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More than an academic exercise
Published:  03 March, 2010

The Norwich Open Academy takes building in cross-laminated timber panels to new heights and in new directions. Mike Jeffree reports

This is the big one for UK timber building. The £21m Norwich Open Academy, now taking shape at a rate of knots on the north-east outskirts of the city, will accommodate 900 pupils when it opens in September, possibly later rising to 1,100. The striking oval main block is 71m across at its widest point and three storeys high, while the domed roof over its central atrium soars up to 18m. Together with the adjoining more conventional rectangular sports hall, the school has a total floor area of 9,500m2. And, bar a few steel beams, the main structure is entirely wood, a blend of glulam beams from Wiehag and prefabricated cross-laminated timber (CLT) panels from fellow Austrian manufacturer KLH.

It is, says structural engineer Ramboll, the biggest solid timber panel building in the UK to date. Moreover, its sweeping curves have pushed the boundaries of what can be achieved with CLT. Small surprise then, that the site has attracted droves of curious visitors including developers, architects and other would-be school builders.
The seeds of this timber showcase were sown a couple of years ago when contractor Kier Eastern worked with Ramboll on the St John Fisher School in Peterborough, then the UK’s biggest CLT building, comprising 1,000m3 of the material.

“Kier first bid for the Academy with another engineer,” said project engineer Gavin White. “The pitch to the client, Norfolk County Council (NCC), was a traditional structural frame solution but, after working on St John Fisher, Kier thought it could be done in timber. So, several weeks into the bidding process, they came to us.”

It was, he admitted, eleventh hour stuff, with Ramboll’s Simon Smith and Tristan Wallwork doing some “frantic” sketching to show how CLT would cope with architect Sheppard Robson’s stadiumesque design. But it worked.
What won NCC over, apart, of course, from the fact that the proposal came in at the right cost, was partly the sheer innovation of the CLT proposal. But the key attraction was its environmental credentials.

According to Ramboll, timber’s low embodied CO2 gives the Academy a carbon footprint half that of an equivalent steel or concrete build. Add 2,900 tonnes of sequestered CO2 in the building’s 3,600m3 of CLT (the volume absorbed by the trees in growth) and it’s carbon negative.

“Planners still struggle with this concept and insist on renewables,” said White. “But it’s not a difficult calculation. The Academy’s operational energy consumption – it will use a wood-fuelled heating system – will generate 30kgCO2/m2, or 285 tonnes a year. So the stored carbon is the equivalent of 10 years’ operational carbon.”

And, he stressed, this sum takes into account that the panels, which were up to 3x13m, had to be transported across Europe in 70 truckloads.

Because CLT is relatively lightweight, the Academy could also use strip foundations, with pads only where column bases are highly loaded, leading to more carbon savings.

Ramboll’s team mastered full engineering design with CLT for St John Fisher. For Norwich it took on connection detailing too, making co-ordination between architect and contractor easier, and also modelled the building in 3D.
“The model was used in design co-ordination for such aspects as planning where panels needed to be drilled for services,” said White. “Ultimately, we believe it could be used to streamline the production process further by linking direct with KLH’s cutting machine software.”

The design team describes the Academy’s core structure as a ‘racetrack’, comprising outer, middle and inner CLT load-bearing walls. The outer ‘lane’, with its full-height Velfac timber/aluminium windows, houses classrooms, while the middle ring accommodates ancillary rooms and toilets. The “wow factor” central drum, with its high ceiling, 1.6m diameter circular skylights and balconies running around all three floors, includes dining and communal areas and a free-standing theatre and lecture structure that can be closed off with an acoustic partition.

The structure of CLT, with each layer of boards bonded at right angles to the next, enables it to span two ways and the 230mm panels used for flooring in the Academy go up to 7.55m without downstand support. For wider spans, they are supported by glulam columns and these are also used where the upper floor steps back, to transfer the load to the walls below.

“We also use steel in specific areas,” said White. “For instance, in the ‘destination area’ there’s a steel truss as the client wanted an uninterrupted, column-free space.”

The diaphragm action of the floor and roof plates, he added, transfers horizontal wind loading, while the wall panels, which range from 146mm to 182mm, provide axial, racking and bending load resistance. Combined with the geometry of the curvaceous design, this enables the building to have minimal designated shear walls.

Ramboll acknowledges that CLT is really best suited to rectilinear structures, so where the Academy really stretches the material is in the curving balustrades of the atrium balconies and the domed roof. “For the balconies, the panels were pulled round the curves using angle brackets and fixed to the slab edge,” said White. “There was concern this would stress them, but KLH found it didn’t affect structural performance.”

In the atrium roof, besides getting its most dramatic application visually, the CLT also fulfils a vital structural function. The 12 600x300mm Wiehag glulam beams, arcing over the space in spans up to 28m, are fixed at the ends with steel ties to prevent outward thrust to the walls. They also cant over at increasing angles along the building and are held in place horizontally by the 163 78mm CLT panels which act as “a rigid diaphragm”. Ramboll calculated the curves of the panel edges and KLH based its cutting plans on Ramboll’s figures – and it all fitted, with the material again ‘moulded’ into shape on site, in this case by screwing to the glulam.

The initial panel erection at the Academy was undertaken by engineers from London-based KLH UK and, as at St John Fisher, Ramboll and Kier were struck by the build speed. “They had eight guys and two cranes and finished the superstructure in 18 weeks,” said White. “Using steel and concrete would have taken twice as long.”

“The sports hall was quickest,” said Kier Eastern’s contract manager John Claydon. “It is the simpler structure, but it’s 36x18m and it was weather-tight in four days.”

The work has also continued at a pace since the KLH team left, and with “minimum bodies”. “It really is a straightforward and deskilled system and we’ve seldom had more than 12 people on site, using a mobile crane and some cherry pickers,” said Claydon. “It also eliminates most wet trades and, because it’s rapidly weather-tight, follow-on trades can come in that much quicker – plus their job is easier as they’re fixing onto timber.”

On St John Fisher, Kier had not quite believed the 1-2mm tolerance claim for the panels’ pre-cut openings and so had not pre-ordered windows. On the Academy it did, shaving several more weeks off construction time.

The prefabrication accuracy also resulted in good airtightness – 5m3/hr/m2 “without really trying” – and just two skipfuls of waste for the superstructure erection.

Claydon also said there were savings from not using scaffolding, with floor panels forming the build platform as the structure went up. There were health and safety benefits to the system too, as railing was fixed to panels before erection, creating an instantly safe perimeter.

White acknowledged that the build programme has not been glitch-free. Most problems have been minor, such as a panel going in back to front, so single and double doorways had to be reversed by cutting out and moving a slice of CLT. But issues with M&E proved more challenging. “We maybe tried to be too clever in pre-cutting lots of openings for different services,” said White. “In the future, we’ll probably have bigger generic openings and leave it to the M&E contractor as to how they use them.”

Despite this issue, however, the building remains on schedule for its autumn opening. Much of the external finish, brick on the sports hall and untreated cedar cladding elsewhere, is in place and interior decoration has also started. While internal surfaces are extensively plasterboarded, large areas of timber are also left unfinished, bar a surface spread of flame treatment, as a contrast and to highlight the structure of the building.

Overall, the success of the build has reinforced Ramboll’s and Kier Eastern’s view that CLT construction has a bright future in the UK and, in fact, the two are already working on their next project.

The erection of the superstructure, including the CLT and glulam roof, was undertaken by engineers from KLH UK

In the interior, large areas of timber are left unfinished as a contrast to the plasterboard and to highlight the building's structure