The UK’s biggest cross-laminated timber (CLT) building project to date was originally going to be in masonry, concrete and steel. That makes it even more satisfying for structural engineers Ramboll UK.
The firm are self-confessed evangelists for CLT. In fact, for the St John Fisher School (SJF) in Peterborough, where nearly 1,000m3 of the solid wood structural panel has been used, they were instrumental in getting the client to make the switch from ‘conventional materials’.
The Roman Catholic school has undergone a £11m revamp, involving refurbishment of some buildings and construction of a new two-storey, 60m-long teaching block and 1,000m2 sports hall and drama studio.
“Initially the new classrooms were going to use load-bearing masonry and pre-cast flooring ‘planks’ and the hall steel frame and masonry infill panels,” said Tristan Wallwork of Ramboll’s Cambridge office. “But, at outline proposal, we put forward the alternative – using CLT as load-bearing floors, walls and roofs, combined with glulam roof beams in the sports hall.”
The case for the panels for the teaching block quickly won over the client, the Roman Catholic diocese of East Anglia, and architects GSSArchitecture, but initially they stuck with masonry and steel frame for the hall. Ramboll, however, would not take no for an answer.
“For the classroom buildings, there was immediate recognition of the suitability of CLT to the design and the savings it gave in build time,” said Wallwork. “But the QS decided the figures wouldn’t stack up so well for the sports hall. We saw that as a challenge and took the architect and client to Austria to visit the CLT manufacturer KLH. The client was so impressed that within minutes of talking to them, they wanted it for the sports hall, even though the material cost was about £50,000 more.”
Ramboll itself first got to grips with CLT at Cambridge Regional College’s SmartLife “sustainable skills centre”, where students learn about modern methods of construction. Since then, it has increasingly championed the material, with its offices in Cambridge and Bristol particularly focused on honing their expertise with it.
“Previously we’ve taken CLT projects to scheme design stage, then subcontracted final design to the panel supplier, but with SJF we took on full design responsibility,” said Wallwork. “It was an important step. Putting ourselves between clients and panel supplier we can reduce the perceived extra risk of using, what is for them, an untried material. It also underlines our confidence in its future.”
Key attractions of CLT, according to Ramboll, are robustness, speed of erection, and the fact that it offers architects a “new form”. But increasingly the “headline” sales hook is carbon saving.
According to Wallwork, the 990m3, or 475 tonnes, of CLT used at SJF add up to just 310 tonnes of embodied CO2, even taking into account trucking it half way across Europe. That compares with around 750 tonnes for the equivalent structure in steel or concrete. Throw in the sequestered CO2 – the amount absorbed by the timber as it grows – and you go into carbon negativity to the tune of minus 480 tonnes.
“Compared to the other materials you’ve got a total carbon saving of 1,200 tonnes. How can you argue with that?” said Wallwork.
CLT, he added, was also a better match to the design of the SJF teaching block, with its large classrooms and voided central corridors. The original spec would have required a lot of primary and secondary steelwork and “extremely awkward” detailing. CLT gave a “more efficient structural solution”.
Cellular layout
The building has a simple cellular layout, with CLT forming internal and external structural walls, and stud walls dividing the classrooms. The roof, wall and floor panels are half-lapped at each end and screwed together, giving full horizontal and vertical load transfer. Because they run in two perpendicular directions, they provide racking resistance, with wind loads transferred by the diaphragm action of the floor panels to internal walls and concrete slab foundations.
The panels used for the walls were up to 5m high and 95mm thick, and for the floors and roof 7m long and 230mm and 140-160mm thick respectively.
“KLH make them up to 13m long, 2.95m wide and 500mm thick,” said Wallwork. “The thicker they get, the better the thermal and acoustic performance, but the less efficient and cost-effective they become. It’s a trade off.”
The material, he added, is less stiff than the pre-cast concrete plank equivalent and analysing deflection and vibration formed a major part of detailed design.
CLT is also used for stairs, lift cores and downstand beams in the upper floor and in some locations the first floor wall panels act as deep beams supporting floor and roof loads to provide column-free space below.
“The beams aren’t as structurally efficient as solid glulam, but meant we were using just one material,” said Wallwork. “The contractor, Kier Eastern, appreciated this. In fact, they got into working with CLT generally and the project manager asked why on earth they hadn’t used it before!”
KLH’s original tender design for the sports hall was to use CLT as infill panels in a glulam beam and column frame. But the architect rejected this and instead Ramboll devised a load-bearing wall solution using 95-162mm thick CLT panels for the up to 11m-high walls, and the combination of panels and glulam beams for the roof.
“The 18m roof span was too wide for CLT,” said Wallwork. “So we used 900x240mm glulam, which is screwed into the wall panels, with 60mm-thick CLT panels over this and the aluminium outer layer fastened to these with top hat supports.”
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The timber structures took 90 days to complete |
Exposed timber
Another contrast with the teaching block, which is plasterboard lined, is that the hall has the CLT exposed internally. “It’s not to everyone’s taste as the finish is quite industrial,” said Wallwork. “But the client liked the aesthetic and wanted the pupils to see how it’s built.”
The sports hall walls haven’t been sealed or varnished either, but Building Regs did stipulate a surface spread of flame treatment. Wallwork described that as “frustrating”.
“They don’t have this requirement in Austria or Germany and we question its application for CLT here. It’s something we’re looking into for future projects. CLT is actually extremely difficult to ignite due to its density.”
Externally the hall was finished in brick, while the teaching block mixes brick, render and Trespa panel cladding. Both sandwich Homatherm rigid insulation between timber and outer skin. “The timber is a very good insulant and, as the joints are half-lapped and sealed, it does well on airtightness,” said Wallwork. “Kier report airtightness of 3m3/hr/m2 for the hall and 5m3/hr/m2 for the teaching block. Indications are that the envelope is effectively over-performing. The heating is hardly used; it may be possible to use just the ground source heat pump.”
The panel system might have exceeded expectations on build time too if Kier had pre-ordered the windows. “The erection of the timber structures took 90 days and Kier estimates that, overall, CLT saved 14 weeks on traditional construction, making the change from masonry and steel cost neutral,” said Wallwork. “If they’d pre-ordered the window frames they think they’d have saved another seven weeks. They didn’t as, due to inexperience with CLT, they didn’t quite trust the pre-cut openings would be right.”
It will be interesting to see if Kier does things differently on the latest project it’s working on with Ramboll. Construction on the Sheppard Robson-designed Norwich Academy is under way and, when complete, it will easily wrest SJF’s crown as the UK’s biggest CLT building. It will use 72 truckloads of the material, a total of 3,600m3.
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The interior of the sports hall is left unfinished, except for a fire retardant application |
