Architectural & Structural Design
The curved and stepped subterranean retaining wall running along the building’s rear is propped at first floor level by the thermally separated first floor slab, which in turn is stabilised laterally by the in-plane rigidity of the retaining wall. In order to form a structural connection whilst minimising any cold bridging effects between the cold retaining wall and warm first floor slab, a proprietary insulating shear connector was cast in. The retaining wall is backfilled with a locally sourced recycled free-draining granular material. A triple glazed faÃ§ade runs along the south / west elevation of the building; the glazing system utilises stacked timber frames sandwiched between timber mullions, with a timber ‘loose tongue’ to create a shear connection between the two elements.
The FSC timber roof structure is made up of sawn timber purlins and specially fabricated twin glulam beams, S-shaped in elevation and sheathed with a plywood diaphragm. As the soffit of the roof is primarily exposed to view from the first floor classrooms, the connection details have been sensitively detailed and fabricated, utilising steel dowel type connectors in lieu of conventional bolts where possible. The primary glulam beams to column connections employ specially fabricated ‘pinned’ connectors. Given the complexity of the roof geometry, a 3D computer model was built to aid the fabrication process.
The design of the roof diaphragm was needed to be achieve global building stability, the curved plan and section building geometry dictated that a degree of double curvature was required, and the internal soffit was to be exposed to view. Consequently FSC birch plywood was the preferred material. In order to achieve the required curvatures, the maximum panel thickness needed to be 9mm, which meant that three layers of plywood where required. Given the potential difficulty of persuading a plywood panel into double curvature, butt joints between the ply sheets were situated over the main glulam beams, reducing the residual stress in the plywood. In order to accurately predict the roof stresses and fixing requirements, a 3D Finite Element Model was generated to accurately determine the effects of in-plane forces due to horizontal wind loads.
The fabricator, Dalton Joinery, built a full-scale model of a typical bay in their workshop prior to commencement on site, enabling the team to proceed with confidence that the curvature could be achieved on site. This also allowed us to agree a plywood sheet lapping arrangement, which was then fed back into the design of the fixings. The joints on the lower layer are expressed with a routed 10mm wide x 5mm deep groove to reduce the visual impact of the plywood cut edge in the exposed internal ceiling soffit.
The team felt it was important for an award winning project to demonstrate its true performance to the wider industry, to learn how it is achieved in practice. So the project has secured funding from the Technology Strategy Board to carry out a Building Performance Evaluation covering two years of operation. A programme of dissemination will follow to reveal the findings as widely as possible. The building has already been part of an early stage monitoring programme by the Carbon Trust.
The RHS’ new Bramall Learning Centre is highly sustainable and geometrically complex with many bespoke details, yet it was completed on programme and within budget. This was facilitated, in part at least, through close team collaboration and partnering between consultants and contractor, for example the design and detailing work that went into the roof structure meant that the doubly curved diaphragm was relatively straightforward to fix on-site. The demands of the brief required a tight specification to be accurately executed on-site, and a high level of craftsmanship was maintained throughout, including working through one of the toughest winters in recent decades. Local feedback has been very positive, the building has received six design awards and it is hoped that the building will inspire and educate in equal measure.
Text by Eco Arc & Peter Corbett ( Corbett-Tasker formally of Gifford ) Pictures Rachael Meyer
Client: The Royal Horticultural Society
Architect: Eco Arc Ecological Architecture Practice
Project Manager and Quantity Surveyor: Turner and Holman
Structural Engineer: Gifford LLP
M&E Engineer: Gifford LLP
Main Contractor: William Birch and Sons
Timber Fabricator: David Dalton Joinery
Photography Rachael Meyer SIRA Studio Ltd