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Where engineering and architecture meet The success of Anthony Hunt Associates, the smallest of the 'big three' engineering practices, is attributable to its constant search for a new way of doing thi

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Sometimes it is very hard to tell engineers and architects apart. Engineers now share architects' mannerisms of dress and affectations of appearance, the same watering-holes, and even, in extreme cases, the same patois. It's a nice reversal of the early days of Modernism when architects wanted to be like engineers, but a swing started several decades ago with what one engineer, who went through it, calls 'a movement where architects have worked with leading engineers'. And it has flowered in the recent proliferation of inventive young engineering practices.

Three firms, though, have remained at the undisputed peaks of their profession: Arup's, Happold's and Anthony Hunt Associates. Hunt's is the minnow of this trio, and not large by the measure of its newer competitors. But Hunt's has honed its innovative skills, producing memorable engineering solutions for projects like Huddersfield's MacAlpine Stadium and the extraordinary roof to the motorway-spanning Galleria shopping centre at Hatfield. Its current workload probably represents its broadest ever clutch of innovative structures, from rail bridges to glass domes to Alsop and Stormer's London Assembly scheme.

The main office is in the unlikely setting of the upper floors of a Georgian- faced town house in Cirencester. Their downstairs neighbour is Knight Frank, which, together with a Waitrose that looks slightly like an illegitimate child of Michael Hopkins' David Mellor factory, bears out the town's image of affluence and second-hand 'good taste'; symptoms of the English upper class as much as the other pursuits - of foxes, hunt balls and polo - for which Cirencester is best known.

So, if it's neither size nor location that accounts for aha's consistency, what is it? Much, of course, must come down to the combined skills and interests of Tony Hunt himself; the way he 'went native' among architects, as one former associate puts it. But no individual can handle so much invention and development work, and there has always been a string of partners, directors and associates whose contributions have been enormous. 'Tony and David [Hemmings] devised the concept of the cable stay at a time when it was unheard of in buildings [for the classic Inmos silicon- chip factory designed with Richard Rogers],' says then project engineer, now director, Alan Jones. He and others did the number-crunching that made it work in practice. The structure is still there and relevant long after it has become obsolete for its purpose.

Jones's career path shows how the practice is mutating from one generation to the next. Developing skills and responsibilities is one way it has kept its edge. But Hemmings, who joined aha in 1971 to work on Willis Faber Dumas and is now managing director, is adamant. The staff, he says, 'are the crowning glory of the company'. In the spring of 1997, when the ill-starred venture with yrm was coming apart, he had to tell staff of the threats of receivership. 'I thought they'd all be out of the door,' he says, but they all stayed, including several agency employees. That vote of confidence bred more confidence in the team which facilitated a management buyout, and in clients, who sustained the workload which made the buyout successful. Since then the Sheffield office under Allan Bernau has grown, with the expansion of the Don Valley stadium - aha's first stadium when it was built for the student games in the early 1990s - into the United Kingdom Sports Institute. London, under Les Postawa, is also expanding, with involvement in the new Terminal Building at Madrid Airport and other notable projects. The sixth director, Bjorn Watson, works from Cirencester and travels regularly to London to oversee Lloyd's Register of Shipping, as well as a number of new projects including the Terminal Building at Bristol Airport.

Less than two years on from the buyout, a whiff of expansionism is in the air: not dramatic - Hemmings sees growth through small regional or continental offices set up by people from the practice who might want to move - but justified by the range of work. aha may be most strongly associated with its steel structures, cables and masts. But Willis Faber and the Truro Crown Courts are concrete: Will Alsop, whose Blackfriars Bridge is engineered by aha's London office, praises Hunt's 'knowledge of materials and ability to work across the range from timber to steel and concrete'. Its current projects innovate in steel, concrete and glass.

