Wind sock: the integration of structure, form and fabric creates London's first environmentally progressive skyscraper.

While Charles Jencks may state that big is boring in his theory that measures design tedium against floor areas (AR August 2002), it cannot be denied that Foster and Partners' 30 St Mary Axe is an impressive sight. Regardless of whether or not you agree with Jencks' proposition (which states that for every additional ten floors added to a skyscraper, the design becomes twice as tedious), as the first truly sky-scraping tower built in the City since 1979, this building cannot be accused of being a monotonous skin-deep icon. Clearly visible from far afield, most impressively perhaps from the MII as you approach London from Cambridge, the more people you speak to, the more you realize that this is becoming one of London's more popular landmarks. While acquiring many nicknames, being likened to a fat banker in fish nets to the slightly more fitting city attire analogy, namely an Argyle sock, Foster and Swiss Re's environmental aspirations have been delightfully interwoven with its form (regardless of whether or not you believe the pursuit of these priorities preceded the form's rationalization).

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As essentially a commercial office development, 30 St Mary Axe is designed to be lettable on the open market. But it is unlike the norm, where architectural design effort focuses predominantly on the outermost six inches of the facade. With St Mary Axe, structure, form and fabric have been integrated, and Foster and Partners have produced one of the City's first large-scale office buildings which genuinely has the capacity to be passively ventilated.

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The building's distinctive pattern is a direct reflection of its internal organization and its environmental strategy, where six orthogonal fingers of flexible office space are punctuated by radial atria: a series of two and six storey voids that spiral around the building, increasing perimeter desk space, and bringing light and air deep within the heart of the building's circular envelope.

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In cladding the tower, Fosters were able to continue their innovative relationship with German cladding contractor Schmidlin, with whom they collaborated on London's City Hall (AR August 2002). Through parametric modelling techniques, deriving the critical co-ordinates of each panel mathematically rather than relying on traditional drawing techniques. Foster and Schmidlin demonstrate that material and component efficiency no longer rely on monotonous repetition. With emerging production line methods, where units are fabricated from palettes containing bar-coded precision cut components, it is no longer the case the incremental variations send costs through the roof. In this case, with the cladding geometry changing at every level as the floor plates increase from the 50m wide first floor to 57m on level 17, before diminishing to the 25m wide private dining room at the summit, an economic solution was reached. Within the principal diagrid established by the 36 steel columns that spiral around building (which form an independent self-bracing structure), each floor level is broken down into 72 five-degree modules. Within this subdivision, the envelope on each level is then formed by just two variant diamond-shaped units; a flat unit that spans between floor plates, and a folded unit formed by two triangular panels, which neatly incorporate a floor plenum air intake slot.

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Unlike County Hall, however, St Mary Axe does not have an axis, and therefore has no north/south condition to respond to. So, environmentally the building had the inherent problem of a circular plan, in that the facade could not be orientation specific if a single cladding system is wrapped around all 360 degrees. The challenge was therefore to develop a solution which while working in all orientations would maintain a uniform external appearance.

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The architect's response was to break the facade into its component parts, allowing each environmental control component to operate at its optimum performance when and where required. Instead of a high, performance glass system throughout, the workspaces are glazed using standard clear low-E units, with a responsive retractable layer of blinds and secondary glazing that could be deployed internally. Conversely, the atria breathe through a clutter-free single skin incorporating opening lights, which has a high performance solar control glass that does all of the work all of the time. While clearly there is a degree of redundancy, as the solar control glass has little benefit on the northern section of the facade, visual continuity had to be prioritized. Clear glass within the workspaces exploits views and increases daylight levels, while the double skin forms a thermal cavity, within which, when deployed, blinds reduce glare, stop sunlight reaching the inner skin, and also assist air recirculation.

While for commercial reasons the building had to offer a base condition of mechanical ventilation, with provision made for ducting routes etc, it is hoped that tenants will choose to naturally ventilate their spaces. The natural ventilation mode was a significant driver in refining the building's form and skin, and following extensive CFD and wind-tunnel modelling it has been proved that, as the atria cut across the high and low pressure zones created by the curved form, a pressure gradient is created that will improve cross ventilation between two-pack atria, and boost the natural buoyancy of the stack effect through the six packs. As a result it is anticipated that the building could be naturally ventilated for at least 40 per cent of the year, setting the standard for other commercial developments to surpass.

Architect

Foster and Partners, London

Structural engineers

Arup

Environmental engineers

BDSP Partnership

Mechanical and electrical engineers

Hilson Moran Partnership

Photographs

Nigel Young

Skyline view (no 1)

Smoothe