution and extending to the top of the building, with fine views out, it is a somewhat vertiginous route, lacking the elegance of the Reichstag. Structurally, the ramp is a continuous steel box carrying concrete treads and contributes to a sophisticated exercise in acoustic design, ensuring that no speaker in the chamber will need to use a microphone - quiet conversation can be heard from the upper levels of the ramp. The chamber is a surprisingly intimate space, appropriate to the scale of operations it houses.

Little need be said about office spaces, remarkable only for being straightforward in character with few cellular spaces. The space, christened 'London's living room', has terrific views of London. It is likely to be heavily used - will private lettings limit public access?

Ken Shuttleworth stresses that this is a public building, set in public space. The square and 1,000-seat open arena surrounding it are beautifully done, using Irish limestone and excluding any extraneous elements - lighting is concentrated on a single tall mast. Designed in the spirit of the square around the Carré d'Art in Nîmes;

this is a space which a local authority would be unlikely to fund or to maintain properly.

But will CIT tolerate the demonstrators who might see it as a natural extension of a public building? There are no gates on More London, but the land remains private - you enter with the consent of the owners.

City Hall cost £40 million and was completed to schedule - compare that with the Scottish Parliament, says Shuttleworth. A rent of £34.50 per sq ft seems reasonable value for a quality building in this location.

Neither Foster, nor, for that matter, CIT can be blamed for the government's choice of procurement process - after all, the GLA could have been shunted into a spec office block by a government which remains attached to the principle of privatisation.

City Hall is a building which has obvious roots in Lord Foster's perennial concern for lightness and the manipulation of natural light. Though not vintage Foster, and not quite the crystalline sculpture promised back in 1998 - the slightly clumsy exterior reads as something of a testbed for the more sophisticated SwissRe tower - City Hall will doubtless become a popular landmark.

Foster has given London an indicative monument reflecting the mood of a City stumbling slowly towards a realisation of its own worth and dignity.

Facade engineering

Arup Facade Engineering became involved in the GLA project shortly after Foster and Partners and Arup won the design competition. Three separate glazing packages were tendered: the unitised office cladding; the glazed outer skin to the chamber; and the internal atrium glazing.

Through the use of freehand sketches backed up by 3D CAD modelling, we were able to define the visual intent of the design. This enabled the contractor to minimise his risks and provide visually and technically compliant tender returns.

The finished glazing systems remain virtually unchanged from the initial tender drawings, having been through a contractor development phase and various value engineering sessions. The project, therefore, did not suffer a dilution of visual intent and design concepts, which is often witnessed in technically demanding projects of this nature. The cladding design responded to a demanding cost plan, and met very onerous environmental requirements to minimise heat gain and loss, combined with transparency. Off-site fabrication and mock-ups reduced complications on-site and maximised the quality of the works.

The office cladding is unitised, with every panel being different and asymmetric to cover the non-spherical complex curvature of the building in a system of facets.The panels are individual thermal flues (see Working Detail, page 32), which limit solar gain significantly and maximise window area available for the user. High-performance glass coatings and acoustic laminates are used comprehensively throughout the scheme, making the most of currently available technology. Safety considerations resulted in widespread specification of laminated glass in inclined and overhead conditions, in some cases beyond the requirements of current UK standards.

From the beginning of design it was clear that the GLA building would be unusual. An inclined structure is needed to create the curving and steeply inclined shape. This generates large horizontal forces that need to be resisted by the building's stability system.

A number of structural forms were investigated. The inclined column solution was chosen for its simplicity, buildability and cost-effectiveness.

The 508mm CHS columns are straight between each floor, and kink within the structure of each floorplate.A network of 675mm-deep primary steel beams, tied to the central reinforced concrete core, carries the horizontal forces that are needed to restrain the columns. These horizontal forces, which exert a net pull to the south, are about five times the building's design wind loads. Their effect dominated the design of the building.

The development of the shape and supporting structure was an iterative, collaborative process that relied upon the rapid exchange of electronic 3D models between Arup and the architect. Components and details were standardised by the Arup design team wherever possible. For example, the 'cotton reel'column node is standardised throughout the building giving efficiency in the fabrication of the structure. Its design can accommodate incoming beam and column connections from any angle.

