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PROJECT REPORT: TIMBER BUILDS
“The combination of this client and site meant that we could push the boundaries in a way we’d never been able to before” Stephen Hodder
Timber PROJECT FACTFILE
Architect: Hodder+Partners Client: Royal Horticultural Society (RHS) Contractor: BAM Construct UK Timber: HESS Timber Environmental/M&E engineers: Hoare Lea
Structural engineers: RoC Consulting Quantity surveyor/cost consultant: Arcadis
Landscape architect: Tom Stuart-Smith
The roof structure – spruce with a cross laminated timber deck, plus the larch external wall cladding and louvres, were all manufactured and supplied by HESS. Their calculations show the roof structure sequests 320 tonnes of carbon, and the cladding an additional 26 tonnes. “The use of timber was absolutely intrinsic to the approach to sustainability,” Hodder says. The biggest challenges that came with the timber were around the structural ‘trees’ that support the 90 by 24 metre roof. Initially the building was designed utilising a portal frame, but when the structural engineers became involved it ended up increasing in girth beyond what the practice were happy with, hampering their desire for something more elegant. Hodder says he viewed the classroom, admin, and kitchen ‘pods’ as buildings in their own right, and that the overarching roof structure should be separate, and “should almost feel as if you’re outside; delicate with lots of natural light flowing in.” To resolve this structural conundrum, the design went through a collaborative, iterative process which resulted in a diagrid structure initially with two-dimensional Y-frames, which slightly reduced the depth of the portal frame – but not enough. They then discussed looking at a more ‘three dimensional’ solution, and came up with the idea of using the structural trees. Columns have four branches at the
top, allowing each column to support a prefabricated six by six metre cassette.
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“There’s a hierarchy in the size of timbers within the roof, but nevertheless it’s much finer, more elegant,” Hodder says. As well as enabling the roof structure to be more refined, the branch approach also meant fewer columns were required, which allowed the designers to keep the main space in the building flexible. “You go through this process until you achieve what you want from the building,” says Hodder. The structural trees, combined with the way the roof sits, have become design features in their own right, says Hodder. A triple glazed rooflight runs down the centre – helping conjure the ‘outside’ feeling – with the diagrid structure beneath creating a variety of shadows. With the building seeing an unprecedented number of visitors (currently operating at levels not predicted until its eighth year of operation) it can feel chaotic at times, but Hodder says that the oversailing warm timber roof helps visitors relax. “For me it’s calming; and it’s quite rewarding when you see people enter that building and the first thing they do is look up at the roof,” Hodder says. “Wherever you are in the building, the roof is that common reference point.”
There were further structural challenges.
The ‘trees’ all sit inboard of the main roof structure, meaning the edges of the roof canopy are cantilevered, and therefore susceptible to additional movement, beyond what is normal with timber. So managing that movement within the design was “critical,” says Hodder. The ‘trees’ were originally designed
ADF SEPTEMBER 2022
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