Perhaps the most dramatically inventive is an all-glass bridge. An rca- educated furniture designer called Thomas Heatherwick - 'a very clever young man,' says Tony Hunt - approached aha with a model and sketch drawings for the bridge. Its geometry is 'slightly peculiar' - inclined and curved on plan - and there could be no mechanical fastenings; everything has to be glass. Hunt thought it could be feasible and brought in one of the firm's bright young engineers, Wolf Mangelsdorf - who is also trained as an architect - to help. Together with Heatherwick and help from suppliers such as Hewlett Packard, Lighting Design Partnership, Davis Langdon & Everest, adhesive people and St Gobain glass, it is evolving as vertically laminated sheets of glass spanning 20m. Cynthia Grant at the lddc almost found a site for it, but that fell through; instead a 1:13.3 model has been built as a tempter for people with a 20m-wide stretch of water to be bridged. Meanwhile, those small niggles of using a material to do something beyond its normal scope are being ironed out, such as how do you make an all-glass handrail, slanting in one dimension and curving in the other, without gluing it so tightly that should one part break, it could never be replaced. Easy, you just . . . but perhaps secrets should not be given away.

'Ours is more radical than the other,' says the director in charge of the Sheffield office Allan Bernau, about his glass bridge, designed with Swiss architect Fillipo Broggini. This one is made of pressed glass domes which could float on water. 'Fillipo's into conceptual design,' says Bernau. They previously collaborated on the shortlisted design in a competition for the Millennium Footbridge in York, an extraordinarily elegant and cleverly balanced proposal, and a third-placed conference and exhibition hall in Como, Italy.

Such rivalry within a firm is generally healthy. Individuals are not satisfied with re-using a standard Hunt solution but look to find a new way of doing things. Not all the ideas emanate from one source. To stimulate this they run regular internal competitions: one, by director Alan Jones for an acoustic barrier at Gatwick, was entered for the open competition and won. 'It's an idea from pure engineering,' explains Jones, 'baa wanted a maintenance-free wall so it had to be concrete, but concrete only stands up if you give it form. It has to be self-stabilising and made from repetitive parts to be economical.' It has a flowing form, its shape generated by a simple, singly curved concrete block; when laid on alternate sides it creates a serpentine plan to add deflection of sound to its effect as an acoustic barrier.

At the Gatwick wall an idea from 'pure engineering' worked because its function is straightforward. Lloyd's Register of Shipping, another project with the Richard Rogers Partnership in the City of London, has the far more systemic set of functions found in a modern office. Consequently, its structural approach cannot be as 'pure'; but devising a solution around the myriad operational, aesthetic and legal conditions is in many ways more challenging. Here, too, the structure is concrete - exposed, so its thermal mass helps the energy strategy - but a composite frame of precast and cast-in-situ elements. 'We think it's never been done before in this way,' says Hunt. 'It's technologically advanced and very clever,' he adds before handing over to director in charge Bjorn Watson.

Watson explains that rrp wanted a structure which left as much clear wall space as possible, partly for aesthetic reasons and partly to get as much daylight as possible into the building. A model of the end of the frame became a 'logo' for the building: essentially its structure is just horizontal and vertical elements with two 'k frames' at either end to make it stable without infill panels. 'There is no other 14-storey precast-concrete building which is completely frame-stable that I know of,' Watson says.

The frame was originally designed to be constructed of in-situ concrete. But O'Rourke's, the specialist concrete subcontractor, made a mock-up which showed that it could not achieve the required quality of finish consistently. It suggested a composite system, and aha worked with it to get it right. The precast parts are hollow columns, U-shaped beams and elegant shell ceiling vaults 'which are only 90mm thick in the middle'. A new German technique which gives variable vibration ensures the minimum of air pockets to scar the finish, leaving the very clean surface which is necessary for exposed concrete. When assembled on site 'with tons of reinforcement to make it fully frame-braced', concrete is poured into the voids. So it has a high thermal mass and enough strength to be frame- braced (leaving walls as free as possible), with a fine finish, and the precast components are not over-heavy. aha is so pleased with the system that, together with O'Rourke's and Imperial College, it is looking at developing it further. And the drawbacks? 'It's very complicated to get the calculations right,' says Watson. He had to develop a theory of thick shells to show how it all works.