Arup developed a construction strategy to control tolerances in which each type of building component - eg structure, cladding, etc - was erected to its absolute 3D geometry, rather than relative to the as-built position of the preceding component. This approach allows each element to be constructed to its intended geometry - vital for a building of this complexity.

Our proposal that three-storey column sections were pre-fabricated and checked before transportation was adopted. This ensured that the frame was erected quickly within allowable tolerances, a critical process since the actual geometry affects the sizes of the horizontal restraint forces.And the structure needs to be the right shape for the cladding to fit.

The circulation ramp spirals through the atrium space. The ramp is supported at three points per revolution - the landing and two intermediate hangers. Its structure is a continuous steel box - 1.5m wide by 0.4m deep - which carries the concrete steps. Control of dynamic performance was an important element in its design. Arup introduced a constrained-layer damping solution in which a damping membrane is placed between the steel box and concrete treads to control the dynamic response of the structure. Tests on the complete ramps showed they performed very well and exceeded the agreed design criteria.

Environmental engineering In setting an example for low-energy design within the framework of a commercial marketplace, the challenge was to demonstrate that lowenergy design does not have to cost more.

The unusual geometry of the GLA building dictated that virtually every glazing panel faced in a different direction and elevation, and the standard thermal analysis tools were not capable of optimising the facade design for each situation. Instead, using 3D lighting analyses, all of the building facade panels were modelled using a daylighting simulation technique. The calculated illuminances were then converted to heat gains on each panel.

A target maximum solar heat gain per linear metre of facade was set. The solution comprises a clear vision zone of high-performance solar control glazing, highly insulated opaque panels and low-level and high-level openable vents. Occupant-controlled blinds are provided in the external cavity, potentially stopping the majority of the solar heat gain outside the building envelope.

Fresh air is supplied to the office areas via displacement floor grilles during occupation times. The floor void acts as supply plenum. In winter, heat and moisture will be recovered from the extract air using a hygroscopic thermal wheel, and this will be used to pre-heat and humidify the incoming supply air. In the summer, the same thermal wheel will pre-cool the incoming fresh air when the external air temperature exceeds the internal air temperature. Openable vents are provided to all perimeter offices. When open, the local air-conditioning and heating systems will be deactivated.

Hot water for building heating is generated using two high-efficiency gas-fired boilers. The distribution systems are low pressure and variable volume, reducing electrical energy used by the pumps.

Passive chilled beams, coupled with a low-level air supply, provide the main cooling to the offices. Chilled beams cool the surrounding air, its density increases and the air flows down into the occupied space using natural buoyancy.

The central chilled water is cooled by aquifer water supplied by pumps in boreholes. After use for cooling, this borehole water is stored for flushing WCs and irrigation before any surplus is discharged to the Thames.

The committee rooms and debating chamber are cooled by displacement ventilation systems when in use. During peak summer conditions, when the chamber is not in session, opening large air vents at the top and bottom of the debating chamber facade will naturally ventilate the space. In the winter, the direct exposure to the external wall of the chamber will create cold downdraughts as heat is lost through the facade. In order to combat this downdraught, the diagrid structure forming the facade to the chamber is used as a large radiator and convector heater.

London's Living Room, located at the top of the building, is air conditioned when in use.

Natural ventilation can also be provided via opening doors at low level and by high-level vents above the doors.

Lighting is switched by a control system working in conjunction with presence detectors, together with internal and external light level sensors. Zones of control are linked so that only an occupied area is lit, together with a 'corridor', a route back to the lift core, ensuring that a member of staff working late has a clear escape route lit at all times.

Independent environmental appraisal Design successes that stand out are:

lpenness - it feels open and you get a real sense of transparency, like walking through a crystal;

low-energy skin - this is one of the cleverest facade systems around, offering huge energy savings and natural ventilation capabilities while following a highly complex geometry;

daylight - a daylit building was high on the agenda, successfully achieved; and lborehole cooling - London is flooding and water tables are rapidly rising. The design relies on borehole water abstraction for all cooling demands, thereby meeting low-energy targets while contributing to reducing water table levels.

The apparent abundance of glazed surfaces suggests a high solar gain building, with no visible solar protection and seemingly clear glass.

However, clever geometry tries to hide the building from direct incident solar gains. In the north, the glass can face up to the sky while the southerly surfaces are angled down toward the ground. Lower-angled sun protection is needed here and to the east and west.

