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Watertight design

For its new operations centre, Wessex Water wanted a green design to reflect its commitment to sustainability.Bennetts Associates delivered an unintrusive, eco-friendly solution

It is hard to imagine a more beautiful site.

The new operations centre of Wessex Water faces south on the edge of the Limpley Stoke Valley, an area of outstanding natural beauty on the outskirts of Bath, and an appropriate place for a very green building. A BREEAM assessment has shown the Wessex Centre to be the greenest commercial office in the UK, consuming less than a third of the energy required to power a standard headquarters office building.

Bennetts Associates has snuggled a substantial 10,000m 2headquarters for 580 staff into the landscape. The building follows the contours of its sloping site in three steps, and nowhere is it higher than two storeys.

The plan is E-shaped, with the long spine of the E - the 'street' or main circulation route - running north-south and stepping down the slope with two staircases. The three horizontal legs are open-plan office spaces. These spaces are set on an east-west axis to catch the prevailing wind, enhancing natural ventilation and giving everyone the chance of views over the wooded valley.

Communal spaces, such as the restaurant and meeting rooms, are set at the west side of the 'street' and act as an environmental buffer zone, protecting the offices from the afternoon sun.

The layout has produced a building with a great diversity of space and a wonderful quality of light. You arrive (served by a shuttle bus to reduce car use) at the upper level.

Through the glazed entrance is the hall, which is roofed with a series of vaulted coffers, supported by a delicate steel framework, and floored with natural stone. The hall flows into the width of the 'street', revealing glimpses of the open-plan office spaces on the east side. It steps down, with pauses for a waterfall sculpture, to continue at a lower level, where it opens out to meeting rooms and a glass-walled restaurant.

The three wings of office space open off the east side of the 'street'. Each has an identical layout, with its own business centre, kitchenette, meeting rooms and WCs. To optimise natural ventilation and daylight, the floors are only 15m wide. On the south elevations the facade is protected from solar gain by internal blinds and a two-storey external brise-soleil.

The structure of a typical office wing consists of a box-section spine beam of paired channels, supported on a central row of steel columns. Secondary beams run at right angles to the spine beam at 3m centres, and extend to the north and south facades to rest on delicate 120 x 120mm SHS columns.

Precast concrete coffer slabs run between the secondary beams. Their solidity contrasts with the delicacy of the steel structure.

Wessex Water has a commitment to sustainability and wanted its headquarters to be 'an exemplar of environmentally sensitive architecture'. It commissioned Bennetts Associates because the firm has long recognised the link between economic prosperity, low-energy issues and the quality of the workplace. Bennetts Associates' approach to sustainable architecture has been continually refined since it designed Powergen, its first low-energy headquarters building (AJ 2.3.95), followed by the John Menzies headquarters (AJ 30.11.95) and the BT headquarters (AJ 7.10.99) in Edinburgh.

On its previous low-energy office structures, Bennetts used in-situ and precast concrete coffer slabs to achieve the required thermal mass to cool the building.At Wessex Water the aim was to achieve similar thermal performance but using less concrete, to reduce embodied energy and consumption of resources. In conjunction with Buro Happold, the structural and building-service engineers, a system of precast exposed concrete coffer units was designed. The coffers and structural topping screed use 50 per cent less concrete than earlier designs, giving savings in the cost of foundations and allowing the use of a lightweight steel structure.

CREDITS

ARCHITECT Bennetts Associates

STRUCTURAL AND SERVICES ENGINEER Buro Happold

LANDSCAPE ARCHITECT Bernard Ede/Grant Associates

CONSTRUCTION MANAGER Mace

SUPPLIERS steelwork and composite precastWescol Slosford Structures; cladding, structural, solar shading Merlin Specialist Products

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