Still basking in the success of the Norwich Open Academy, the largest timber panel building in the UK, completed in September 2010, the same project team is now erecting another equally impressive academy on the west side of Norwich.

With a timber tally of 3,100m³, the new City Academy, with a 10,000m² GIA, has 400m³ less than the first structure, but it is no less a triumph of modern timber engineering.

Like the Open Academy, the City Academy is built from cross-laminated timber (CLT) but this time it is combined with an internal steel spine to provide the flexibility of use essential to the client, Norfolk County Council.

“The Open Academy has a cross-walled solution internally, but City Academy has an internal spine so you can have an almost clear floor plate with steel columns. That was one of the key drivers on the project,” said project engineer David Stone of Ramboll.

“A concrete and steel solution was considered,” said architect James Lumb of Sheppard Robson, “but timber was chosen largely because of the success of the Open Academy.

“The main contractor [Kier Eastern] was impressed with the speed of construction, and the ability to fix in the windows at a much earlier stage of the programme, thus enabling an almost watertight envelope from which to begin internal works,” he said. “It could have been a full timber solution, however, by using the inherent properties of timber and steel together, it meant it could be built more efficiently with a steel ladder frame down the centre of the building.”

The superstructure has gone up in about 22 weeks and the project is due for completion in spring next year.

Challenges
Ramboll had not used the combination of CLT and steel on this scale before and the hybrid construction did throw up some challenges.

“You have to think about different deflections and different vibration performance. It required detailed engineering to see how they behave and what that means to the overall building,” said Stone.

Ramboll did a full vibration analysis of the floor plates, using a finite Element Analysis Model, which, the company believes, hasn't been done on this scale before.

“It provided a guide as to where the stiff points hadto be so we rearranged the internal supports and the stiff points where we needed them,” said Stone. “It guided the design in certain areas where the spans are quite long. If we had just looked at it simplistically, all the elements would have been bigger but doing a specific detailed investigation meant we were able to value engineer it and make the size more efficient.”

The Academy is effectively three buildings – a curved single-storey building housing the main entrance, library and sixth form area and, behind it, two three-storey buildings connected by an atrium. The width of the latter meant the project has also further developed Ramboll's expertise in finding timber solutions for longer spans.

Cassette solution
How to address a 10m span caused some head scratching because one timber panel wouldn't provide the required stiffness. The solution, developed by Ramboll and  produced by CLT manufacturer KLH, was a cassette comprising CLT panels either side of glulam ribs.

“We used timber solutions wherever we could; that's been the philosophy,” said Stone. “We needed  a steel spine inside because the client wanted it to be as small as possible, but everywhere else we said 'how can we make this in timber; how can we add the stiffness in timber?'.”

The atrium houses two curved bridges made from CLT, but in much thinner panels than those used in the building’s structure

Curved bridges
The atrium houses two pleasingly curved bridges made from CLT, but in much thinner panels than those used in the building's structure.

“We used thinner, 57mm CLT panels. The curvature of the bridge was designed around the maximum bending radius of the timber,” said Lumb.

The bridge is supported by three-armed CLT posts, forming 'timber trees', and the connectors again called for Ramboll to find a new approach.

“We worked hard to make the exposed connections buildable but also visually pleasing and easy to erect, because that saves time and money,” said Stone.

The result is a hooked plate on each of the tree arms which drops into a slot on the trunk and is then covered with a timber infill.

“The connections are supporting some significant loading but you're able to erect it on site just by dropping it in and locking it,” said Stone. “We are really pleased with the result.”

Externally the academy will be finished in brick and render, while internally some of the CLT will be left exposed and here it is treated with spread of flame retardant. The internal non load-bearing walls are stud and plasterboard, allowing the school to alter classroom spaces in the future.

BREEAM rating
The Academy is required to achieve a BREEAM rating of Very Good but the project team is targeting a higher, Excellent rating. And the emphasis on timber certainly helps this.

“Its thermal performance is excellent,” said Lumb. “Some people were sceptical about not using concrete because of the loss of thermal mass. However, when timber is used in the sense of CLT, rather than the typical ‘timber frame’, it should really be thought of as medium weight construction.”

In designing the City Academy Ramboll and Sheppard Robson applied lessons learned from the Open Academy, and this latest project has added to their expertise – and may well allow them to extend the use of CLT further.

“We've learned a lot about how to do vibration analysis,” said Stone. “Timber doesn't behave in the same way as a concrete slab, where it's the same in all directions. With timber you have the direction effects of it and it's screwed to the walls at the edges rather than being monolithic.

“We have an ongoing research project with Cambridge University investigating more fully how to best model vibration performance of large timber buildings. We're pushing on with what we can do with it, pushing longer spans in timber.”

The bridge is supported by three-armed CLT posts

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