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Taking the high ground

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Entrance arrangements at Hounslow East station on the western reaches of London's Piccadilly line were once far from perfect, but architect Acanthus Lawrence & Wrightson has solved the problem with its redesign

Hounslow East is one of the last overground stations before London's Piccadilly line dives underground into the Heathrow nexus.

More 'overground' than most Tube stations, it is up on an embankment, the continuation of a bridge over an adjacent cross road. Buying a ticket and getting to the westbound platform once involved going out of the station, under a bridge and then up an embankment to the platform where, not surprisingly, ticket collection was a bit hit-and-miss.

In 1999, architect Acanthus Lawrence & Wrightson won a competitive tender to redesign the station. Due to be finished later this year, the practice's scheme had a groundlevel ticket hall with stair and lift access to the platform, which had to remain at its old, high level. In this new scheme, a tunnel under the tracks leads to another set of stairs and a lift up to the westbound platform.

The idea of a relatively free-form roof in green copper arose when the design team, led by Michael Watkins, decided on the lofty, ground-floor ticket hall. The green roof was a kind of metaphor for the embankment the ticket hall was replacing. The roof could be extended to provide shelter for the platform and its general arrangement could be echoed in the rather smaller stair/lift structure to be built for the other platform.

Appropriate language

There were some design tweaks, such as rounding the visible end of the main building in a gesture to the great 1930s Tube architect Frank Pick, and swooping the roof down to the height of the predominantly two-storey buildings round about.

The fact that old overground Tube stations are normally constructed from wood suggested that exposed timber represented 'an appropriate architectural language', says Watkins. 'We were aware of the work Buro Happold had done with timber, so we asked them if they would be interested in looking at the problem.

'We had a meeting with Buro Happold's Ian Liddell, who talked us through the possibilities and some solutions for the two-way spanning roof. Ian came up with the idea of a diagrid, a timber system that Cowley, the timber people, had done before. The base element is an almost square grid with a leg projecting from each corner of the square.

Ian said it was a relatively flexible means of covering a complex shape using internal tree supports at high level and resting on external walls elsewhere.'

The senior Buro Happold engineer was Ken Jones. He explains that the sectional shape of the two roofs is a constant barrel vault with a radius of 23.8m. The best way to achieve a diagrid structure was to use a lamella structure - which is the 'square grid with a leg projecting from each corner of the square'. Between the two World Wars, timber lamella structures, first patented by Friedrich Zollinger in 1921, were common and very successful - although steel versions were not so popular. One of the virtues of this kind of structure is that the basic elements are small in size and can be handled easily by operatives assembling them on site.

Working on the Hounslow East station roof design, Jones's team decided on a 1.25m grid with each individual lamella 2.5m long.

Half the lamellas are handed versions of the other half but are otherwise identical, so there are no specials apart from wall plates and edge beams. The engineered wood product that the design team chose for the lamellas was Kerto LVL - laminated veneer lumber - from Finland.

Jones says: 'This is an exciting material. It comes from managed forests, it comes in enormously long sheets - 40m by 1.8m is the maximum - it's dimensionally stable and for the lamellas the strands are oriented in the same direction. It's not plywood, but you can get it in a cross-ply formation and we used that for the roof decking.'

Ingenious solution

Though the lamellas are all the same, they each need to twist very slightly if they are to make a solid edge-to-face junction with the decking that stiffens the structure. Cowley Structural Timberwork had an ingenious two-part solution. It slightly offset each lamella at its junction with a cross member and planed the edges of the lamellas so that the decking always lay flush to the top edge of the lamella and the wall plates on the bottom.

The architect wanted the timber structure to be exposed. Inter-war lamella structures were normally bolted and, on a big structure, the fixings are not so visually prominent.

On this relatively small structure they could only look clumsy and here another piece of Cowley ingenuity came to the rescue: the Cowley Shearlock connector.

This involves a long, threaded bolt sleeve glued into the end of a lamella with a threaded receiving tube in the other. On site, bolts are threaded through a drilled hole in the cross member into the end of the next lamella in line and the joint tightened up at an access hole in the side of the lamella.

Normally there are two Shearlock bolts in each end and, where the shear forces demanded it, as many as four. The offset angle between lamellas needed to allow the deck to lie flat is achieved by slightly offsetting the two Shearlock connectors in the ends of the lamellas either side of vertical.

The Shearlock connector not only enables the structure to work, it does so quite invisibly. The joints were tested at Bath University, Buro Happold's local higher-education establishment.

Generally, the edges of the roof are supported on steel stanchions. But the cantilevered top edge of the roof over the platform is propped by a succession of triangulated timber struts sitting on specially shaped steel stanchion tops - and there is a main 'tree' support in the middle of the roof, made up of oak struts attached to the slender steel 'trunk' branching up and fixing to the grid using simple bolted bent plate joints.

Some time ago a Buro Happold/Edward Cullinan team developed a joint that involved drilling a hole in the end of a timber tailing strut and fixing a steel bolt/clevis pin arrangement in the end with epoxy resin. A similar technique was used here, with the end grain being protected by a stainless-steel collar.

The structural decking consists of 27mm-thick tongued-and-grooved sheets of LVL screwed to the tops of the lamellas.

There is waterproofing, a vapour barrier, insulation, battens, a ply skin and then the pre-patinated Tecu copper standingseam roofing by KME, which is rolled over the edge and a metre or so back under the eaves.

Ken Jones's paper about the lamella roof is on the CD of the recent 'Time for Timber' conference, available from Alison Kelly at Buro Happold on 01225 337510. A more mathematically detailed paper by Jones is to appear in an issue of The Structural Engineer later this year.

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