A WELCOME ADDITION TO SUSTAINABLE CONSTRUCTION

RHS Garden Bridgewater is the largest gardening project in Europe with its new Welcome Building showcasing a number of different timber construction components and techniques to award winning acclaim.

The Royal Horticultural Society (RHS) is the world’s leading gardening charity, inspiring passion and excellence in the science, art and practice of horticulture. In November 2015, the RHS, in collaboration with Salford City Council and Peel Land and Property, announced the vision to create the new garden at the site of the 154-acre Worsley New Hall estate. RHS Garden Bridgewater will become a major new tourism and horticultural destination which aspires to welcome and inspire up to 700,000 people a year within a decade. The new ‘Welcome Building’ saw Hodder + Partners invited to design a new world-class visitor centre providing a gateway to the gardens as well as a visitor meeting and interaction point, restaurant, gift shop, offices and educational spaces. The Welcome Building has been delivered by BAM as the main contractor with HESS TIMBER providing the engineering, production and assembly of the structural timber elements.

The Welcome Building is designed predominantly as one open space, allowing the visitor to flow between the various uses and functions: ticketing, a learning centre, retail area, indoor plant sales and café. All of these public elements are contained under a single overarching timber diagrid roof, supported on structural glulam timber columns, allowing the space to flex in response to seasonal demands. The roof structure consists of three different timber construction components: ‘cigar-shaped’ glulam columns, timber diagrid cassettes with diagonally arranged inner grillages and a straight

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outer glulam timber rafters, as well as cross laminated timber (CLT) panels to tie the cassettes together.

The sanded ‘cigar-shaped’ glulam timber columns rest on 16 reinforced concrete columns, each of which has a diameter of 450 mm. The glulam timber columns measure 3850mm in length and 300mm in width (in their centre) and are tapered to 200mm at both ends to ensure an attractive, filigree and light overall appearance of the roof construction. The columns are structurally relevant and support the entire roof, which measures 2160m in total. The timber diagrid consists of individual cassettes dividing the whole roof into 6m grids. Each individual cassette measures 6m x 3m and consists of diagonally arranged inner grillages and straight outer rafters which provide stability and bracing to the whole roof structure.

Natural light permeates the building either through Siberian Larch louvred curtain walling or filters through the diagrid roof via two roof lights, one

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running centrally 6m wide x 54m long and one 18m wide x 12m long creating a glasshouse environment. Sustainability is at the heart of the proposals and embedded in the work of the RHS, the building benefits from rainwater harvesting, green roofs, ground source heat pumps, underfloor heating and cooling and natural cross ventilation. The visually impressive use of timber as the material of choice in the roof construction provide a constant reminder of the environmental aims as well as sequestering an approximated 350 tonnes of carbon into the structure of the building. The timber used across in the project is the PEFC-certified and was manufactured with zero wastage production methods. State-of-the-art CNC technology ensure that the timber is cut to precise measurements with off-cuts reused for other purposes down in the value chain such as chipboards, pellets or bio-fuel power.

3D modelling was used extensively as an important tool in understanding complex junctions between structural components. A particular example would be the junction between the concrete columns, steel head connections and glulam timber columns. The 3D modelling was also used to communicate ideas, ensuring all structural, servicing and aesthetic requirements were met within very fine margins.

Due to the very large timber components it was crucial to plan the production, logistics, preassembly and assembly just in time to reduce the installation period in total and to be able to handle the heavy components on-site effectively and without taking up too much space as around the building. High use of offsite fabrication minimised site set-up and reduced waste and kept the project within a very tight time schedule, with the whole roof structure installed within eight weeks. Another key success factor for the overall sustainable and energy efficient approach all project participants had in mind was the high degree of offsite fabrication of timber components delivered to site. The smart and wellthought-out fixing of the cassettes to a whole 24m long unit with respect to the challenges of the assembly of such a structure benefited a quick assembly immensely and therefore minimised energy consumption compared to conventional on-site work. With the use of timber as the main roof construction material it has been possible to sequestrate approximately 350 tonnes of carbon into the structure of the building.

Highly co-operative and collaborative team work allowed developing a precise programme based on HESS TIMBER’s wide experience to optimise delivery dates and to achieve continuous progress on site without significant interruptions. The high-quality surface finish demanded by both the building owner and the architects has been one of the most challenging parts. A milky white transparent varnish had to be applied to all the timber elements to get an even and very exquisite finish. While low in embodied carbon, the timber elements require only very minimal maintenance

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throughout the building’s lifetime as the timber does not trap dust or dirt as for example a steel framed equivalent.

The Welcome Building can be seen as exemplar in the holistic approach to meeting a range of requirements whilst delivering a sustainable and high quality spatial experience. The dramatic roof and flexible, column-free environment showcase the spatial opportunities available when using structural timber whilst also highlighting the sustainability of the building. This will be visible to an anticipated 700,000 visitors per year, increasing the understanding and acceptance of timber construction. As the building will be highly frequented both by locals and tourists the visual and constant display of timber can build confidence among all visitors towards timber in bigger structures.

RHS Garden Bridgewater won Architect of the Year and Project of the Year 2020 at the Structural Timber Awards.

www.hess-timber.com www.hodderandpartners.com

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01-04. The Welcome Building is a significant timber and sustainable addition to the largest gardening project in Europe. Courtesy RHS/Hodder+Partners