Critical mass.

Using principles of natural heating, cooling and ventilation, this colossal building in Harare provides an intriguing model for a more sustainable form of commercial development.

Originally developed in the temperate climate of Europe, the air-conditioned glass tower continues to be exported indiscriminately around the world. From Buenos Aires to Beijing, central business districts are characterised by these homogeneous symbols of corporatism and profligacy. Even in Africa, with its extremes of climate and scarcity of resources, the model is unthinkingly replicated. So it is encouraging to report on a new building in Zimbabwe's capital Harare that could provide an alternative and inspiring paradigm for future development. In this case, concern for energy use has resulted in an architectural form far removed from the received image of a modern office block.

Lying 18 degrees south of the Equator, at an altitude of 1500 m, Harare experiences a tropical, high altitude climate. An eight-month-long, warm dry season is followed by a four month spell of increased heat and humidity. Clear skies throughout the year, combined with long hours of sunlight, generate extremes of temperature during the day and night.

The new building is located in the eastern area of Harare's central business district, occupying nearly half a city block. It provides 26000 sqm of office space, combined with 5600 sqm retail space and parking for 450 cars, making it the largest mixed development in the country. As such, it might be expected to adopt conventional commercial forms of architecture and servicing. However, at the instigation of the architect Pearce Partnership, the developer became convinced that the building should, as far as possible, be ventilated, heated and cooled by natural means.

Using computer simulation of heat gains, air flows and temperature gradients, engineers Ove Arup sought to devise an alternative passive services strategy. The principal aim was to provide ventilation, since Harare's climate effectively reduces the need for an elaborate heating system. Certain parameters were established, concerning the building mass (to be heavy as possible, with much of this exposed internally), the external walls (shielded from direct sunlight) and the windows (to be small and not exceed 25 per cent of the surface area on the sunny north face). In addition, windows were to be sealed to avoid unpredictable temperature fluctuations and all ventilation was to be ducted.

The architect's response was to create two narrow nine-storey office blocks oriented east to west and linked by a 145 m long glazed atrium. The heroically muscular appearance of the blocks reflects the thermally massive structure of in-situ concrete, clad in brick and precast concrete units. The floor slab extends to form continuous balconies, providing shade to the windows and articulating the elevations.

The glazed roof of the atrium is supported by a network of lattice beams. Additionally, four sets of bridge beams span the atrium at 35 m intervals. From these are suspended the vertiginous open bridges that link the blocks together at each floor, along with the vertical circulation cores of lifts and stairs. At each end of the four bridges is an entrance lobby, making a total of eight access points on each floor. This gives increased subdividisional flexibility -- office suite sizes can range from 75 sqm to 5000 sqm.

The vertical circulation cores are linked longitudinally by a suspended concourse at second floor level, which is also the main entry level for the offices. Access to the concourse level is by sets of escalators rising from the pedestrian street at ground level. Both ground and first floors are given over to shops, bringing life into the base of the building.

The enormous thermal mass of the building is used to regulate the air temperature. Only the first two floors and the parking basements are mechanically ventilated. Immediately above the first floor, behind an open cross-grille string course is a service storey containing the large low-speed air intake fans that provide forced air to the offices above. Air distribution is through 32 vertical supply ducts located along the central core of each block. The ducts supply plenum chambers under the office floors and the offices themselves via grilles below the windows. The hollow floor structure was formed by placing precast concrete stools on the in-situ slab and screeding over. The stools have dentellations on their underside to increase the inner surface area of the voids and thereby encourage air turbulence. These devices serve to increase the rate of heat transfer to and from the mass. As the concrete temperature is expected to remain at around 20 deg C, this mechanism can be used alternately to cool air during the day and heat it during the cool nights. The slab soffit is vaulted to increase the surface area exposed to the offices below, thereby increasing the slab's ability to absorb local heat gains. Exhaust air from the offices enters high level bulkheads parallel to the core and then moves horizontally to vertical extract shafts. Eventually, the slow moving mass of waste air is discharged from the building at roof level, through 48 splendidly bulbous chimneys.

Optimum performance is expected to be achieved by supplying air at about two changes per hour during the day. This is increased to seven changes per hour at night, to accelerate the cooling of the slab. Large fans are used for the night-time pre-cooling and smaller fans for daytime ventilation. The timing of the day/night changes varies with the season to take advantage of changing ambient temperatures. Artificial lighting is by fluorescent tubes, directed upwards towards the vaulted concrete ceiling. As well as generating less glare, this ensures the heat gain is absorbed by the slab rather than the room. Room heaters are provided for the cooler winter months. These are activated both locally and centrally, so that occupants may control the heat in the rooms and load shedding can be regulated by the building management. Peak electrical demand using this system of natural cooling and heating is estimated to be 20 times lower than conventional air conditioning.

The design and servicing of the building clearly illustrates the importance of environmental responsiveness. In rejecting the ubiquitous aggressively technological approach (particularly inappropriate in the context of Zimbabwe's lack of resources) Pearce Partnership sought to adapt vernacular responses to environmental control. The most evident examples of this are the creation of mass, by assembling modestly sized precast concrete units (unit size is limited by production facilities and available cranage) and the shading of walls (achieved by cantilevering shaped sunscreen units off the structure). Juxtaposed against the great brick and concrete mass is the angularity of the structural steelwork.

Public reaction to the building has been largely positive, following initial bemusement. Inevitably, an informal process of re-education has taken place, but both tenants and visiting public are beginning to accept that the building is very different and are responding accordingly. For example, all but one of the shop tenants have been persuaded not to install local air conditioning. Office workers accept an ambient temperature range of between 21 deg C and 25 deg C, against the air-conditioned absolutes of before. More adjustments lie ahead as the building goes through its first annual cycle and the behaviour of both building and its occupants is closely monitored.