York's historic Clifford's Tower has been radically transformed with a modern timber staircase and engineered timber viewing area that breathes new life into this ancient monument.
Clifford's Tower is the largest surviving structure from the medieval royal castle of York, built in the mid-13th century but has stood as a roofless ruin since 1684. The tower offers superb views over the city and following a £5million project to conserve and improve the interior of the 800-year-old landmark for English Heritage, it now incorporates a freestanding timber solution that protects the ruin, carries a timber viewing deck, and sees new walkways and stairs give public access to features unseen for over 350 years.
Timber was ideally suited for the new structure, with its low carbon impact and lightweight nature. The glulam timber structure was designed so that all connections were concealed, and a neat appearance achieved. The main structure comprises primary beams supported on four columns, carrying a timber viewing deck and central atrium. The columns have recesses concealing drainage and services routes. New services were ducted through the new raft foundation and concealed within the glulam columns, allowing the deck to be used for functions and night-time events with LED lighting concealed in structure, finishes and handrails.
The design is 'circular' with the timber frame demountable and reusable, if needed at any point in the future. This gives further sustainability benefits where the carbon captured within the timber can stay locked into the material for a significantly longer period of time. Using timber supplied by leading French softwood supplier Piveteaubois, Buckland Timber installed the engineered timber elements alongside Douglas fir decking. Buckland chose to work with Piveteaubois as they could supply the sapwood-free Douglas fir that was used to make the glulam for the larger elements. They were looking to cut down on in-house manufacturing work and Piveteaubois could supply the floor joists thanks to their cuttingedge CNC machining capabilities.
Piveteaubois provides a consistent grade of sapwood-free Douglas fir. Being dried to 12% moisture content it is perfect for glulam manufacture, so it worked well for Buckland to call on them for any large volumes that are needed quickly. For the Clifford's Tower project, having some of the beams delivered already machined was useful as it allowed a reduction in manufacture time.
Piveteaubois supplied 36m3 of 140mm x 350mm GL24h sapwood-free Douglas fir glulam, comprising of 55 beams, with 38 of them further processed by Piveteaubois' state-of-the art SMC OIKOS 5-axis machining centre. This delivered precision cutting to any required length plus a wide range of machining capabilities. The 38 beams were cut into 129 pieces of various lengths and machined with mitre and bevel cut, as well as drillings and counterbores for easier installation.
Piveteaubois produces 50,000m3 of glulam and finger-jointed Douglas fir, Spruce and Pine glulam annually. With a permanent stock of around 3,000m3 in its 5,000m2 warehouse, they are ready to supply customers with standard 13m lengths or bespoke orders. This also includes a range of glulam treatment options including pressure treatment of Douglas Fir to Use Class 3.2 or prepressure treated UC4 Pine glulam for higher risk environments.
Douglas fir combines remarkable mechanical properties with excellent natural durability as its duramen is naturally Use Class 3.2 without treatment. Sapwood-free Douglas fir glulam, cladding and decking are excellent for use outdoors and age gracefully just like Larch and at a lower cost. Piveteaubois also offers Douglas fir C24, finger-jointed timber as well as CLT, making it a hugely versatile building material. All Piveteaubois Douglas fir comes from sustainable forestry and is PEFC-certified.
The non-slip timber deck was constructed using PEFC-certified Marley CitiDeck®, which is manufactured from premium grade European Redwood and was specified by Hugh Broughton Architects Ltd. The decking was installed, and carefully cut to a smooth curve following the line of the new metalwork guarding at the edge. There is also a large opening in the middle that allows visitors to look down to the floor of the keep. The boards were also used to create seating areas and the guard structure around the central opening. CitiDeck® is an attractive and robust anti-slip timber decking solution ideal for use in public areas, the non-slip surface ensures the deck is safe to use all year round.
Working with Hugh Broughton Architects and conservation architect, Martin Ashley Architects on the detailed design, Ramboll provided the engineering design solution for this sustainable timber structure. This unique project introduced an architecturally stunning, structural timber intervention within the tower, to allow the public to fully access and appreciate the monument whilst demonstrating exemplary conservation practice by respecting the significance of the monument and site. Clifford's Tower re-opened in April 2022 and has been shortlisted and won several design and construction awards including Best Retail & Leisure Project and Winner of Winners Award at the Structural Timber Awards 2022.
www.uk.ramboll.com
www.piveteaubois.com/en/
www.bucklandtimber.co.uk
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