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Devonshire cream

BUILDING STUDY: Bennetts Associates' redevelopment of a former BT site in Devonshire Square in the City of London offers an alternative to the standard masonry-clad and glazed office facades

As late as the 1920s Houndsditch, on the eastern fringe of the City of London, was described by one of the leading guidebooks of the day as 'the headquarters of Jewish brokers and dealers in secondhand clothes';

nearby Petticoat Lane was not the tourist attraction it has since become. Just to the north, the great brick warehouses around Cutler Street, built by the East India Company, contained 'vast quantities of carpets, porcelain, tea, tobacco, feathers, drugs, spices and other Oriental wares'.

During the past three-quarters of a century, the relentless growth of the City has pushed back the boundaries of the East End.

The Cutler Street warehouses were (to the disgust of conservationists) partly demolished, partly gutted in the early 1980s to contain offices. Houndsditch - the place where medieval Londoners deposited their dead dogs (and doubtless much else) - is now an inert street of largely post-1960 vintage, its frontages featuring alternately blank tinted glass or witless Post-Modern detailing. The Heron Tower, set to replace a particularly dismal pair of buildings at the Bishopsgate end of the street, offers the best hope for Houndsditch and would form a dramatic contrast to Foster's Swiss Re tower, rapidly rising on the former Baltic Exchange site just to the south.

As Rab Bennetts of Bennetts Associates explains, Houndsditch, even today, lacks cachet - hence the rather curious address (2.5 Devonshire Square) for the Premier Place office development that occupies a site between the busy street and the still-tranquil square tucked away behind, that retains a few Georgian houses. Agent Simon Harris, acting for BT - Bennetts Associates' initial client for the development - saw the Devonshire Square address as a useful marketing tool, and it was established that the natural point of entry to the building was on the north-west corner of the site. From here, it is a short hop to Liverpool Street station, via a pedestrian alley used by thousands of commuters daily. There is a steep fall in pavement level from north to south, hence the decision to locate the main reception area at first-floor level - areas of the ground floor are occupied by the service yard and by a bar, accessed from Houndsditch.

There were a number of problems associated with the redevelopment of the site, formerly occupied by a telephone exchange.

A BT cable chamber, wide enough to carry a Tube line, extended diagonally across the western edge and had to be retained intact at basement level, and there were several points where cable ducts pierced the site vertically. After planning consent had been obtained, BT sold on the site to AXA, which retained the architects (though Waterman Partnership replaced Whitby & Bird as structural engineer for the scheme).

Construction began late in 1999 and the building is now being fitted out for occupation by the Royal Bank of Scotland. AXA's commitment to the ideas in the project, says Rab Bennetts, ensured its quality was not compromised by the design-build procurement process.

The use of a steel frame for the scheme was in line with recent City practice. From the upper floors of the building there is an excellent view of SOM's mammoth Bishopsgate development, where panellised PostModern detailing is applied to a steel frame.

'Stick-on facades' don't come naturally to Bennetts - 'we wanted to avoid a facade job', he says. 'But architectural integrity is not a high priority in the City - our ideas had to be pragmatic and commercially viable.'

The City, Bennetts says, is a place for 'selective innovation - you have to choose your moment'. There was no scope for the highly progressive environmental agenda of Bennetts Associates' Wessex Water headquarters (where the concrete frame is part of the equation). Conventional air-conditioning was mandatory.

Bennetts Associates' early buildings were, in fact, mostly concrete-framed, though Rab and Denise Bennetts had a preoccupation with the role of the frame which extended back to their student days (and the influence of the late Andrew Jackson), their travels in the US, where they were in awe of the work of the Chicago School, Mies, Kahn, and Roche Dinkeloo, and Rab's time at Arup Associates under the late Peter Foggo.

In 1996, the year that design work on Premier Place started, Bennetts Associates' proposals for a new factory for Cummins Engines came under the scrutiny of Kevin Roche, acting as advisor to Cummins - Bennetts' return to steel for that scheme informed the City project.

