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Whether the building has begun to attain a life of its own
since occupation is a matter of judgement.
This was an attempt to design a building based on the metaphor
of a living system more like a termitary.
An ecosystem not a "machine for living in."

MICK PEARCE

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Sustainable architecture must satisfy the needs of present users without diminishing the prospects of future generations. It must also be embedded in its natural and social environment. Eastgate in Harare is an expression of two architectures; the new order of brick and reconstructed stone and the old order of steel and glass. The new order moves away from the international glamour of the pristine glass tower archetype towards a regionalized style that responds to the biosphere, to the ancient traditional stone architecture of Great Zimbabwe and to local human resources.

In the new order massive protruding stone elements not only protect the small windows from the sun but also increase the external surface area of the building to improve heat loss to space at night and minimize heat gain by day. These are made of precast concrete, brushed to expose the granite aggregate that matches the lichen-covered rocks in Zimbabwe’s wild landscape. The horizontal protruding ledges are interrupted by columns of steel rings supporting green vines to bring nature back into the city. The model used was the termitary; an ecosystem not a "machine for living in."

The old order comprises the lattice steel work, the hanging lift cars, the glass and steel suspension bridges and the glass roof. It is the architectural expression of the technology brought to Zimbabwe by the mineral hungry settlers in the late 19th century.

Eastgate comprises two buildings side by side linked together by a glass roof. Below this, steel bridges and lifts suspended on cables from steel lattice beams span over the atrium below. The lifts connect with a suspended glass skywalk which runs the length of the atrium at level 2. The centre of the skywalk is connected to street level by escalators and the street leads to the city’s web.

Along the ridge of the red tiled roof are 48 brick funnels topping internal stacks which pull the exhaust air out of the seven floors of offices below. Under the office floors is a mezzanine plant room behind the cross chevron screen where 32 banks of low and high volume fans draw air from the atrium through filters. This air is pushed up through the supply section of vertical ducts in the central spine core of each office wing. From the duct the air is fed through the hollow floors to low level grilles under the windows. As it is warmed by human activity it rises to the vaulted ceiling where it is sucked out via the exhaust ports at the end of each vault through a system of masonry ducts to the exhaust sections of the central vertical stacks. In the office space uplighters use the concrete vaulted ceiling to reflect light downwards and to absorb their heat.

The sandwich of the vaulted ceiling and the voided floor above acts as a heat exchanger. The cold night air passing through the void festooned with concrete teeth removes the heat of the previous day and on the following day warm external air is cooled about 3°C by the same teeth before entering the room. Normally the high volume fans run at night to give ten air changes per hour and low volume fans run during the day giving two air changes per hour. By timing the change over from low to high air velocities the optimum use of the diurnal swing of the biosphere can be utilized.

The engineers, Ove Arup & Partners, have installed a data logger which continuously records air temperature at five critical positions.

Eastgate uses 35% less total energy than the average consumption of six other conventional buildings with full HVAC in Harare. The saving on capital cost compared with full HVAC was 10% of total building cost. During the frequent shut downs of mains power, or of HVAC due to poor maintenance in the other buildings, Eastgate continues to operate within acceptable comfort levels with its system running by natural convection.

EASTGATE AND PASSIVE COOLING PERFORMANCE

The big question everyone wants to have answered is ‘does passive cooling in Eastgate work?’

The answer is summarised below and is based on a data logger which has been recording daily temperatures since occupation in April 1996. This measures:

the outside air temperature.
the concrete structural temperature.
the room temperatures at different levels within the rooms.

The buildings performance has been equal to or slightly better than that originally predicted by Ove Arup the Engineers.
The data logger graph shows that in average conditions covering ten months of the year 3°C of cooling between outside and inside temperature is achieved. Optimum cooling is achieved when the external night temperature falls below 20°C. When 20°C is exceeded at night; mainly due to cloud cover (which prevents efficient heat radiation back to space), and when the following day is clear especially in October or November the office temperature remains fairly warm.

The peak internal temperatures recorded at 4:00pm on occasions during 2 to 3 weeks of the whole year have been up to 27 - 28°C on certain days within that period. Usually during the rains in November onwards cloudy nights are followed by cloudy days and the internal office peak temperatures remain below 26°C.
People and machines add 1.5°C to the internal temperature each day which if not flushed out each night tends to accumulate until the week ends when the building cools down sufficiently. However, by altering our control system we have reduced this week day build up.
The difference between low level and high level temperatures in the room is 1.5°C due to stratification in a 3m high room.
The challenge for improvement is in the following two areas.
a) Control system which maximises the use of advantages provided by the unpredictable outside air temperature swings.

b) More research into designing of the heat exchange inside the concrete floor slab, involving studies of the behaviour of air and concrete surfaces to optimise heat transfer.

Already Eastgate’s success can be measured by comparing the annual energy consumption with other conventional buildings in Harare. The bar chart shows that Eastgate consumes less than 50% of the energy used in conventionally air conditioned buildings while achieving very satisfactory comfort conditions for all but 2 weeks in 52 weeks of the year.

bASIC DATA OF EASTGATE.

TIMETABLE

Commissioned March 1991
Design April 1991- Dec 1992
Construction Jan 1993 - April 1996
Occupancy April 1996

AREAS AND SURFACES

Site area 9313m sq
Ground Floor Area 9313m sq
Total combined floor area 55000msq

COSTS

Costs (without land) Z$ 354M / US$30M
Cost per sq m Z$ 6545 / US$ 595


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THE DESIGN AND CONSTRUCTION TEAM.

Clients Old Mutual Properties.
Architects Pearce Partnership.
Engineers Ove Arup and Partners.
Quantity Surveyors Hawkins, Lesnick and Bath.
Contractors Costain Sisk joint venture.

INTERNATIONAL AWARDS

International Council of Shopping Centres Certificate of Merit 1997 - Eastgate was awarded a Certificate of Merit by the International Council of Shopping Centres, New York. This is the first such award ever to be made of a building in Africa (including South Africa).

Fulton Award 1997- For excellence in the use of concrete.Concrete Society of Southern Africa

Steel Construction Award 1997 - Certificate of Merit.Southern African Institute of Steel Construction