Far Moor Bridge
Structural Award Highly Commended 2011
Location: North Yorkshire
Building Owner: Yorkshire Dales National Park Authority (client)
Architect: Civil Engineering Design Services (formally Forestry Civil Engineering)
Builder / Main Contractor: Houseman and Falshaw Limited
Structural Engineers: Civil Engineering Design Services (formally Forestry Civil Engineering)
Joinery: CTS Bridges Limited (responsible for joinery and construction)
Other Associated Company: Marsden AES Limited (responsible for landscaping/holding areas)
Wood Supplier: James Jones (wood cut, sized and drilled by Forestry Commission)
Wood Species: Scottish Larch
Far Moor Bridge is part of the new 350-mile Pennine Bridleway National Trail, funded by the Countryside Agency (now Natural England) and Sports England. The section in the Yorkshire Dales at Far Moor Common, Selside, is a new section of bridleway with a new bridge, across the River Ribble. The commissioning Authority saw an opportunity to build an iconic structure, which would delight walkers and riders while being environmentally sustainable.
Timber was the preferred material for construction being a natural material that would sit comfortably with the landscape in colour and context but it was thought that the required span was too great for this material. However, on seeing a stress laminated single arch bridge at a site in the North York Moors, Yorkshire Dales National Park (YDNPA) approached Forestry Commission Enterprise (FCE) to see if they could provide a design solution for a much larger bridge using this technique.
The brief required a bridge that would be aesthetically pleasing, complement the surrounding rural upland, be constructed with regard to environmental constraints and be of a high quality standard as expected by users of National Trails The design of the bridge, a flat arch structure, reflects the ‘egg shape’ drumlin landscape of the locality. The type of timber construction is, as far as we can ascertain, unique in the world. It is a combination of stress laminated arch construction acting compositely with a screw laminated stressed timber deck. Assembled on site the main construction material is Scottish larch treated with the wood preservative Tanalith E with oak used on the outer section of the lower arches as this is able to withstand and spread the stress loading of 150KN. The arch to concrete pad foundation support brackets, stressing rods, washer plates are galvanised steel. The design loading was 5kN/m2 with as high a fundamental natural frequency as possible to ensure comfort for horses.
FCE have considerable experience of constructing bridges for this type of use and knew that a 50m long bridge could become a major obstacle for some types of horses. It was for that reason that the design linked the deck and the arches to form a stiff composite structure. FCE achieved a FNF of greater than 5Hz. The resulting design required a three dimensional analysis never before attempted for this type of structure. The central span over the river is 24m with only a 200mm deep section. The bridge required 2 side arches of about 15m to maintain the integrity of the flood plain. Trial pits revealed that the ground at either springing of the central arch would not support the significant lateral thrust of about 300kN. The design was modified to ensure the thrust was transmitted through that support and into the side arches. This transferred the lateral thrust to the outer foundation which are in drier, better ground.