The eventual solution adopted by Arup was offices planned around a central atrium with air movement generated by solar gain to the atrium.
A series of tests were carried out and found that in summer the system worked well, mainly due to the thermal mass of the structure. In winter the surprising result was that the occupants did not open the windows as expected, resulting in under-ventilation and fairly high concentration of CO2 at up to 900 ppm1.
The original brief for Gateway House was to service a new 14 000 m2 office development with a low budget which was to stand out from other offices buildings at that time. This brief was achieved by the use of low energy, self-regulating systems using a minimum of simple, tried-and-tested technology.
But times change, and like many owner occupiers, Pitney Bowes wanted to realise the financial benefits of selling the building to a property developer, in this case Castlemore. The developer aimed to create a modern office space for multiple occupiers. This meant that the engineers had to create a building that not only meets the developer's current criteria, but that can also be adapted by future occupants.
The structure of the original Gateway building has not been fundamentally changed. The site slopes down from north to south by two stories over the width of the building. The lower two levels are given over to car parking, plant areas and supplementary office space, above which there are five stories of office space. At the centre of the main office space is a large, full-height atrium. In the original Gateway building, office space was separated from the atrium by wood panelling partitions.
The natural ventilation system was based on opening windows around the perimeter of the building providing ventilation to office areas. Air entered the atrium via gazed louvres in the wood partition, and was ventilated at high level. Mechanical ventilation was limited to areas such as toilets and the computer suite.
The major refurbishment work on the Belvedere consisted of stripping back the building to its existing shell. The partitions between the office space and the atrium have been removed on all four floors, creating a very light open space, although areas of the atrium were filled in to provide additional office accommodation. The existing floor provided limited access and flexibility, and this has been replaced with a new raised floor with tiles on pedestals co-ordinated with the coffers below.
The visitor entrance to the Belvedere Basingstoke has been one of the main areas of refurbishment. The north west facing entrance has been retained, but remodelled into a double-height space. The new reception desk provides facilities for the switchboard and the adjoining security suite contains the bems terminal, security, and fire alarm panels. The entrance area is heated through new ducts rising from below to discharge at floor level around the full height perimeter glazing.
The atrium is large enough potentially to be used as a cafe area, and there is an area on the ground floor which could serve as a kitchen. This atrium had underfloor heating which has been retained. There are cantilevered circulation walkways running around it from floors one to five which have new balustrading. There are also high level central walkways running above the atrium. The underside of the walkways has been treated for sound control. The glass roof creates a very pleasant, well lit and ventilated area. At the east and west ends of the atrium there are hydraulic lifts.
Adding cooling
The most radical change, in terms of services, was the addition of a chilled beam and mechanical ventilation system. The system, provided by Essex-based Halton, is a passive beam system, with air floor outlets. The design criteria are maximum internal summer temperature of 24°C±2°C in the office spaces only; and internal winter temperature of 21°C, again in office spaces only.
Peter Bull, associate director of Arup, comments: "Using a chilled beam method of cooling, with a low level air supply, compliments the original low energy and passive environmental design intent."
Initially, the plan for the building was to add false ceilings, hiding the pre-cast concrete coffers. However, it was eventually decided to leave these coffers exposed. "This decision maximised the available floor to ceiling height, and provides a feature rather than a bland suspended ceiling," says Bull.
The beams were bespoke manufactured to suite the requirements of architect TTSP Fookes, and Arup. The longest is 7 m, the shortest 5 m. The beams are split which allows for future partitioning in the office space. Halton sales manager, Nigel Atkinson, says: "Because there is no false ceiling here, the water pipes come down the dropper into the coil from the floor above. The depth of the coffers means that this is largely invisible to occupants. The beams are suspended by wires and grapple hooks." Atkinson explains that acoustic baffles have been added to the coffers in order to achieve sound reduction.
With chilled beams there is always the problem of potential condensation to deal with. At Belvedere, chilled water is normally distributed to beams at 14°C. The bems monitors arrays of beams to detect the formation of condensation, at which time the flow of water to that array is stopped. The bems also monitors space conditions and can raise the chilled water temperature to the various zones when possible, which saves energy as well as reducing the risk of nuisance condensation.
Air enters the occupied space through floor outlets. Supply air is delivered at 1·8 litre/m2 of which the fresh air provision is 1·2 litres/m2. At each corner of the building and on each floor, four large supply air ducts can been seen which bring in the supply air from outside. Again, these have not been disguised, but used as an architectural feature. Two air handling units have been placed on the roof, with the chiller beneath the building adjacent to the car parking area.
New for old
In the plant room, use has also been made of existing equipment, with additions of new kit where necessary. Three lthw atmospheric gas-fired boilers of 400 kW each provide heating for the building. Existing pumps were given a full service by their manufacturers and reused. The plant room also has an empty space which could be used in future for an uninterruptible power supply, for example. Some items were in working order, but spares would not have been available. These have been replaced.
The trend towards refurbishment of offices is a growing one. There will doubtless be times when engineers are faced with the challenge of converting a naturally ventilated space into one which meets the demands of property developers and their clients for cooled offices.
The question is not whether this should be done, but how sympathetically and efficiently this can be achieved. Belvedere Basingstoke offers a good example of how engineering techniques can help clients meet their needs without taking the most obvious route.
Reference
1 Results of tests written up in the Advanced Case Studies Seminar, April 1991, published by The Renewable Energy Promotion Department, Energy Technology Support Unit, Harwell Laboratory, Oxfordshire. The document was compiled by Databuild Ltd, 4 Venture Way, Aston Science Park, Birmingham.Source
Building Sustainable Design
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