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Spanning the light fantastic

Cezary Bednarski's Thrapston and District Community Centre is an imaginative structure with a curved roof working as a combination of stress skin and catenary to bring much light into the building

Pragmatism can be as important as imagination in architecture. One of the most important times to be pragmatic is when you have to accept that a ground-breaking idea is not as necessary as it seemed at first.

This was something that architect Cezary Bednarski had to accept when he realised that his initial plan for the Thrapston and District Community Centre, to create one of the largest-span unsupported timber roofs ever, would not be appropriate. He has refined his concept to give the client a solution that will still be elegant and still involve a considerable amount of innovation, without breaking any records.

Bednarski won the project in 1999, in a competitive bid with a design emphasis, when he was still with Studio E Architects.

The client, East Northamptonshire District Council, requested that he take the scheme, which had changed completely due to budgetary constraints, with him when he set up his own practice, Studio Bednarski. The community centre, which overlooks the River Nene, includes a 25m swimming pool and splash pools, a community hall and meeting room, a bar and other ancillary facilities.

Looking at the project, Bednarski came up with the idea of using a Kerto LVL (laminated veneer lumber) catenary roof. With support from the client, he looked for a contractor who would be able to fulfil his vision.Not surprisingly, he ended up with Gordon Cowley, who is the salvation of almost every architect who wants to do something unusual with timber. Working with structural engineer John Westmuckett, who at that time was with Parkman, Cowley developed the design and produced a firm price which came to little more than the cost of doing a standard portalframe building with a metal sandwich roof.

Initially, Bednarski wanted the building to have a full-span catenary roof, similar to that used by Alvaro Siza at the Lisbon Expo.

But he realised that this was not appropriate, since the building did not have a single space that demanded to be spanned in this way.

Movement across this span would have been up to 300mm which would have meant that the wall-to-roof details would have been both expensive and prone to failure.

So Bednarski abandoned his romantic idea, and developed instead what he describes as 'a morally and economically appropriate solution'. This supports the roof on a series of columns so that it works as a combination of stress skin and catenary.

The roof is 40m wide and 56m long. It sweeps up from the central gutter line to the glazed eaves of the building, with a radius of curvature of 100m. The effect will be of a light-looking, light-coloured structure, which brings light into the building.

The principal structural members of the roof are 300mm-deep glued timber I beams running along the curve of the roof. These are fixed to the top and bottom boarding at 1.72m centres to form a composite panel.

Connection between the I-beam flanges and the decking is made with high-capacity Timberlock screws. The Kerto LVL, which forms both top and bottom skins, is 32mm thick, giving a total structural depth of 366mm.

There is no metal in the roof at all, says Bednarski, apart from the screws. He believes that there are 72,000 of these, beating even Daniel Libeskind's pavilion for the Serpentine in terms of the number of screws per m 2.Loads are transferred from the ends of the ribs to the columns using solid timber cross beams set within the depth of the roof. These columns are 300mm-diameter hollow glulam columns, which Bednarski finds exciting as they work out cheaper than steel square hollow sections. They are, he says, 'sold by the metre like sausages'. The columns have exposed, fabricated steel head and shoe details.

Massive, reinforced-concrete shear walls provide lateral stability in the longitudinal direction and also provide horizontal restraint for the remaining catenary action.

In the cross direction, lateral stability is provided by braced steel frames within the walls of the plant room.

Because the underside of the roof structure will be expressed internally, and there will be no brittle ceiling finishes, deflection of the roof has had to be considered in the design only where partitions will be built up to join the roof.

This design, for which construction should start shortly, is a vindication of the belief that an enlightened client (yes, even within a local authority) and a determined architect can achieve an elegant, imaginative structure even within a limited budget. The decision not to stick with a purely catenary roof means that the building will not enter the record books. But the creative pragmatism of the design will have far more impact if it persuades other, more timid, clients that imaginative structures can work and can be affordable, given just a touch of vision.

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