Timber construction looks set to take flight in more ways than one from a corner of Cambridge Regional College’s campus.
A few months ago a small crowd gathered to watch a crane stretch over a new L-shaped building and hoist a 24m square PVC canopy into place. For a while it resembled the raising of a circus big top. But slowly the fabric unfurled between its moorings and stretched out taut as a drum under 80 tonnes of tension. The finished article is the biggest single-span hypar canopy in the UK and floats between the arms of the building like a giant wing about to swoop over the surrounding fenland. This is undoubtedly the signature feature of the structure and it has already become a local attraction, inducing gawps from campus visitors and motorists passing on the A14.
The rest of the building may be less flamboyant. But in its way it is no less of a showpiece, demonstrating a unique blend of sustainable timber construction that could exert an influence on future building in the surrounding region and beyond.
The £2m development at Cambridge Regional College will house a construction skills training centre and was commissioned by the SmartLIFE initiative. The latter is a national government and EU-backed scheme involving a range of stakeholders, including local authorities and businesses in the UK, Sweden and Germany. The aim is to develop the next generation of sustainable housing using modern methods of construction (MMC) and train the workforce needed to build them. The UK lead partner is Cambridge County Council which estimates its region in the next 15 years will need an extra 70,500 homes. The SmartLIFE Centre’s job is to build up the skills base needed to kick-start the development programme, aiming to train 600 students a year.
The key players behind the building are construction company Bluestone, architects Annand & Mustoe, electrical and mechanical engineer WSP and structural engineer Whitbybird.
The latter is making its mark as something of a UK timber construction pioneer, with recent projects including one of the country’s biggest timber frame buildings, the 8,312m2 Mossbourne Academy in London which has three storeys of concrete floors suspended from a frame comprising 1,000m3 of glulam. The SmartLIFE building may not be quite on this scale, but Whitbybird clearly sees it as yet another step along the timber building road.
“In the tender process we were asked to come up with two design ideas, with the criteria that they had to fit in with the SmartLIFE’s programme, using fast-track sustainable and affordable modern methods of construction,” said Whitbybird’s Cambridge office director Simon Smith. “We were surprised that they chose the design they did because it was the more innovative. We thought the one they turned down was ‘safer’!”
The rejected design comprised adjacent blocks linked by a barrel-vaulted canopy. It would have done the job, but the latter would have had nothing like the neck-craning impact of the PVC awning. This forms the roof for an ‘external’ practical area where students will build MMC housing using lightweight steel, prefabricated concrete and timber frame systems – the kit for the latter supplied by Pinewood Structures. Beneath the canopy there is room for a pair of two-storey houses.
The practical area is not heated, but students are protected from the worst of the elements by two arcing walls rising to the edge of the canopy. From the outside all you can see of these is aluminium cladding but, like the rest of the building, they too have a wooden heart. Beneath the metal skin are dozens of BCI I-Joists made of laminated veneer lumber and OSB. á
Ü “One of the key aspects of the brief was to use sustainable materials, which put timber at the top of the list by some considerable margin,” said architect Jeremy Mustoe. “But in this instance, I-joists were also the logical solution.”
“I-joists have been used for walls before,” added Smith. “But what sets the Centre apart is the height. The wall goes up to 12m high and it’s curved on plan so involved some complex calculations. It was also built without scaffolding, with the carpenter working from a platform on a scissor lift.”
The I-joist walls have an OSB skin under the cladding, cement composite panels internally and are topped with 660x140mm spruce glulam waling beams. The latter, said Smith, act like a picture frame, tying the structure together, and were engineered by manufacturer Lamisell at its CAD facility in Holland.
Lamisell glulam also forms the exposed frame in one of the 500m2 blocks, which house lecture and conference rooms, offices and a visitor centre, with spruce used internally and darker, more durable larch outside.
Highlighting the strength and design potential of the 200x400mm beams, some of the rooms have 8.5m column-free clear spans, creating light and airy open spaces.
The roofs use Unilin Unipur SIPs (see p60)) imported from Belgium by Milbank Roofs Ltd. They were also considered for walls and floors, but this would have pushed the very tight budget. Instead, stick-build timber frame was used – studs and plasterboard for the walls and I-joists for the floors.
A key attraction of glulam-frame building is speed, but at the Centre it was beaten to the punch by the construction system used in the second block. This is based on Lenotec panels, which are prefabricated in Germany and comprise multiple cross-laminated layers of spruce. The engineers sent over by the manufacturer Eurban got the shell up in four days, with the result that the second building was finished first, even though the panels arrived some weeks after the glulam frame.
“Eurban worked hard to give us what we wanted and performed very impressively,” said Mustoe.
The project team were also struck by the solidity of the Lenotec. Some of the panels weigh three tonnes and they make up the main, load-bearing structural element of the block, including the floors, which have a layer of concrete screed floating over the top. “Timber buildings are criticised á Ü as having springy floors, but this is very resilient,” said Smith, stamping on the screed to demonstrate.
There are load points above doorways and window openings which the Lenotec panels alone couldn’t support, but Whitbybird and Eurban put their heads together to resolve the issue. “We could have used thicker panels, but we wanted to stick with 115mm,” said Smith. “Instead we reinforced load points with LVL.”
The blocks’ main façades are clad in untreated western red cedar and, after some negotiation, the architects got the clients to accept timber for the windows too.
“They were dubious about this on maintenance grounds,” said Mustoe. “But with finishing supplier Sikkens we demonstrated that, with correct detailing, materials, construction, factory finishing and maintenance procedures, wood windows could be given a life cycle comparable to the best metal-based systems and miles better than PVC.”
The final, environmental-credential-enhancing touches to the building are a 15m electricity-generating wind turbine and ground-source heat pump which, between them, will provide 75% of its energy needs.
Perhaps inevitably for such a novel construction project, the Centre was not completed without a few glitches. The waling beams proved difficult to get right first time and, despite Lamisell’s engineering expertise, needed on-site tweaking.
Another headache was some water-staining of the spruce glulam. The instant assumption might be that the problem was down to the beams being mishandled or poorly protected by site workers, but Smith takes a wider view. “We’re all learning about this type of construction and designers and engineers also have to take responsibility,” he said. “Design and scheduling have to take into account the nature of the material. The beams basically got wet because the structure was not water-tight quickly enough.”
Another complexity was chain of design responsibility, with the overall structural scheme design and specification set by Whitbybird, then most of the final design of the two blocks sub-contracted to Eurban and Lamisell.
“This is not an uncommon approach in the UK because these forms of construction are relatively new,” said Smith. “But, as engineers become used to this sort of project, the process will be simplified and we’ll take on full design responsibility.”
Despite these complications, however, the Centre was handed over on budget having taken just over a year to complete. Most importantly, the client is happy with the result. SmartLIFE chief executive Kevin Scobell described it as a “lot of building for the money”. More than that, he clearly believes the structure itself can be used to enthuse students and others about MMC. “It’s already attracted tremendous interest and we’ve had visitors asking to see the building in mid-construction so they can see how it goes together,” he said. “We’ve also had webcams on site and we’ll make a stop-frame video to show the project from start to finish.”
