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Design of the times

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STIRLING PRIZE 04 - Addressing issues from energy efficiency to staff transportation, 30 St Mary Axe has been labelled London's first 'environmentally progressive' tower.Barrie Evans nvestigates what makes his 'gherkin' ick

There is a history to streamlining buildings, not least by Foster-hero Buckminster Fuller.

This approach, applied in towers for environmental reasons, can be seen notably in the work of Ken Yeang, in the circular-plan RWE Tower in Essen by Ingenhoven Overdiek Kahlen (AJ 15.5.97) - the practice which came second to Foster in the Commerzbank competition - and in the Frankfurt Commerzbank itself. That building's planning logic is triangular but its envelope is rounded for air flow (AJ 20.2.97).

Foster's 30 St Mary Axe is described by Matt Kitson of engineer Hilson Moran as London's first 'environmentally progressive' tower. The big items here are the lifetime energy costs of running the building and of transporting the staff (though the reducing net-to-gross ratio as you build higher is a disadvantage). In the short term, the extra demands on London's creaking public transport system may be a problem, but a centrally located, 3,500-workplace building with cycle storage for one in 12 staff, but no car parking, is a good start.

The tapering cylindrical form promotes smoother airflow than would a rectangular building, with lesser air speeds at ground level to trouble pedestrians but higher negative pressures as the airstreams sweep around the building, providing greater pressure differences to drive natural ventilation (and natural smoke evacuation too).

The earlier RWE and Commerzbank towers needed to naturally ventilate directly through their double facades in order to comply with the German insistence on cellular offices. It is a difficult trick. At 30 St Mary Axe the spaces fitted-out so far are about 85 per cent open plan, so natural ventilation airflow can be across the floor-plates generally.

The spiralling atria, separated into six-storey packs, have the only opening windows in the facade; the atria then act as buffers, with office windows opening off them. As the atria spiral upward, on the side of an atrium where each floor steps back as it rises - the visually most open - there is a small balcony on each floor edge (and an air inlet route to offices behind).

On the opposite side of the atrium, the floor edge is the potential location for cellular offices with their own opening windows.

This is a mixed-mode building, switching from air conditioning to natural ventilation whenever possible. The switchover is informed by weather stations monitoring air speed, air temperature and rainfall. It is estimated that the building will run naturally ventilated for 40 per cent of the time. The assumptions here are that occupants work, say, 8am-6pm, five days a week at a density of 10m 2 per person (for late-night working, any one of the six plan-sectors per floor can remain operational selectively), and the tight institutional standards of temperature and humidity normal for air conditioning can be relaxed to 20-26¦C and 30-70 per cent relative humidity.

Maintaining internal conditions is almost entirely about cooling. While there is little exposed thermal capacity - floors are carpeted, ceilings suspended - there is the facility to flush-through the building at night by natural ventilation. The double-glazed cladding sits outboard of the structural gridshell. Inboard, for offices, there are louvre blinds, then a vertical laminated glass screen (convenient for attaching partitions); the void between the two sets of glazing is ventilated. Overall envelope U-value is 0.8W/m 2K. With motorised control of the blinds' louvre blade angle to one of four positions, the double facade is predicted to keep out 85 per cent of solar heat gain while admitting 50 per cent of daylight.

The triangular atria, which have the planning benefit of leaving the office spaces almost rectangular, also provide greater daylight penetration. Photocells in the office ceiling trigger the switching-off of banks of lights when daylight is adequate. This is not a building with user control.

The atria don't have the sense of scale and openness of Commerzbank, a reflection in part of Commerzbank's genesis in a climate that included a Frankfurt city government that was a coalition of Social Democrats and Greens, though Swiss Re did actively push the environmental agenda, beginning six years ago. One disappointment is that while both buildings show sculpturally where the atria are, from the outside the openness of the spaces within is masked, in the case of St Mary Axe, by dark tinted glass - a marked contrast to Yeang's buildings.

The dome at the top of the tower is permanently conditioned using displacement ventilation for the two floors within it. There are designs for revolving shading, spanning some 160-170¦ of arc, and servicing for this has been installed. Time will tell whether it needs implementing. Plant, such as for heat recovery, is distributed locally through the building. But major items are housed below ground in a separate building nearby.

Overall, the building's energy consumption is predicted as 150W/m 2/y, assuming full occupancy and casual gains of 50W/m 2/y. This consumption is for HVAC, lighting and small power. The energy consumption, and ensuing CO 2 emissions, would then be 50 per cent of those of a typical office building (as defined in 'Energy Consumption Guide 19') of the same type, size and usage. Of course, any tenant could choose to run its floors in permanently air-conditioned mode. But Swiss Re and the design team have made something better possible.

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