The aim, in true Chicago spirit (and even Mies could sometimes be capable of a form of facadism), was to express the structural frame externally, creating well-detailed facades which are an elemental composition of post, beam and window void. In the early phases of the project, Corten steel was proposed, 'but it made our clients nervous', says Bennetts. 'The idea of a facade 'maturing' over some years doesn't make sense in a context where the aim is to let the building quickly, ' he says. One quality Bennetts specifically did not seek was that of the minimal or understated; 'the modern City office building is really a financial factory', he says.

'Solidity and a certain weightiness are appropriate.'

As constructed, Premier Place includes nine office floors and three basement levels, which are used for parking, services and storage. The office floors are conventional enough in terms of their scale and appearance, providing the requisite open-plan flexible spaces - the cellform beams which gave the interiors a strongly industrial character at pre-fitout stage have now been covered by ceiling panels that conceal a plethora of services running through the perforated beams. A stepped series of atria, rising above level 3, provides increased natural light, but these remain standard City floors, the product of an essentially North American office culture, where the comparison with a Victorian factory is not inappropriate. Service cores are located at three points, on the western perimeter, where the main bank of lifts is adjacent to the reception area, and on the south-east and north-east corners of the building. The strategy has obvious advantages in terms of delivering unencumbered office floors but also reflects Bennetts' desire to animate the edges of the floors, to allow the building's occupants views out and visual contact with their surroundings.

The fine materials and careful detailing seen in the staircases (framed by stone-clad 'bookends'), with their granite treads and landings, reflects this idea, as well as the overall air of quality in the completed scheme. The transparent glazed boxes that top the lift-shafts are another distinctive touch which could have been lost in the procurement process. Rab Bennetts was equally keen to challenge the general tendency of City office schemes to iron out individual or non-uniform spaces in favour of universality - the placing of the cores defines a series of such spaces, ideal as meeting rooms, though they might be requisitioned as executive offices, which have striking views out.

Again, he praises the client for its support of quality and distinctiveness - though these surely count in terms of the perceived commercial value of the building.

Seen from the surrounding streets, for example, up St Mary Axe, the building is a sober but far from oppressive presence. On Devonshire Square it provides a welcome contrast to the bulk of an adjacent sub-station, clad in crude Post Modern brick which does nothing to reduce its impact.

Rights of light issues, of course, explain the set-backs which occur from level 5 upwards. On levels 7 and 8, which are conventional rectangles, the conflicting grids in the scheme, reflecting the vagaries of the site as found, are resolved.

Though the building is air-conditioned, there is an attempt to minimise energy use by means of sensible low technology - incavity blinds, for example, and sunshade louvres on the southern elevation (which appear so solid from the street as to give the unfortunate impression of a dense layer of plant rooms).

For those who come to work in this building, the elements of 'delight' will hopefully leaven what is, on the whole, a typical rather than exceptional contemporary working environment. Where the building contributes most is in its external persona, in stating clearly the potential for a rational City architecture which has clarity, integrity and quality: what sort of impression do some of the flashy frontages in the surrounding area give of the institutions they house?

With the impending redesign of Devonshire Square, which Bennetts sees as a necessary adjunct of the project, its full urban potential will be realised. Leaving aside exceptional projects such as Swiss Re, current City of London architecture seems to be dominated by two approaches to facade design: those where masonry cladding or screening predominates (Foster's City Gate, Finsbury Square, is a good example), and those where glazing technology is exploited to the fullest degree. Bennetts Associates offers an alternative way which, though rooted in Classic Modernist ideals, has relevance for the ongoing reconstruction of the Square Mile.

Structure, services and procurement Office buildings in the City of London are bought and sold like any other commodity. Each transaction brings with it an army of lawyers and construction consultants with a brief from the purchaser to show that the project is not as perfect as the vendor maintains. In these adversarial conditions, it is hardly surprising that design is sometimes under threat or that some of the designers fail to stay the course. It is quite normal for one architect to obtain planning consent, only for another to execute the scheme for a new owner and for a third to carry out the tenant's fittingout. It goes without saying that the more radical a design, the more robust should be its design strategy in terms of technical rigour.

The planning consent scheme, obtained by Bennetts Associates for BT, was based on an exposed Corten steel frame (model photograph above), with relatively long internal spans and a single, linear atrium.

