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Opening a window on innovation

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Designed to highlight the structural, aesthetic and environmental benefits of timber, Velux's recently completed regional offices and training facility in Kettering is also a showcase for its products It is a sad fact that when it comes to considering materials for primary structure, timber is still not thought of in the same way as steel, concrete and masonry. But it is catching up fast. Timber's credentials as one of the most sustainable building materials on the planet - indeed, the only renewable building material - are giving it a slowly growing market share in an increasingly ecologically minded construction industry. Timber becomes an even more attractive option when all its many advantages are considered, particularly structural performance.

According to the Glue Laminated Timber Association, a 550mm x 135mm softwood glulam beam has the equivalent performance to a 305mm x 165mm steel 'I' beam.

Yet the glulam beam requires only a sixth of the energy for its production, and only a fifth of the energy required to produce a comparable 400mm x 250mm reinforced-concrete beam. In terms of weight, a structural steel beam can be up to 20 per cent heavier and a concrete beam 600 per cent heavier than an equivalent glulam beam. Yet the production of steel and concrete results in polluting byproducts.

One architectural practice that has capitalised fully on the benefits of timber is White Design Associates, architect for Velux's recently completed regional offices and training facility in Kettering. Designed mainly in timber, the striking building engineered by Buro Happold, not only has a low embodied-energy glulam structure and a highly insulated envelope to minimise energy consumption, but also natural cross ventilation and maximum daylight to optimise user comfort. As such, it is a fitting advertisement for the client's environmental credentials.

Highly photogenic In addition to the durability, sustainability and economy targets the client had set, the building also had to showcase the company's roof windows and have the 'most photographed roof in Europe'. Whether this will be achieved remains to be seen. The highly conspicuous structure comprises a gluedlaminated (glulam) timber A-frame configuration which gives the building both its characteristic triangular form and transverse stability. This arrangement has created two large roof slopes accommodating a variety of Velux windows in a way that is not too dissimilar from the company's Glenrothes HQ.

Two opposing lines of glulam ribs spaced at 6m centres form the sloping sides of the Ashape. This configuration is asymmetrical both in height and in shape: the north-elevation ribs are cruck-shaped and overlap the ridge on a higher level than the straighter ribs of the south elevation. This creates a convenient gap which is exploited both for ventilation and clerestory lighting, but it also creates two distinct roof forms: on the south elevation, an elegant concave sweep clad in natural slate; on the north, a double-convex slope covered with shingles of Canadian cedar. Arranged along a broadly east-west axis, the plan curves as the grid rotates by 4infinity and 9infinity in response to the site boundaries.

Imported from Denmark, the impressively engineered glulam beams were chosen by the architect as a high-quality, structurally efficient material that could be formed easily to create the shape and scale of the ribs.

Spanning 15.5m and reaching 12m high to the apex, the 800mm x 200mm glued laminated components define the form of the building and provide the structure from which the steel second floor is hung.

Each pair of opposing glulam ribs is connected by a circular steel column rising from the ground-floor slab and repeating along the central axis of the building. Horizontal glulam purlins on each side of the building give lateral stability and these are formed into continuous members running the entire length of the building by carefully detailed steel connectors bolted to the primary ribs.

Where ribs meet the ground concrete slab, they fit into a steel-foot fixing detail. Connections such as these, which transfer stresses from timber components into concrete, are often the most difficult aspect of an engineered timber structure to detail.

The pleasing aesthetics of exposed glulam structural members allow them to protrude here and there to add to the overall Scandinavian aesthetic - clearly important to the Danish client. Penetrating the external envelope is not a problem for glulam, as its insulation properties are sufficient to eliminate the risk of cold bridging. In addition, energy consumption is further reduced as glulam has a low thermal mass and does not readily absorb space-heating energy.

Breathable structure Timber is the predominant material throughout the building. The external vertical walls are made of a breathable construction which comprises 250mmthick softwood framing with Rockwool insulation infill. This is faced with a breathable, 15mm bitumen-impregnated fibreboard onto which a rainscreen construction is applied, comprising 25mm x 70mm untreated horizontal cedar slats on treated battens. Ample ventilation is provided by 15mm air gaps between each slat. Two layers of plasterboard on a vapour barrier are applied to the framing on the internal side.

This construction, which borrows much from breathable roof constructions, is broadly similar to that used on the northand south-facing slopes, where a Tyvek membrane was substituted for the impre nated fibreboard. The resulting high insulated construction plays a great part saving energy and providing comfortab working temperatures.

The building's highly insulated, low mass timber envelope is built around th thermally massive concrete construction the ground and first floor, which helps regulate internal air temperature. Environ mental performance was high on the agend of the building's design, and this extended all timber used, which had to be fro responsibly managed sources. The archite was keen to enforce such a strict policy.

Project leader David Noble of Whi Design Associates says: 'Where possible, w insisted that all timber for this projec whether hard- or softwood, should be fro sustainable sources. We inserted a clause this effect in the specification. However, w had to reject a delivery of plywood as was from an unknown source and did n conform.'

The architect also wanted to avoid th inclusion of products that contained tropic hardwoods. Such species would only b allowed if the contractor could furni information that included: the speci and country of origin; the name of th concession/plantation within each of th origin countries supplying the produc copies of appropriate forestry policies confirm adherence to sustainable policie and, evidence to confirm the UK suppli actually obtained the timbers from the give plantation.

It may sound rather involved, but suc detail has resulted in a building of exception al design quality, where the use of glue laminated timber components fulfils stru tural, aesthetic and environmental roles. Th client is clearly impressed, as is Ketterin Planning Committee, which has alread highly commended the building. This loo to be the first of a series of commendatio and awards which this innovative desig should win in the coming months.

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