Facade design of perimeter office space is a clever double skin which facilitates natural ventilation of perimeter zones, while providing solar protection and glare control (see Working Detail, page 32). The cavity between the outer single skin and the inner double-glazed skin is ventilated at both high and low level. This ensures that the majority of heat absorbed by cavity blinds is ventilated away before affecting perimeter office space. Mechanically operable ventilation flaps below each window, linked to corresponding motorised high-level exhaust air flaps above each window pair, provide an excellent level of perimeter natural ventilation.Glazed areas are less than they appear from the outside, while maintaining reasonable levels of daylight.

Office space is cooled by passive chilled beams, fed by borehole water, providing a highly efficient means of cooling. Even IT loads are dealt with by this system. Spaces are zoned to ensure that perimeter beams are isolated once natural ventilation is deployed. Fresh air is introduced using low-level ventilation, but with no apparent corresponding means of isolating these zones during natural ventilation.

The payback for photovoltaics would extend well beyond 10 years, so they are not cost-effective. But the sun shading around the top floor could accommodate PV panels in the future.

There is some office waste segregation and a small use of recycled materials, such as for mats and floor tiles. Beyond these, the team struggled to find other reliable supply sources with suitable quality standards. No embodied energy assessment was carried out for the building, which is a slight disappointment.

The team claims a 75 per cent reduction in energy consumption, compared to a typical City of London high-specification office building. Though the GLA is outside the normal office use on which BREEAM is based, a full assessment achieved an 'Excellent' rating. And while designed to the old Part L regulations, the building is well within the new version. Post-occupancy studies and energy monitoring are planned.

There are a few elements that have not been totally thought through:

while the chamber does have a natural ventilation mode that will work well under dry conditions due to high stack effect, there is not a convincing strategy during wet conditions as roof level openings are not rainproof and air has to find its way through 'London's living room'on the top floor via propped doors;

while the architect has carried out a maintenance analysis, showing overall costs reduced due to innovative environmental systems, the additional glass layers may start to collect dust early and require an increased cleaning schedule; and lthe building is about access and no compromise has been made on this account since 11 September. It will be interesting to see if this can be maintained during operation.

Overall, one cannot fail to be impressed, not only by the building itself but by the government team which set the brief, the architect who stuck to his beliefs and the client/developer who kept his nerve. It has set new standards for responsible and sustainable government design. If only the PFI process could now deliver buildings as good as this.

Costs

Cost analysis based on final account. Costs rounded up or down to the nearest pound

SUBSTRUCTURE FOUNDATIONS/SLABS £112/m 2Large pile cap to core with straight-shafted piles, reinforced concrete basement slab and retaining walls with pad and pile foundations as required

SUPERSTRUCTURE FRAME £221/m 2Reinforced concrete core, steel frame comprising circular hollow section columns, nodes and standard section beams

UPPER FLOORS £146/m 2135mm-thick reinforced concrete floors on metal decking

ROOF £33/m 2Concrete roof slab plus Bituthene waterproof membrane for hard landscaping by others

STAIRCASES £114/m 2Complex helix steel-framed and concrete ramp with stainless-steel handrails and glass balustrades.Precast concrete stairs with in-situ concrete landings and painted mild steel handrails

EXTERNAL WALLS £540/m 2Triple-glazed system, opening vents, spandrels and integral glare blind

EXTERNAL DOORS £5/m 2Double doors 4mx4m.All-glass revolving doors.Single, 2.5m-wide x 3m-high secondary revolving door

INTERNAL WALLS AND PARTITIONS £198/m 2Single-glazed acoustic atrium screens.Flush-glazed demountable partitions to office fronts.Plasterboard partitions generally with masonry to plant areas

INTERNAL DOORS £32/m 2Flush spray-lacquered timber doors with stainlesssteel ironmongery

INTERNAL FINISHES WALL FINISHES £50/m 2Painted plasterboard generally.Laminate panels to WCs.Acoustic linings to conference rooms

FLOOR FINISHES £90/m 2Terrazzo tiles to public areas.Ceramic tiles to WCs.

