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THE BUILDING RETAINS ITS DISTINCTIVE FORM: A GREAT RECTANGULAR CATENARY ROOF SWOOPING BETWEEN TWO GLASS FACADES

SPECIFIER'S CHOICE / NENE COMMUNITY CENTRE

In 2002 Studio Bednarski won a limited competition for a £3.8 million swimming pool and community centre at Thrapston, east Northamptonshire.

Nene Community Centre is scheduled to be completed at the end of August.

But, as practice principal Cezary Bednarski explains, the near-completed building has not lived up to all the expectations of the design.

In October 2002, the AJ wrote that Studio Bednarski's design for the Nene Community Centre was 'a vindication of the belief that an enlightened client? and a determined architect can achieve an elegant, imaginative structure - even with a limited budget'.

Earlier this year it was given a Best Safety Innovation - Designer commendation in the 2005 Health and Safety Awards. Yet, as sometimes happens, it has ended up not as good as predicted, and practice principal Cezary Bednarski says ruefully: 'As a result of the brief and budget changes, the realised building does not resemble the competition-winning design.' Still, the building retains its distinctive form: a great rectangular catenary roof swooping between two glass facades.

The swoop reflects the fact that the changing rooms, stores and plant rooms, with their conventional ceiling heights, are in the middle, with the 25m pool at one end and two community halls at the other. Bednarski explains: 'We had a client with lots of desires but little money. So you do as simple a building as possible:

you put the services zone in the middle section, with two large boxes on either side and the line that links these building masses is the design.'

The long, straight back wall and two-part angled front wall, which appear to brace the roof, are of in-situ concrete construction. Each set of the Schüco curtain walling at each end of the building has a row of eight Gordon Cowley timber columns standing outside the glazing in a rather Classical arrangement.

Bednarski is no Classicist, but the deployment of the bases and capitals to these columns justifies his half-joking description of them as 'the Thrapston Order'. The columns are arranged on an eight-by-five grid, so that three rows are inside the building. Their presence gives the clue that, although the roof swoop looks like a catenary, it is actually a simply supported roof. Still, it is a Cowley roof and, as you might expect, a structurally interesting one.

THE ROOF The original idea was for the roof to be a catenary structure hung from the north and south walls. The architect and roof engineer, John Westmuckett and Alastair Massie, considered steel-cable and steel-beam structures, but eventually opted to build an all-timber roof; not least because it would have been the longest clear-span timber roof in the world. Bednarski explains: 'Gordon Cowley said, 'yes we can do it', and the price was within a few per cent of the price of a conventional steel roof. It was exciting. But we would have ended up with a huge deflection and very expensive detailing for the wall-roof junctions.' And, while the services rooms in the middle read as structures within the main structure, it was not absolutely necessary to have a visible clear span along the entire length of the building. The architect and roof engineer added three internal rows of supporting columns and redesigned the structure of the roof.

The roof itself is a stressed skin/semi-monocoque sandwich of two 33mm-thick Kerto LVL (laminated veneer lumber) skins from Finnforest, connected together by internal LVL I-beams at 1,720mm centres. These beams are 300mm deep at the middle and 320mm deep at the ends. They have wide lower flanges, which Cowley technicians screwed through and into the lower skin, leaving the underside entirely free of metal (which would soon corrode in the chlorine-laden atmosphere of the pools).

The ceiling panels were coloured with an Akzo Nobel pigmented stain, in harmony with the columns, and protected by an acrylic vapour-control coat. Additional moisture protection in the form of Crown Breathline was laid on top of the lower skin.

Cowley made up the lightweight base panels in units 17m long and 1.7m wide, which, when they arrived on site, turned out to be difficult to handle in windy conditions. They were joined together using the Cowley technicians' favourite American Timberlok hexagonal-headed screws. Rocksil insulation from Knauf in thicknesses of 175mm and 125mm was laid on this lower structure, allowing a 125mm unventilated air void above.

The top skin was then screwed into place.

With computer design, you might expect perfection, but the longest spans deflected 20mm more than the anticipated 35mm. It was reckoned on re-analysis that this was due to an overestimate of the diaphragm effect and an inherent difficulty in knowing how a mix of glue joints in the LVL joists and the screwed inter-panel junctions would behave. But this was not a real problem, and only minor adjustments had to be made to the internal connections between the roof and the walls.

Bednarski originally wanted to waterproof the roof with a single-ply polymer, but the client - East Northamptonshire Council - had reservations about such a design. A Plannja profiled aluminium sheet was considered, but a Keybemo standing-seam roof was chosen instead. The change had the effect of thickening the roof edges more than the architect wanted, and, as there are no perimeter gutters, this led to detailing and waterproofing issues.

In its final form, the roof's standing seams run down each slope, guiding water to the wide gutter across the lowest point of the roof. There are three downpipes concealed in hollow Cowley columns. The single transverse gutter would not have worked without a syphonic system and, although the architect looked at Geberit, Fullflow was prepared to design a simpler system specifically for this building. The gutter is accessible using a cherry picker, and the condition of the roof can be inspected using binoculars - two factors cited in the Health and Safety Executive commendation.

The hollow 300mm-diameter Cowley columns could be bought by the metre. They are made up of 12 vertical segments glued together with a roughly circular cross-section, then machined to a smooth cylinder on what was effectively a giant lathe.

The design of the columns, with their concrete base, timber shaft and steel capital, has been influenced by Bednarski's interest in Japanese architecture. The precast bases are knee-height and are resistant to chlorine-based corrosion. The galvanised mild-steel column heads, which provided better protection than using just stainless steel, were individually fabricated by Cowley to accommodate the changing angle of the ceiling.

The two end walls facing north and south are made from Schüco System FW50+2 profiles in an anodised natural finish. The CBS blinds to the south glass wall were originally located outside the glazing, but Bednarski says: 'For some reason, the price trebled and the client decided to put them on the inside, where they work, but aren't ideal for heat gain.' The client chose a grey colour for the blinds, when the architects, following the advice of BRE, had specified sunfl ower yellow.

The east and west in-situ concrete walls have a white insulating render originally specified as Sto's StoTherm Classic.

But the contractor's proposed alternative, Permarock EPS, could not achieve an even surface. Bednarski says: 'With oblique sunlight, it looks, as one visitor put it, like a 'cellulite bottom'.' Some notable client changes have been the purchase and installation 'in the wrong way and wrong place' of cycle racks, which had originally been specified as Broxap Mawrob racks. The client similarly insisted on using its pet brand of lockers in the changing rooms. Their performance to date has already left something to be desired. The original specification was for the Link Lockers Ambassador anodised-aluminium range.

Equally unsatisfactory was the small saga of the low, green glass screen between the main pool and the toddlers' pool. It was a lamination of two sheets of 10mm toughened glass either side of a coloured PVB laminate, but it had a manufacturing flaw.

When the client decided, without any consultation, to take out the screen, it shattered. Another has been sourced, but the council is now worried about the possibility that a replacement might shatter too. This fear remains, despite, as Bednarski points out, the successful use of similar glass in most of the world's airports.

Then the client decided, against Bednarski's commissioned masterplan for the locality, to abandon a small civic square in favour of building developer housing right up to the edge of the centre. Now, rather than having views across the Nene River valley, users have brick boxes to look at.

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