Whitby Bird, Cundall Johnson and Davis Langdon & Everest - all of which had worked with Bennetts Associates on previous projects - were responsible for the structure, services and cost respectively. Whitby Bird's sympathy with architectural detail and its experience in fire engineering provided the initial confidence for Bennetts Associates to pursue an exposed steel structure that would withstand the attention of those who would inevitably scrutinise the project at a later stage.

Although there were changes along the way, the innovative use of steel as the principle element of the facade has survived intact, despite the obvious questions raised about the need for fire protection, durability, thermal breaks and quality.

The past 20 years have seen steel become the dominant structural medium in the City on account of its potential for rapid construction and prefabrication for congested sites, but it is surprising that few buildings make a virtue of this form of construction. The combination of steel and the standardisation that characterises office planning imposes a strict discipline on the structural frame, with repetitive column grids based on 1.5m multiples and secondary beams supporting metal decking for the slab at regular three-metre centres.

Most beams are of the 'cellform' type, with frequent circular holes that allow the flexibility for a multiplicity of services penetrations and with studs welded to the top flanges to achieve composite action through the slab. The service/circulation cores at Devonshire Square are in the optimum structural locations for lateral bracing.

Unlike some of the classic exposed steel-frame buildings in the US, where the handsome external frame can disguise a concrete-encased structure inside, the external structure at Devonshire Square has the integrity of its load-bearing function. To express this, columns and beams on the facade are not covered with fire protection, but are shielded by the cladding design. The column flanges and the beams - the dimensions of which are large enough to cover the raised floor and ceiling zone - incorporate a degree of redundancy in the structure that assists with fire resistance. The necessary thermal break between each internal beam and the external structure is achieved by a special insulated connection. Tolerances are about half that of a normal steel frame and workmanship on the steel facade itself is of a very high order.

Deep-plan office floors for IT-intensive dealer rooms are not the place for low-energy solutions such as natural or mixed-mode ventilation systems. The need for artificial cooling was evident from the outset and, in consequence, the four-pipe fan-coil system follows fairly conventional lines. Nevertheless, measures to control solar gain include external louvres on the south elevation and in-cavity blinds on those facades subject to low sun angles.

BT sold the site with the benefit of planning consent to AXA, which reviewed the design and the team. It was also able to buy a neighbouring building and enlarge the development. Bennetts Associates was reappointed to complete the scheme, together with Cundall Johnson and Davis Langdon & Everest, but Whitby Bird was replaced by Waterman Partnership on account of the latter's greater City experience. AXA also asked for a more dramatic entrance and for the single atrium to be divided into two, as it gave greater flexibility for the tenant market. At the same time, the internal structural grid was modified to avoid the longest spans. Not unexpectedly, AXA was nervous about Corten steel and, as there were some real difficulties with avoiding rust run-off, a painted solution was adopted instead.

New planning consent for the modified design was quickly secured and construction began shortly afterwards, in late1999. The main contract was carried out on a design-build basis, with Bennetts Associates novated to the contractor.

Changes of this kind reflect the pressures of property trading and, although they call for a certain resilience from the design team, they also provide the opportunity to improve the design. The sequence of internal spaces, the finish to the steel, the massing of the corners and upper levels, the landscaping of Devonshire Square, have all benefited from AXA's involvement. The building has since been sold again, with Sheppard Robson carrying out the fit-out for the Royal Bank of Scotland.

The integrity of structure has been fundamental to Bennetts Associates since the foundation of the practice in 1987. A particular influence was a meeting between Rab Bennetts and Kevin Roche in 1982, when Roche described architecture that dislocates structure and facade as 'like designing the wrapper on a chocolate bar'.

The practice's on-going research into the integral frame led to the design of Wessex Water Operations Centre (AJ 22.11.01) where the combination of slender steel framing and lightweight concrete floors plays a key role in the passive servicing strategy. The exposed steel structure of 2.5 Devonshire Square was informed by the steel frame at Bennetts Associates'design for the Cummins Power Generation Factory in Marston, Kent (AJ 22.11.01). In turn, the portal frame structure of the Loch Lomond and Trossachs National Park Gateway and Orientation Centre (AJ 4.10.01) was conceived as an abstraction of the more commercial structural vocabulary employed at Devonshire Square.

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