Carpet to balance of areas

CEILING FINISHES £117/m 2Plasterboard ceilings to general floors between service zones.Rod ceilings to perimeter.Acoustic ceilings to public areas. 'Spiral'stainless-steel ceilings to reception

FITTINGS AND FURNISHINGS FURNITURE £56/m 2Perimeter desks to typical levels.Two reception desks, chamber desks and seats

SERVICES

SANITARY APPLIANCES £16/m

2SANITARY APPLIANCES £16/m 2White vitreous china standard WC fittings

SERVICES EQUIPMENT £1/m 2Mobile facade-cleaning equipment

DISPOSAL INSTALLATIONS £26/m 2Soil waste and vent

WATER INSTALLATIONS £30/m

2Hot and cold water supplies to toilets, kitchens, etc

SPACE HEATING/AIR TREATMENT £150/m 2Borehole water for chilled beam installation.Supply air within floor plenum

ELECTRICAL SERVICES £ 137/m 2Standard and feature lighting throughout.Power to riser and landlord areas

LIFT AND CONVEYOR INSTALLATIONS £65/m 2Four passenger lifts at 13 person, two visitor lifts at 13 person and one 3T goods lift PROTECTIVE INSTALLATIONS £22/m 2Sprinkler system and smoke alarm

COMMUNICATION INSTALLATIONS £38/m 2IT cabling, flood wiring and BMS controls BUILDERS'WORK IN CONNECTION £16/m 2Coring and penetrations, fire stopping and plinths

PRELIMINARIES AND INSURANCES PRELIMINARIES, OVERHEADS AND PROFIT £308/m 2

CREDITS TENDER DATE September 2000 CONTRACT START DATE May 2000

COMPLETION DATE May 2002 GROSS

EXTERNAL FLOOR AREA 19,814m

2TOTAL COST £50 million

PROCUREMENT Construction management/bespoke contract

CLIENT CIT Group

ARCHITECT Foster and Partners:Lord Foster, Ken Shuttleworth, Andy Bow, Stefan Behling, Sean Affleck, Richard Hyams, Niall Monaghan, Max Neal, Frank Filskow, Ken Hogg, David Kong, Bruce Curtain, Graham Longman, Mario Pilla, Alice Asafu-Adjaye, Louise Blackler, Elodie Fleury, Attilio Lavezzari, Sam Harvey, Tomer Kleinhause

QUANTITY SURVEYOR Davis Langdon & Everest

M&E QUANTITY SURVEYOR Mott Green & Wall

STRUCTURAL, M&E, TRAFFIC, FIRE, ACOUSTIC AND FACADE ENGINEER Arup

LANDSCAPE ARCHITECT Townshend Landscape Architects

LIGHTING Claude Engle Lighting

LANDSCAPE LIGHTING Equation Lighting Design

FACADE MAINTENANCE Reef UK ACCESS CONSULTANT RADAR Promotions

LANDSCAPE WATER FEATURE CONSULTANT The Fountain Workshop

CONSTRUCTION MANAGER Mace

SUBCONTRACTORS AND SUPPLIERS Atrium cladding, (level 9) lens, scoop, skirt Selle; external cladding Schmidlin; steel frame Wescol; concrete substructure Geoffrey Osborne; fit-out Sherlock Interiors Constructing; lifts Thyssen Lifts & Escalators; fire service Hall & Kay Fire Engineering;mechanical pipework systems Axima; electrical Phoenix Electrical Company; chamber stepped ramp Waagner Biro Aktiengesellschaft;waterproofing Coverite;

blockwork Swift Brickwork; revolving doors Rush Entrances, Blasi;

drylining Fireclad;

mechanical air systems Senior Hargreaves; BMS and controls Sauter Automation; architectural metalwork Glazzard (Dudley); raised floor Hewetson; general metalwork Glenworth Fabrications;

demountable partitions, reception desks Ergonom;chamber 'underbelly' ceiling Jordan Fabrications; hard flooring Grant Ameristone; ceilings Astec Ceilings; chamber furniture, preimeter desking Isis Concepts;

services commissioning Quantum Associates; soft floor Rees Flooring; voice and data cabling J Grant & Taylor; concrete Edmund Nuttall; light mast Siteco Lighting Systems;

infrastructure fit-out FB Ellmer; landscape paving McNicholas; lighting Erco Lighting; chilled beams Trox Technik

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