Constructive urbanity: an attempt to generate a large urban building which relates to the form and experience of city life, while offering varied office accommodation, has generated a result surprisingly related to Constructivism.

The larger the building, the greater the danger of oversimplified forms, anaesthetizing repetition, deep plans dominated by structural discipline, and the creation of hermetic artificial worlds. As Rem Koolhaas pointed out in Delirious New York, sheer density can even force a complete change of approach. Manhattan's buildings became too tall and too deep to articulate their parts, so content and image were severed, paving the way for imposed and invented forms as nice as the Chrysler building or as nasty as Johnson's AT&T.

Building as landmark became autonomous. In a memorable German competition of the late '80s, Stirling as judge dubbed one entry 'lipsticks' as though a collection of towers imposed on the city was of no more consequence than the layout of the cosmetics counter. Now we see such 'lipsticks' jostling for attention in city skylines across the world. But the density of Manhattan or Hong Kong remains exceptional, and in European cities, the bulk of the fabric is much lower. Towers have remained the protruding gestures of power they always were. Koolhaas's romance about the swimming pool on the 30th floor remains a rarity. Even if the modest towers of European cities in the last half century do not reflect Manhattan's pressures, most have been dull to look at and worse to inhabit. Under orthodox modernism, structure and flexibility seemed to demand identical floors, lifts were the compulsory means of access, the perimeter was kept as short as possible, and air and light were supplied mechanically and electrically.

Things have changed. In the twenty-first century computers make complex structures and irregular component assemblies easier and cheaper. Sophisticated, well-insulated, layered facades prompt a policy of exchange and control as opposed to the former drive to minimize surface area. The growing energy crisis has provoked criticism of heavy servicing as unsustainable, suggesting that the 35 per cent cost of mechanical plant might be better spent in other ways. Instead of air-conditioning we might have shallow plans, daylight, opening windows, and end up using less energy. In the process, buildings might break down their large scale to articulate their contents and respond to place. Behnisch, Behnisch (pere et fils) & Partners show in Hanover--regional capital of Lower Saxony--that it is possible to extend their well-established principles of situational design to their largest urban building yet.

The architects understood that the building's duty was evenly divided between the locality and the city skyline, a much more balanced role than that of Foster's Commerzbank tower in Frankfurt (July 1997).

Energy in urban context

Set at the south-east corner of the old city, the site lies just beyond what was once the line of the medieval city wall. It faces the inner ringroad which, as so often in Continental cities, took the wall's place and today carries heavy traffic. To the east is Aegidientorplatz, a major node and traffic intersection dominated by an axially placed theatre. To the west is Willy-Brandt-Allee, across which is seen the imposing nineteenth-century town hall, with the beginning of Masch Park in between, a green lung stretching south. So the building occupies a transitional zone between old centre and southern suburbs. The plot was L-shaped and included the listed Siemens House at its south-west corner. The architects' strategy was to build a six-storey ring of double-loaded offices around the outside, with the 17-storey tower at the centre. They have given the ground floor back to the public realm as a series of shops and cafes, starting at the bank accommodation at first floor level. The central part of the site un der the tower could serve as the bank's cafeteria, divided from the public perimeter by a series of ponds. These improve the microclimate by evaporation in hot weather.

Since the street to the north is so loud with traffic, and Aegidientorplatz in the north-east corner such an important urban node, the architects decided to make entrances at the corners rather than classically at mid front. The main approach from north-east was defined by cutting away the two lower levels of the perimeter building in favour of a broad diagonal promenade leading to the main entrance. This diagonal gesture sets up the skewed orientation of the tower and prompts the rotation which differentiates it from the conventionally orientated perimeter. The varying but centripetal disposition of the upper floors creates a strong sense of interaction between centre and periphery, between unexpectedly rising landmark and conventional city block. Entering the foyer, the main stair leading to the bank's territory presents itself unavoidably to the right, with the lifts behind and reception to the left. The whole entrance foyer is covered by a sloping glass plane like that at Stuttgart, again emphasizing the direction of the stair with the upper level presented as balcony over. Beyond the cluster of vertical communication shafts--which also serve as structure-- lies the cafeteria with its lake, and south of the central node are kitchens, services and car ramps--definitely the

back.

