Swiss tectonics.

Stations have always offered services beyond their obvious function. The Zurich Hauptbahnhof has been enabled to engage more creatively with the city by an ingenious moving fence.

The extensive Swiss railway system has played a crucial role in the country's development, with stations now assuming a broader function as centres for out-of-hours shopping and services. It is this important relationship with the city that Swiss architects Marcel Meili and Markus Peter (in collaboration with Axel Fickert and Kaschka Knapkiewicz) wanted to strengthen through their competition-winning entry to roof over the outermost tracks of Zurich station. Though readily comprehensible as a monumental gesture, the new addition is also a finely judged analysis of the relationship between form and construction. It is massive yet hollow, gestural yet rigorous, structurally exacting but not expressive.

The extension meshes the city with the station, which people approach from all sides and use as a thoroughfare. Extruded in a U-shaped plan, the new roof runs 240m along the north and south sides of the platform and between the tracks and station hall. Night security gates are positioned along the line of the roof edge, so that the extension and outermost trains become part of the city. The ingeniously devised gates unfold like an arm, cantilevering out of a covered trough set in the ground.

In cross section, the roof is clearly made up of two parts. Thrusting towards the city, the larger part spans 15m and concludes the row of six existing arches. The smaller, upper roof cantilevers beyond the station, terminating the gesture outwards. Materials are contextual, though the architects admit that at present, the timber lattice is especially startling. Laid in the long direction, it reinforces the roofs length and will acquire a patina from the large amount of steel dust produced by the trains. The underside of the upper roof is clad with plywood sheets.

The lattice on the lower roof's underside filters daylight and acts as a veil through which the skeletal construction dimly appears. This glimpse of what lies behind the form, or, more specifically, the play between form and construction, is a recurring proposition. The 900mm square concrete columns, for instance, are approximately four times as large as structurally necessary and are hollow. Their scale and apparent structural clarity are made ambiguous by canting them, and making the lower roof perpendicular to them. In contrast to the existing steel structure, which is riveted and clearly additive, the columns disappear into the roof and touch the ground without any articulation.

The practical and structural constraints were considerable. For a start, building at a busy train station without interrupting its operations meant that most of the construction had to be prefabricated. Moreover, the points where columns could hit the ground were predetermined, because a commuter rail station runs under the pavement and part of the platform bridges the river Sihl. The resulting span of approximately 40m means that each column carries a considerable load, so the roof construction had to be as light as possible. Two parallel trusses, which act like a tube, span between these columns, taking up most of the tension and torsion. The upper roof cantilevers off these trusses and the lower roof hangs from them, touching the existing steel columns which are spaced approximately three times as closely as the new ones. Between these columns spans a 2.5m tall Vierendeel truss which facilitates the change in grid between the new and existing columns and avoids diagonal bracing in what is essentially a clerestory.

The extension's monumentality relates both to the scale of the city and the neighbouring Neo-Classical station. It also affords the opportunity of unifying a station complex that had evolved piecemeal. But the architects faced what Meili describes as the 'Semper-like problem' of achieving monumental form with contemporary (i.e. non-massive), building technology. Embodying almost purely structural and tectonic considerations, the new roof elegantly resolves this dilemma.