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Sustainable timber design at Achray Bridge

Achray Bridge, Queen Elizabeth forest park, Perthshire, Scotland.

The bridge over Achray water is one of four bridges on an important country access through Queen Elizabeth Forest Park in Perthshire. The Forestry Commission had begun a research project to develop a design technique to build long span bridges using short lengths of Scottish plantation timber. The project leaders showed great insight when they agreed that two of the bridges could be of this revolutionary and untried design. The aim was to construct bridges which would demonstrate maximum sustainability with an aesthetic presence which would interest walkers and provide them with a reason to stop, explore and absorb the surroundings.

"The specification required the design to cater for walkers, cyclists and horses while providing all ability access. This determined gradients, handrail heights, loading capacities and so on which determine the materials that are necessary to span the rivers. There was a will to use timber but available timber sizes cannot span 20 m. A stress laminated timber arch, however, could provide the structural resistance for such a span so the design began along those lines. The design criteria was untried anywhere in the world so we first carried out a load testing trial at Glentress Forest.

Timber is the only renewable structural resource available to man. It is carbon neutral because at the end of its useful life the carbon dioxide which it puts into the atmosphere is equal to the quantity that it removed while it was growing. Timber is easy to work and to fix and many of the traditional skills still exist. It is the Forestry Commission’s material and the forgotten primary structural element."

The future

"The construction of low cost sustainable bridges is a worthy aim. The world is entering a period where specialist materials are in short supply, energy costs are escalating and atmospheric pollution is getting out of control. In this context anything that can be done to help the environment is politically, socially and morally justified. Throughout the world, bridge construction uses a significant amount of material and much of it requires energy-intensive processes to make it, transport it and build with it. As population rises, demands on the available materials increase and therefore costs escalate."