A fanciful notion? Perhaps. Yet such a concept forms the key element of one of Europe's most ambitious railway station redevelopment projects. Credibility for such an outlandish structure comes from the world's most acclaimed collaborators on tensile structures: German architect Professor Frei Otto and British structural engineer Buro Happold, which has an office in Berlin.
Germany is involved in a multibillion-pound redevelopment programme of its railway network to cater for high-speed trains. In several cities, such as Berlin, Frankfurt and Leipzig, this involves total redevelopment of the main terminus and its approach lines.
In Stuttgart, one of the most ambitious of these station redevelopments was submitted for planning approval in July. The scheme entails closing the existing terminus and developing a new station at right angles to it. Crucially, Stuttgart will no longer be a dead-end, as it is at present. High-speed trains will pass through the new station, which will have platforms almost half a kilometre long to accommodate them, and will then be able to hurtle through the city in either direction on Deutsche Bahn's new high-speed rail network.
As well as building the new station, Stuttgart's £2bn redevelopment involves laying 50 km of new track and boring 30 km of new tunnels. As at London's King's Cross, the proposed redevelopment is to be 30% subsidised by the sale of 50 ha of redundant railway sidings for lucrative city-centre commercial development.
The new station design was chosen as the result of an architectural competition. The winning scheme, by Düsseldorf architect Ingenhoven Overdiek und Partner with Otto and Buro Happold's German arm Happold Ingenieurbüro, was effectively a non-building. They proposed an entirely subterranean station, which would allow the city's historic central park to extend over it. The existing station hall would be converted to a spacious ticket hall with escalators leading down to the new station below.
As Christoph Ingenhoven, senior partner of Ingenhoven Overdiek und Partner, explains: "The new station will no longer just be 'Stuttgart's gateway to the world' [as the existing station was described by its architect in 1927], but also the key to the regeneration of the new city."
Obviously, the new station roof and base for the extended park will be the most prominent element in the building. A flat concrete slab would have had little presence, so Ingenhoven opted for a more baroque vaulted structure.
In addition, large, clear-glazed skylights will funnel daylight and views from the park down to the station below. These skylights, which resemble large tilted bullseyes, are the holes left by "tearing out" the columns from the deck above.
In terms of functional engineering, the sculptural vault structure is by no means as fanciful as it looks, despite the concept originating in tensile membrane design. The curvilinear forms correspond with parabolas, so that, in theory at least, all stresses operate in compression and run along the structural lines rather than bending and cutting across them. The effect is that the concrete decks can be reduced to a relatively slender thickness of 420 mm, with the columns tapering to a cross-section of 3 × 1.5 m at platform level. Direct compression also means that there will be minimal cracking under long-term permanent loads.
Scores of models were developed to devise the most efficient and elegant structural form. These started by blowing soap bubbles through wire hoops of varying shapes, with smaller hoops within them to represent the skylights. Models of hanging chains helped devise the most efficient paraboloid forms, with the shapes then transferred to plaster models. For detailed design, the models were transferred to Buro Happold's sophisticated 3D CAD software package.
In practice, the roof structure will not entirely act as a pure shell structure in compression. The edges of the roof will be cantilevered outwards and therefore subject to bending forces. Buro Happold is working out ways to post-tension the top layer of the insitu concrete structure to counteract the bending stresses.
The planning process is not due for completion until next spring. After that, development will stretch in phases over several years until 2007 and beyond.
Client Deutsche Bahn, Stuttgart Architect Ingenhoven Overdiek und Partner, Dusseldorf Structural Engineers Happold Ingenieurburo, Berlin; Leonhardt, Andra und Partner, Stuttgart Services Engineer HL Technick, Munich Special Consultant Professor Frei Otto, Warmbronn