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Oak Bridge

Oak Bridge

Special Award: Best Use of British Timber 2009

Location: Northumberland
Building Owner: Private
Architect: McMorran & Gatehouse Architects
Builder/Main Contractor: Jeremy Cosmo Davies
Structural Engineers: Price & Myers LLP
Joinery:
 Jeremy Cosmo Davies
Wood Supplier: Stuart H Somerscales LTD
Wood Species: Oak (Herefordshire)

The brief required a pedestrian and light vehicular bridge spanning 10m at the head of a stream through a spectacular mature Northumbrian landscape garden. Constructed entirely from oak, the commission is for a private client and completed in November 2008.

The design is a collaboration between the green wood furniture designer and maker, Jeremy Cosmo Davies, McMorran and Gatehouse Architects and Price & Myers Engineers. Previously engaged by the client to design and build a set of Windsor style office chairs, Jeremy has also developed a design range of ‘elliptic’ garden seats inspired by North East clinker built boats and silhouetted fragments of old beached timber hulls along the coast.

These influences, together with a visit to the local Eyemouth Boat Museum, convinced Jeremy and architect, William McMorran, that here was a rich seam of craft construction technique to be explored. A single span, fan ribbed arch was developed into a series of three dimensional reflexive curves, utilising the natural ability of solid wood to take up complex geometry.

Initial sketch designs by McMorran and Gatehouse, were developed into a card model. Resourcing main beam timbers in the lengths and girth necessary was a major concern. A design review looked at fabrication in sections or lamination. Both possibilities were ultimately ruled out, due to doubts about the long term durability of exterior adhesives applied to air-dried oak, and the denial of the natural flexure of timber, if sectioned and then joined by metal connectors. The team was determined to retain the original conceptual purity of a curved all timber bridge. A computer model was prepared by the engineers. It was at this point that Price & Myers developed the diagonal bracing system and the galvanised steel beam end support flange geometry. The 3D model also provided a set of full size templates from which the curved beam sections could be accurately cut in preparation for cold bending on site.

Finding the size of trees necessary did indeed prove arduous and involved a national search. Despite concerns about the heartwood integrity of the selected standing trees, exactly the right timber was eventually obtained, by Stuart H Somerscales Ltd., from a sustainable plantation dating back to the last days of timber ships, in Herefordshire. Trees were selected for their natural curvature, ensuring that, as far as possible, grain follows beam camber. Diagonal bracing involved production of 64 noggins, each individually scribed to their locations. An on-site chain morticer machined two pairs of sockets into the opposite sides of each noggin to receive stainless steel coach bolt fixings wound in place through diagonal pilot holes with a torque wrench, directly into main beam cross grain. This system allows end to end alignment of noggins. Shiplap boarding is cut in straight sections curving naturally to the bridge balustrading geometry. Construction was carried out entirely on site by Jeremy, assisted by an ad hoc relay of unskilled but very enthusiastic helpers, one of whom occasionally included the architect.