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Sustainable credentials

Architect Stride Treglown's use of timber in two buildings in Bristol - part of a new office complex in the old Temple Meads railway yard - has highlighted the green nature of this under-utilised building material Glass, steel, concrete yes. But you do not necessarily associate atria with the other mainstream building material, timber. It may be something to do with the perception of fire ratings. Yet concrete has to be fireproof, steel has to be fireproofed - and it is not exactly difficult to impregnate timber with fire retardants. Spread of flame is probably seen to be the big issue. Despite promising work by the BRE on its big, multi-storey timber structure test rigs at Cardington, timber is still likely to find most of its use in commercial buildings as skins and fittings and furniture.

But Bristol-based Stride Treglown has used timber panelling in two new buildings - part of a new office complex in the old Temple Meads railway yard. The first building has a trapezoidal plan with a similarly shaped atrium in the middle, lit from above by a glass roof. The second, just completed, has a roughly U-shaped plan with one side of the atrium a sheer planar glass wall. In both cases the architect has used laser-slotted timber panelling which has, according to the clients, a humanising effect and, with the aid of an absorbent quilt, attenuates sound in spaces which have the potential for prolonged echoes.

BREEAM rating Temple Quay House is the earlier of the two Stride Treglown buildings on this brownfield site and has special sustainability credentials. The client was a partnership between developer Castlemore Securities and the prospective occupant, the DETR - now DEFRA.

Their brief was that the building should be green and, although what that meant was partly down to the architect, they asked that the building should have an 'excellent' BREEAM rating. The architect and services engineer achieved that with a score of 74.7 and an environmental performance index (EPI) score of nine out of 10. This was partly achieved by measures such as harvesting rainwater for WC cisterns and indoor plants - and the use of a brownfield site.But the main measure was designing the office as a naturally li and ventilated building with back-up during extremes of temperature. The atrium's natural stack effect is used to draw air from the offices where it is exhausted via louvres in the glass roof. The exposed soffits of the concrete floors are deployed in the service o absorbing heat by day and releasing it during a night-time cooling cycle when the roof vents are opened.During winter, heat in the atrium is recycled to indirectly pre-heat fresh air and adiabatic, rather than steam, humidification is used during this cold season. Whatever artificial light is needed is controlled by detectors in each light fitting.

Martin Pease, project architect for Stride Treglown, says although green buildings are reckoned to be more expensive to build Temple Quay House shows it is possible to go convincingly green at current market rates.

The DETR/DEFRA had occupied a number of buildings one of which had a steel, concrete and glass atrium. Pease explains:

'They wanted a counterpoint to that, a building which was soft and warm and inviting.We did a lot of work on the kinds of materials we could use to achieve that. Acoustically the space was like a large cathedral.What we needed was something that would absorb sound.'

Part of the design solution was to use curving balconies that extended the edges of the office floors on the south and east edges of the atrium. The glazed office walls remain orthogonal but these flowing walkway routes, however redundant they seem in terms of pedestrian traffic, actually animate the space - and also provide lots of soffit area to which acoustic treatment can be applied.

The basic sound-absorbent element is CNC machine-cut slotted 18mm ply laid over a 50mm absorbent quilt. This is used as flat, shaped acoustic ceiling panels to the balcony soffits, and as rectangular spandrel panels to all the office windows overlooking the atrium. All these came from Kidlington firm Kingerlee. The windows and spandrels are framed in timber painted the same grey as the structural steelwork of the balconies.

According to Pease, the great advantage with CNC machine cutting 'is that you can have curved slots just as easily as straight slots'. The curving slots to the shaped panels of balcony soffits are reminiscent of the filigree on a guitar. The sweeping curves of the balconies, reinforced by natural material, are counterpoints to the man-made steel of the balcony structure.

'We chose birch as the facing layer for the ply. We wanted a consistent colour and finish, one which was quite pale and blonde and which would make the slots look quite dark and show them up.'

Noise absorption On the ground floor, there are low-curving slotted screens demarcating the public realm from user space.These screens have the same construction except that the quilt is sandwiched between two facings of slotted ply.

This is not deploying the economies of scale because they are specially curved and the quilt has the real function of absorbing noise from people walking on the hard marble ground floor and talking at the tables in the adjacent non-public zone of this floor.

A special timber feature is the reception desk. It has a canopy whose structure is a bit like an old-fashioned light-aircraft wing, timber ribs and a veneered ply skin. It is suspended by cables and stabilised by props attached to a wooden mast. It is a single straight piece of kiln-dried ash cantilevering up from a metal shoe cast in the floor. The load of the canopy and the tensioning of the rigging mean that it is bent.Once the loads are removed, Pease promises, the mast will straighten out.

Although it is not related in any way in terms of tenancy, the newly completed adjacent building, No 2 Rivergate, has the same aluminium-faced timber external win dows from Velfac and much use of the sam slotted panelling inside, this time from Dav Carpentry of Bristol. This is a differe building: the office floors are open to th atrium and separated from it at each of th three floor levels by balustrades. These a made up of slotted panels whose 2.7m dep accommodates both the edge of the slab an the depth of the raised floor behind.

Pease used the panels on the sides of th freestanding main staircase as well as on th edges of the slabs. He says: 'The timb detailing is much the same as at Temp Quay House but because of the smaller scal we tried to use a richer, warmer material.W chose Columbian pine.You get colour vari tions but we quite enjoy the fact that som panels are red, some pink, some yellow. Th building has simpler fire issues and we used matt clear surface treatment to cope wi spread of flame.'

Here the flat panels are veneered MD and the curved panels on the staircase a three layers of flexiply which is curved an then veneered and given hardwood lippin before the slots are cut.

A feature of the sheer glass wall is a b irregularly shaped copper-clad entrance bo inserted through the lower glazing. It is line with horizontal panels of American whi oak. Pease says: 'This is an inside-outsid space. The white oak is stable when it's w and, lovely, dense wood that it is, it's like piece of furniture inside.'

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