Vaulting ambition: with its ingenious suspended steel structure and low energy glass skin, this vast office building is a modern version of the industrial cathedrals of the nineteenth century.

In reality, the Berliner Bogen, a 140m-long steel and glass barrel vault of offices, has no site. The bulling is a bridge, hanging over a water basin at the end of a canal. It could be classified as a land reclamation project. The primary structure of steel arches was chosen to span a cavernous underground cistern, part of the city's drainage system, and avoid underwater foundations. The difficulties of the location, which the city wanted to be developed because of its prominence at the eastern gateway into Hamburg from Berlin, resulted in a bargain price for the buyer. The hug project cost only [euro]1300 per sq to complete.

The brief called for a landmark on water. The entrance in the north facade of the barrel vault is over an open, hard paved, elliptical piazza. Set on a raised metal podium in the centre, a glass cube containing a pump house is a multimedia information box publicizing the city waterworks. Opposite, the Berliner Bogen is reached over a footbridge spanning a shallow pool. A milky green glass funnel leads through two glass doors into the long public mall which feeds into lift foyers for the offices above and, at the end, onto an open balcony overlooking the canal to the south.

Eleven tubular steel arches, fixed with pin-joints on either side and rising to 36m, support the structure. Each arch bifurcates as it springs from the ground and is welded at the roof apex to its neighbour, forming a net of steel arches from end to end of the barrel vault. The external envelope is a glazed umbrella, below which tubular steel hangers, suspended from the spanning arches, carry eight concrete floor slabs to form house within a house. By hanging the building from above, as opposed to supporting it from below, floors are left virtually free of columns. Only the weight of the ground floor and two basements, with plant rooms and parking for 190 vehicles, keeps the underground cavern submerged 7m below water level.

The double skin concept dramatically reduces energy use. In plan the floors are two combs, back to back, along a central spine containing four nodes of vertical circulation and plumbing. Between the teeth of the combs, six landscaped courtyards are visible from the 1200 workstations and by passers-by in the street. Every alternate full-height window in the offices is a sliding door (with a protective steel balustrade), opening onto the internal gardens. The glazed vault shelters the inner house and the glazed courtyards are mediating buffer zones for ventilation, cooling and warming. Low-level radiators have only a back-up function. In fact, primary heating and cooling comes from the temperature storage capacity of an exposed concrete structure. An active carpet of mats laid within the slabs, containing heatresponsive fluid, regulates temperatures locally, depending on climatic factors assessed by computer. Only if windows are open overnight will the conventional small radiators be required to heat the offic e the next day. Running costs are so remarkably low that many tenants from conventional office blocks nearby have moved into the Berliner Bogen.

The project's unusual appearance beside a busy road has made it a talking point for locals ever since site work started. Using temporary formwork, the concrete house within a house was constructed before the

OFFICES, HAMBURG, GERMANY

ARCHITECT

BOTHE RICHTER TEHERANI

supporting steel was erected. Steel suppliers and erectors were all from eastern Germany where pay for craftsmen is still lower than in the west. Each arch arrived in five sections and was welded in situ after tolerances had been adjusted to account for settling under load. Above 20m, steel members needed only a coating of fire protective paint. This has allowed the structure to be seen and given the building its powerful image.

The structural engineer and architect together developed the vault glazing system. Horizontal tubular-steel purlins running over the arches in turn support vertically aligned tapered arms ending in stainless-steel fixings. Mounting of the 14000 sq m of glass was carried out by construction mountaineers.

Butt joints produce an uninterrupted glass facade over which rain falls into a border grating on both sides of the vault and then directly into the canal. High-tech and a '60s sci-fi-comic vision of our future meet in the Berliner Bogen. Bright red lift foyers against cool silver neutrality, curved surfaces and sculptural slit windows with rounded corners set in white concrete, metal grid mesh walls and floors, invisible light sources with uplights and downlights so that building elements give the impression of floating. In space or on water? Walking down the mall between two rows of inward leaning oval concrete columns, images of spaceships or submarines come irresistibly to mind.

The Berliner Bogen is a physical experience. The tension of the bow (Bogen) can be felt. A modern steel and glass architecture matches Hamburg's vaulted main station -- with its elevated train tracks that were copied in Chicago and New York -- the city's wholesale market halls, now turned into exhibition spaces, and its river bridges. Berliner Bogen follows in the steps of earlier industrial cathedrals, overwhelming the laity with the awesome power of technology.