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Technical & Practice: The Angel Building, London, by AHMM

AHMM’s pragmatic refurbishment and a striking installation by McChesney Architects have revitalised a 1980s office building

‘The Angel Building is our fourth office project for Derwent London’, says Simon Allford, director of Allford Hall Monaghan Morris (AHMM). Completed last September, it challenges the wisdoms that every AHMM building is a new start and that all architects build their reputations as designers at the expense of one-off clients.

It’s the perfect repeat commission because, as Derwent head of regeneration Simon Silver acknowledges, it is the culmination of its three predecessors.

The key move was the decision to retain the concrete frame of the existing office building, completed in 1981 to designs by Elsom, Pack and Roberts, with generous ceiling heights. This reduced the construction period and enabled Derwent to invest in the building’s principal assets: layout, conservation of resources and architectural quality.

Layout

By adding another floor to the building and extending eastwards (with a steel frame, chosen for speed of erection), AHMM added 9,200m² of floor space and devised a layout that enables it to be let to multiple tenants and subdivided horizontally and vertically.

The original building had three entrances, including one on the corner – a difficult location to handle architecturally. Now there is a single entrance on the east side, flanked by large retail units.

AHMM created a five-storey atrium by enclosing the existing courtyard, which contained a service road leading to a car park and provided access to plant rooms.

This atrium serves as a grand entrance area, improves circulation and orientation and creates breakout areas, where Vierendeel trusses provide column-free space and sunken floor areas add height.

Energy and environment

The building’s environmental performance simultaneously conserves tenants’ resources and hits ecological targets.

By deciding to retain the existing frame, AHMM made huge savings on embodied energy and construction costs, enabling more capital to be spent on green technology.

This is the first time AHMM have designed offices with displacement ventilation – a brave step for an office developer. It enables the offices to be naturally ventilated for 80 per cent of standard operating hours, significantly reducing energy consumption. Mechanical ventilation, using roof-top chillers, kicks in when it’s hot.

The new fifth-floor accommodation is set back from the building’s perimeter to conserve neighbours’ views and rights of light, creating an ample terrace which harvests large volumes of rainwater, used for toilets, window cleaning and bin washing.

With water-efficient taps and non-flush urinals, the equivalent of 455,000 WC flushes per year is saved.

Additional enhancements include variable speed pumps, energy-conserving lifts made from recyclable materials which feed braking energy into the building’s electrical system, two biomass boilers providing 15 per cent renewable energy and office light fittings with daylight and motion sensors.

The ample facilities for cyclists contribute to the BREEAM ‘Excellent’ rating of 74.7 per cent.

Architectural quality

AHMM had high aspirations for architectural quality and visited Chicago with Derwent to look at exemplar 20th-century office buildings.

The layout and environmental design also enhance architectural quality, built on a critique of what project architect Steve Smith describes as ‘soulless, hermetically-sealed office buildings which are difficult for people to inhabit and interact with.’

AHMM continued to develop a language of high quality internal concrete finishes, using pulverised fuel ash, which also lowers embodied energy (see article on visual concrete AJ 02.09.10).

A grid of 1,800mm deep beams casts shadows and diffuses light entering the atrium through super-transparent ETFE pillows, also screening views of roof-top plant which they have the capacity to support. ‘The inspiration for the terrazzo floor with marble inserts was the Olivetti showroom in Venice by Carlo Scarpa,’ says Smith.

The black powder-coated external cladding and curtain walling looks brown because of reflected light. With a low iron glass and film that adds a blue tint, it looks precise and its dark, reflective finishes dramatically yield to the light internal finishes.

Its qualities were tested and refined, using full-size mock-ups at design stage. Mullions are set out at 3m centres, providing more comfortable, horizontal proportions than the customary 1.5m module, although this would reduce flexibility if areas were sub-divided.

Conclusion 

There are some flaws in the Angel Building’s crystal clear logic. The public space to the east is narrower, although it may be animated by high-quality retail units when Derwent finds suitable tenants and it is enhanced by the yielding, though sombre, concave facade.

The concrete cladding to the existing atrium columns significantly increases their bulk.

Perhaps it’s futile to speculate that the narrower, dual aspect floor plates of the original building which was ‘hermetically-sealed’, would have offered higher natural light levels and through-ventilation. This wouldn’t have been commercially viable because of the loss of floor space.

Commercial logic also ruled out exposed concrete soffits which would have improved environmental performance, although concrete slabs in the floor voids do help to regulate displacement air temperatures.

‘Perhaps we’ll have exposed concrete soffits in our next office project for Derwent, says Allford.

A video of the installation can be seen at architectsjournal.co.uk/angelbuilding

Credits

Name of project The Angel Building
Start on site October 2008
Practical completion September 2010
Form of contract Two-stage design and build
Gross internal area 33,224m2
Construction cost £72 million
Construction cost/m2 to Cat A £2,008
Client Derwent London
Architect Allford Hall Monaghan Morris
Structural engineer Adams Kara Taylor
Services consultant Norman Disney & Young
Landscape architect J&L Gibbons
Cost consultant Davis Langdon
Lighting consultant Equation Lighting Design
Graphic designer Studio David Hillman
Fire consultant NDY Fire
Acoustic consultant Sandy Brown Associates
Main contractor BAM Construction
Main contractor’s architect Veretec
Project manager Buro Four
Estimated annual CO2 emissions 33.7kg CO2/m2
Airtightness at 50 Pa 7.75m3/h/m2
Mains water consumption 2.47m3/occupant/yr, including rainwater harvesting

McChesney Architects’ installation

Architect McChesney Architects
Structural engineer Atelier One
Laminates engineer Gurit
Fabricator AM Structures
Form of contract JCT Minor Works with amendments
Contract duration 9 months

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