Office variety

At first floor level the central node connects to the corridors of the peripheral office wings, and two glazed tubes span the ponded court as shortcuts. In five places the access corridors in mid plan are skewed, opening towards circulation nodes and breaking the monotony of endless passage. Often, too, glimpses are provided into inner courts or to outside world. For reaching further levels lifts dominate, but stairs are always adjacent, and local open flights here and there grant local connections from one floor to the next. A triple bank of centrally placed lifts services the tower, arriving in foyers which vary in shape from floor to floor and always allow a glimpse of view. As in other Behnisch office buildings, fully glazed partitions or partitions with glazed tops allow daylight into central passages almost everywhere. Varying plan depths in the wings are exploited to provide different sizes and types of office, from individual cells to big team rooms. A standard glass partitioning system was developed which can be adapted as needs change. The tower culminates, predictably enough, in board room and entertainment suite. It is interesting to observe how naturally the stepped forms and skewed angles blend into the top floor plan while accommodating and expressing pedestrian movement-directional spaces and a lively tower profile at the same time. This simple example reflects a policy which drives the whole.

The pursuit of radical energy and environment policies is a major interest of Stefan Behnisch, and perhaps the most obvious development he has brought to the firm's work. Surprisingly for such a large building, air conditioning is only used for underground rooms and service areas such as kitchens. Elsewhere are opening windows, and a system of largely passive ducts or 'air chimneys' to induce convection. On the outside faces is a typically Behnisch layered facade, with extra glass panels for acoustic isolation to the north, thermal glazing and external solar blinds on the other sides. Deployed blinds are specially designed to reflect a controlled amount of light inward to avoid leaving the room too dark Heating and cooling of offices is achieved by heat exchange through water pipes in the concrete floor slabs, whose mass provides thermal storage, and overall energy demand is reduced by heat exchange with deeply sunk ground pipes. These absorb heat in summer and provide it, stepped up from six degrees by heat pumps, in winter. Exchange with a district heating system is also involved, while solar water heating, photovoltaics and fuel-cells play minor and sometimes experimental roles. With all these measures, consultants Transsolar claim to have saved 1920 tons of carbon dioxide emission per year.

A state of flux 

Some critics find the work of the Behnisch office too chaotic and imperfect, too unmonumental. Yet the advantage of their responsive approach most visible here is that a big building can be treated like lots of small ones, almost like a mini-city. This holds the scale down and allows flexibility, for without a great preconceived plan, there is room to incorporate site influences, existing buildings-Siemens House quietly absorbed-and to allow for growth and change. Most important of all, human scale is retained throughout and numbing repetition avoided. Every floor gains a different character: in the tower the lobby changes shape or an office wing cantilevers further. There are frequent views out for orientation. It was wise to give the ground floor back to the city with shops and cafes to animate the public realm, while the protected water court marks the beginning of the larger institution as well as helping the environmental strategy. Lively and memorable, the tower as landmark is different from each side. Its appearance is enhanced by areas of special colour-coated glass added for their reflective effect, part of a colour design policy which is another late Behnisch trademark. The high-tech brigade may fume about the catholic variety of details in this architecture, but professionalism does not necessarily entail the perfection of the machine. The issue is more profound: Mies's 'God is in the details' reflects a monumental architecture striving for the eternal in its perfection, and flexible only through its sheer indifference to change. Behnisch's situational architecture accepts a state of flux, always open and always developing, allowing for growth, interacting with life.

Architect

Behnisch, Behnisch & Partner, Stuttgart

Project architects

Gunter Behnisch, Stefan Behnisch. Gunther Schaller, Martin Haas, Jorn Genkel. Alexandra Burkard, Martin Gremmel, Dominik Heni, Bettina Maier, Klaus Schwagerl, Jorg Usinger, with Christian Kandzia

Project team

Dirk Anhorn, Chiara Baccarini, Volker Biermann, Andrea Croe, Willy Haberer, Michael Huiss. Eckart Kruger, Birgit Mannsdorfer, Maik Neumann, Severin Russmann, Alex Sargeson, Ann Katrin Schilling, Noa Shatzmiller, Wolfgang Sterr, Roland Zimmerman

Landscape

Behnisch, Behnisch & Partner with Nagel & Schonhoff

Photographs

Christian Kandzia and Martin Schodder