Wessex Water New Operations Centre, Bath

Architects

Bennetts Associates Architects

Structural Engineer

Buro Happold

Steelwork Contractor

Wescol Glosford plc

Constructor Manager

Mace

Client

Wessex Water

Situated on a brownfield site just outside Bath, the £23M bespoke headquarters building overlooks an area of outstanding natural beauty. It incorporates a host of environmental features such as greywater recycling and passive climate control.

Placing sustainability at the heart of the design concept provided impetus for fresh thinking and approaches to the problem of finely-detailed exposed structure.

The office spaces have been designed to facilitate natural cross-ventilation and passive cooling. They feature a soffit formed of exposed sculpted precast concrete coffer units on slender steel beams. The exposed structure provides thermal mass to control temperatures within the spaces. A subtly curved rib is a feature of each coffer, providing strength as well as increasing the surface area available for heat exchange.

The steel frame has enabled a very light appearance. Only the bottom flanges of the secondary beams are visible, as slender dove-grey strips supporting the precast coffer units.

To express the lightness of the structure through the façade, a study was performed with the architect to establish the optimum column spacings.

Whilst the central columns are at 6m centres along each wing, reducing the edge column centres to 3m (one for every secondary beam) meant that the edge beam functions purely as a tie. A small PFC section sits below the edge coffer unit, framing the BMS-operated high-level windows which provide night-time cooling. The shape of the coffer is expressed externally through the glazing.

The primary "spine" beam was designed as a box-section, perforated to maintain the flow of natural cross-ventilation below the soffit.

The internal faces of the spine beam can be seen from the office space. It was decided to fabricate the beam from a pair of PFCs toe-to-toe, to allow shop application of paint to these faces.

Particular attention has been paid to the detailing of exposed steelwork connections. Machined pins are used to provide interest where the spine-beam-to-column connection is expressed.

It was recommended that the steelwork, precast units and painting to both should be procured as one package, under the leadership of a steelwork contractor.

Fire engineering studies carried out by Buro Happold FEDRA demonstrated that the secondary beams did not require applied fire protection, and that only the external faces of the primary beams required intumescent paint.

Whilst undoubtedly not the cheapest structural solution in capital cost terms (steel, precast floors and topping, painting and erection together cost around £220/m² in a typical bay), the system offered numerous holistic cost and environmental advantages:

• The optimal integration of the natural ventilation solution considerably reduces the amount of area required for mechanical plant, saving an estimated £50/m². It removes the cost and programme implications of supplying and installing ductwork (estimated saving £20/m²) as well as yielding a saving in running costs (estimated saving £50K per year).
• Saving in materials embodied energy: used half as much concrete as a coffered concrete naturally-ventilated design (or a third as much as a flat slab), with the same total weight of steel.
• In providing a first-quality exposed soffit with attractive details, the cost of a suspended ceiling was removed.
• The coffers integrate the lighting within the structural zone and maximise the sense of height and space within the storey height.
• The excellent quality of the working environment, developed around the integrated structure/M+E/architecture solution, offers potentially significant benefits in terms of increased staff productivity, and reduced absenteeism and staff turnover.

Judges' Comment

Carefully engineered modular design, creating a high quality office environment with a simple, but well crafted, frame solution. Thermal mass of this steel-framed structure is successfully utilised for internal climate control. A significant step towards the development of fully integrated building design, with good environmental credentials.