Steel is the other great generic structural material of contemporary architecture, and it is no surprise that aha is pushing technology here too. Its most dramatic steel structure is the giant glass toroid dome at Middleton Hall, the National Botanical Gardens of Wales between Carmarthen and Swansea. Designed with Foster's, it measures 100m by 56m at its widest.

A torus is best described as the face of a ball which is swung around on a string in circles. At Middleton the whole form has a radius of 140m, but only a segment is actually built. Placed on a low rise, it makes a pristine glass cap to a low hill which is flanked by higher ones: human intelligence working in the landscape. A wedge of this segment becomes a concrete base, inclined to the south, and covered in grassed embankments. The rest is steel arches and glass panels.

Although fiercely complicated, the toroid geometry does allow the structure to be described mathematically. It is elegant and crisp; the arches expand in width and therefore height closer to the centre, and they also tend to the vertical the closer to the centre they are. Under this complex but regular umbrella, the excavations of the ground add a carefully designed counterpoint.

Where the cleverness and skill lie is in two places. Firstly, there is the mathematical control necessary to describe the shape in the first place, and following on from that is the control of detail design to minimise the number of components and conditions. An obvious example of the latter is the repeated stainless-steel ball joint and steel socket which connects each arch, whatever its incline and span, to the concrete ring beam, at whichever elevation or point on its curve. The arches are standard steel tubes with T-sections welded to them for stiffness. Lateral stabilising tubes brace the structure. At the widest point, 19 sheets of glass make up the arch. So the 800-odd glass sheets have come from 19 standard shapes - along the long axis the condition is the same; shorter arches follow the same radius, they just have pieces removed from their lower sections. The glass sits on recessed glazing mullions which are glued to the structure. Its pared structure ensures maximum daylight penetration for the Mediterranean flora which will fill it, and some panels open for cross-ventilation on sunny days.

The Eden project, with Nicholas Grimshaw & Partners, is another botanical project which shares many of the constraints imposed on Middleton. The high spherical shapes forming the structure to the biomes have been generated by the need to fit snugly into the site profiles. Once again, the structure has been pared to a minimum to ensure the highest transparency possible, but in this case glass was unsuitable due to its dead weight and the complex geometry of the cladding. etfe foil cushions have provided the answer. The sculpturing of the site into an accessible form has presented a significant challenge for Hunt's young civil- engineering team.

Other steel projects are inevitably less dramatic, but with the potential for standardisation and re-application, no less interesting. The Dyson factory, designed with Chris Wilkinson, came from 'an idea to produce something interesting and cheap', says Hunt. The sinusoidal roof over the new building was initially proposed by aha's David Hamilton. With ties at quarter points in each curve, 'it's very efficient', and its weight is a remarkably low 25kg/m2. A very deep profile metal deck spans 10m, adding to the flowing elegance and giving interest to artificial lighting in the office areas. The curve is achieved with pre-formed standard I- beams turned round to give the serpentine form. 'Our aim was to produce an updated equivalent of Reliance Controls,' says Hunt, with 'as much repetition out of as few simple bits as possible'. Foreshortening lends an aesthetic effect: the arches appear to get steeper the further away they are.

And what of the future? Hunt himself may gently pull back in a few years and md David Hemmings might retire about the same time. But since the buyout the firm has consciously brought forward a new generation with considerable autonomy in the Sheffield and London offices. And several projects offer new challenges. In Sheffield, there is the massive expansion of the Don Valley stadium. Six sports are involved, says Allan Bernau, as well as the Northern General Hospital, Sheffield University and Hallam University. It is likely to be on two sites, with a variety of requirements for spans, shapes and sizes. Hunt himself looks optimistically to a kind of Disneyland in south Wales offering another variety of structural challenges. Big spans, large buildings, varied structures, materials, aims and architects - that's probably enough to keep them up with their historic competitors for a decade or two.

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