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Delicate Essen.

This headquarters tower sets out to demonstrate how tall buildings can be made which reduce energy use and allow individuals control of their surroundings.

When the results of the Commerzbank competition were announced (AR May 1992), it was clear that the second prizewinning scheme was in the same league of inventiveness as the first. Christoph Ingenhoven's design consisted in essence of two concentric glass cylinders separated by a wide annular cavity (varying in width) which contained a range of climate modifiers, including plants which would oxygenate the air and shade the interior. Next year Ingenhoven and his partners won the competition for the RWE headquarters in Essen with a very similar design which was ironically completed late last year, a few months earlier than the bank. It is clear that the architects regarded the matter as a contest, for they announced when their building was finished that 'It is not Frankfurt, Germany's banking centre, that can lay claim to having Won the race to build the first pro-ecological high-rise in the world ... The laurels go to the Ruhr Valley ... The city of Essen whose population is slightly less than that of Frankfurt ... is known as the office capital of the region', a region that is recovering from having been one of Germany's rust belts by wholeheartedly embracing ecological policies and new high tech enterprises. This is an example of the traditional rivalries between German cities working at their best.

The basic design has undergone one or two changes in its transition from the centre of Frankfurt to its site in Essen's Opernplatz more or less opposite Aalto's theatre (AR June 1989). The most important alteration is a reduction in the width of the cavity to half a metre and the abandonment of any attempt to colonise it with plants. But the basic principles remain. The plan form of the tower remains circular because that shape offers the lowest surface area to volume ratio obtainable this side of a sphere, a strategic decision scarcely compromised by locating the main lifts in a semi-detached glass tower. The 120m, 30 floor main tower is linked in its lower floors to a seven-storey block which is angled to follow the street line. A great portico through the low piece signals the entrance (a little too grand perhaps, but at least it lets you know where you are). The ground floor of the tower is a double-height hall in which the structure is visually reduced by making the slab above exceedingly thick so that most of the loads of the floors above can be transferred to five massive columns. A stair leads down to the foyer from which the cafeteria and glazed staff restaurant spill out towards the lush garden that occupies the middle of the urban block.

Standard floors have a ring of office accommodation 5.85m wide, within which is a circular corridor that surrounds core activities: meeting rooms, lavatories, stores and so on. The German proclivity for individual offices makes, as usual, for unpleasant mainly artificially lit corridors, which are made the more anonymous and placeless because of the circular plan. How much more genial and orientating it would be if a few of the offices had glazed partitions to the corridor so that you could see out, but this would not be fair on the occupants of the particular cells, so uniformity is imposed throughout, except on the top six floors where the senior management lives. Up there, changes in plan-form are allowed which permit, for instance, small waiting areas to link corridors and periphery; at these levels, the core is partly taken up with open glass staircases that link floors and make the whole place much less claustrophobic.

But in many ways, such areas are not crucial. The building is an argument about the nature of standard office space in towers, and it must be admitted that the quality of the individual cells is outstanding. They are of course wedge-shaped, a form that becomes a little cramped at the narrower end of the smallest offices. But the shape emphasises the stunning relationship to sky and city offered by the floor-to-ceiling glazing. The prospects are so breathtaking that, at first, you do not realise that there is a double skin, or that it can do very clever things.

The metal ceilings curve slightly up at the edges, emphasising views and increasing daylight penetration by a surprisingly large amount. The curve of the ceilings is continued and emphasised in the aluminium horizontal members that form the edge to each floorplate. The architects call these 'fish mouths' because in section they resemble the elegance and complexity of the mouth parts of a herbivorous fish like a carp. Structurally, they carry the glazing: the outer layer of flint glass which is point fixed to its mullions and the double inner layer of heat insulating units. And they allow the building to breathe. Air enters through the fish mouth slot and is directed by baffles up into the cavity through perforations in the top of the section. Hot air is drawn by convection to the top of the cavity through perforations in the bottom of the section and is ejected through the fish mouth. Alternate bays breathe either in or out, so even the smallest two bay office can do both.(1) The inner glass panels can be moved up to 150mm sideways(2) manually (using a winding handle) so that, as in Frankfurt, even the highest offices can have direct access to fresh air without experiencing a gale.

When wind speeds become too high for comfort (7m/s in the lower 15 floors and 10m/s above) an alarm sounds, warning occupants to shut their windows. Glare and insolation are controlled by motorised aluminium venetian blinds in the cavity (activated by switches near the doors) and fabric blinds in the offices themselves. When windows are shut, air is input and returned from ducts in the corridor. Heating is from perimeter radiators in the floors and there is provision for ceiling cooling, though the architects do not believe it to be necessary, or even (being a mechanical system) particularly reliable.(3) Plant floors on the 19th and 20th levels handle all the internal air and hot and cold water.

These floors make an opaque collar round the building, the transparent purity of which is also modified by the vertical streaks of the escape stair stacks. Yet the glazing is elegant as well as innovative (the architects have been able to hold Gartners in check here, whereas the window manufacturers seem to have been allowed too much freedom in Frankfurt). The whole is a notable achievement, according to one German critic, the 'most beautiful tall building in the whole country'.(4) Whether it works as well will be revealed during the next year as the building is scrupulously monitored in all climatic conditions.

1 There are vertical glass elements in the cavity every two bays which reduce horizontal sound transmission from office to office and ensure that there is air inlet and outlet for each room. Vertical sound transmission is cut down by the fish mouth sections.

2 The narrow opening is to prevent haughtiness and accidents. Window cleaners can open the panes fully.

3 See Evans, Battle, 'Through the Glass Cylinder', Architects' Journal, 15 May 1997, p44.

4 Fuchs, Claudia, 'Konzernzentrale von der RWE AG in Essen', in Baumeister 5, 1997, p33.
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Title Annotation:pro-ecological building in Essen, Germany
Author:Pearson, James
Publication:The Architectural Review
Date:Jul 1, 1997
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