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Tall storeys come true

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BDP's special structures group has taken a step forward in timber design by creating a seven-storey structure for London South Bank University's Keyworth Centre

It is only when you are inside the sevenstorey (33m-high) atrium of London South Bank University's Keyworth Centre (AJ 25.3.04) that it really strikes you. The fullheight, seven storeys high by 31m-wide glass wall, is supported on a timber structure. The only metal support visible is the aluminium frame attached to the front of this vast timber grid. According to the director of BDP's special structures group, Farah Jahanpour, the contractor was worried about attaching glass directly to timber, which it was thought might move - and insisted on the aluminium substructure. You can't blame the contractor, when a glazed timber wall of anything like these dimensions had never been built in the UK before.

The other clues to the prevalence of timber are, first: the two seven-storey timber towers either side of the atrium supported on skinny timber legs; and second: the two sets of props in rotated pyramid formation that sail overhead from the glass wall back to the massive concrete structure of the main body of the building. These two open pyramids on their sides act as both three-dimensional diagonal bracing and as anchors to stop the wall flopping around under wind loads.

Steel standing Although timber is what you see, there is quite a lot of hidden steel in the form of Cowley Timber Engineering's patented joints, which were developed for use at Hounslow Underground Station. Here they are used to connect the vertical and horizontal members of the wall, which is on a 2.5 x 1.84m grid. The individual members are 360mm deep x 90mm thick, although in some zones the thickness of vertical members rises to twice that, in order to deal with bending stresses over junctions with props.

The grid is timber, but of course it is a special version of timber: Kerto, laminated veneer lumber (LVL) from Finnforest. Kerto LVL is manufactured using 3mm rotarypeeled softwood veneers glued together, hot pressed and trimmed to size. Panels are made in thicknesses of up to 90mm. The standard panel width is 1.8m and they come in 30m lengths - although LVL panels that long would need to be transported at night with police escorts.What is great about LVL is that it is very stable, because it is effectively a homogenous material, it looks fine in its raw state and is around three times better in bending and shear than comparable timber sections. Rectangular LVL beams have much the same structural performance as similarly dimensioned flanged and webbed steel U beams - and they are not quite as expensive.

Jahanpour is relaxed about the engineering - it is pretty conventional, bending/shear/ deflection stuff in a two-way spanning structure. The pyramid props are located so that there are never more than 56m 2ofunsupported wall. Even the lateral bracing of the timber grid is not a big issue because it has sturdy concrete bastions either side, in the form of stair and lift towers. And the roof is designed so that, although there are only sliding restraints at the junction between glass wall and roof, there is no transmission of forces or loads between the two.

Jahanpour says that in the design meetings there was a general desire to have an atrium and to have curtain walling. 'We looked at the different structural options and we thought: fiOK, we'll go for timber and prop it back to the main concrete structure. fl It all worked out nicely because the pods were much lighter in timber and the timber structure is lighter than concrete or steel, so the loads and therefore the size of the structural members could be much lighter as well. And visually the LVL is a self-finished material and looks good. It can also be used outside, an essential characteristic if is was to be used for the glazed wall.'

The wall was constructed by pre-assembling three-panel, 7.5m-high LVL ladders using Cowley connectors, which are very efficient at transferring around 70 per cent of shear and bending between adjacent timber members. The ladders were joined together on site, using the same connectors.

Column inches The so-called pods on either side of the atrium are supported on very slender timber columns, which, like the struts bracing the glass grid, were turned in a giant lathe by the late and much-lamented Cowley Structural Timberwork. Needless to say the columns have been engineered to do the job and the apparent skinniness is partly an optical effect - they are seen adjacent to the much thicker concrete columns supporting the heavy concrete upper floors of the main block. And they don't have very heavy loads: the pair of two-level pods on the north side of the atrium and the three on the south side have floor space for only a limited number of students, and they are of lightweight construction anyway. There is a physical interaction between the top and bottom of each pod in the form of a tapered timber prop that attaches to the first mullion in from the side bastions. This prop interacts to distribute lateral loadings from the glass grid through selected pod floors to the solid concrete main structure.

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