Far Moor Bridge, Far Moor, near Selside, Yorkshire

Introduction

A unique timber bridge, a combination of stress-laminated arches and screw-laminated deck, crosses the River Ribble in remote moorland near Ribblehead in the Yorkshire Dales. Far Moor Bridge is far from any road; it is designed for horses, cyclists and walkers who take the Pennine Bridleway, a new National Trail in the north of England. Funded by Natural England and with the support of Sport England, the bridleway enters the Yorkshire Dales National Park at Long Preston and weaves through the Dales to Kirkby Stephen. A new section of bridleway, together with the new bridge over the Ribble, has been created near Selside, only a few miles from the famous Ribblehead railway viaduct. Here the commissioning authority saw an opportunity to build a comparably iconic structure which would also be environmentally sustainable and low budget.

The brief specified a bridge that would be suitable for horses, cyclists and pedestrians; it should be aesthetically pleasing, complement the surrounding rural upland and be constructed on sustainable principles in terms of environmental impact, energy use, pollution reduction and whole life cycle cost. It should also be of a high quality as befitted an important national trail.

 

The concept

The idea of using stress-laminated timber techniques to construct rural bridges was developed by Dr Geoff Freedman, who, before his retirement from the Forestry Commission in 2011, ran a specialist design and construction service for rural bridges within the Commission (a service which continues today as Geoff Freedman Rural Bridges). Twenty years ago Geoff had observed the development of stress-laminated bridges in the US; subsequently, as a PhD project, he developed the principles of stress-laminated timber to create an arched bridge and tested the results to full-scale, often to destruction. Stress-laminated bridges are not only low-cost and sustainable but have particular advantages to the Forestry Commission as they can be erected in remote rural locations. In the last ten years about 100 of these types of bridge have been built in Scotland and northern England. The Yorkshire Dales National Park Authority had seen an example and approached Forestry Civil Engineering to see if a bridge across the Ribble could be built using the same technique.

Reference:

Freedman. G. Vertically Laminated Timber Bridges.
Proceedings of the Institution of Civil Engineers, Nov 2009.
Journal of Construction Materials, Vol 162 pp181-190
Geoff Freedman Rural Bridges t: 0131 448 2333 e: geoff.freedman@googlemail.com

 

Structural design

The spans, flood characteristics and topography meant that the bridge deck would have to be supported by three arches. The central span of 24 metres over the river is taken by the central arch; two side arches, spanning 10.8 and 16.8 metres, were additionally needed 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 foundations which are in drier, better ground.

Each arch is of stress-laminated timber construction, a technique in which a series of sawn timber planks – the laminates - are laid side-by-side and compressed together by threaded steel bars. The bars are passed through holes drilled in the wide faces of the laminates and tightened against external bearing plates. The resulting pressure sets up friction forces between the laminates, making the whole into a solid load-bearing timber plate with the ability to distribute load laterally and longitudinally.

In the case of the bridge arches, the laminates were 200 x 50mm sawn timber planks, about 2.5 metres long, with four equally spaced holes in the 200mm wide face. The laminates were laid side-by-side on a curved support scaffold; steel bars were threaded through the holes and tightened against the outer laminates with large steel circular washer plates. Adjacent laminates were staggered by one hole so that, as additional ones were added, a continuous and solid plate was created, faceted to form the curve of the bridge.

A hydraulic jack was used to tighten the bars and compress the laminates. Once compressed, (so that the tension bars were recorded at 85% of the bar’s breaking strain) the jack was removed and the process was repeated along the length of the bridge. The supports of the temporary scaffold were then removed - a potentially nerve-wracking moment - but only a 2mm deflection of the main bridge structure was recorded. The deck, supported by timber bearers, was then built over the arches. The screw-laminated deck construction is based on similar principles to the arch construction, the difference being that the deck laminates - 150 x 50mm timber planks - were each screwed to adjacent laminates to form a solid load-bearing timber plate. Top rails were then added to complete the structure.

The design loading was 5kN/m2 with as high a fundamental natural frequency as possible to ensure comfort for horses. For this reason the deck and the arches were linked to form a stiff composite structure, achieving a fundamental natural frequency of greater than 5Hz.

 

Materials

The main construction material is Scottish larch treated with the wood preservative Tanalith E, with oak on the outer section of the arches to withstand and spread the stress loading of 150kN. All timber laminates, posts and rails were pre-cut, drilled and pressure treated before being delivered to site. On site, a clear coat of wax waterproofing was applied to the soffits of the arches to encourage shedding of windblown rain and to protect cuts around posts. The screw-laminated deck is finished with an epoxy resin and bauxite non-slip surface on bitumen macadam, applied in-situ to provide waterproofing and a cushion for horses’ hooves. The balustrade posts are substantial enough to take lateral load from horses; they also connect the deck with the arches below, additionally stiffening the deck.

The arch-to-concrete foundation support brackets, stressing rods and washer plates are all of galvanised steel.

 

Pollution and disturbance of wildlife and salmon in the river

As the site is of significant ecological value, construction activities were carried out with great care to avoid pollution. During the salmon spawning season - July to the end of September - no machinery was permitted in the water and scaffolding was carefully removed. Crayfish breed in the water and sometimes carry an infection which must not be spread. Scaffold and machinery was disinfected before and after being in the water. The foundations for the arches were kept back from the water's edge to provide a safety gap in case concrete was spilled; the gap also allows otters and other wildlife to pass on dry land while staying close to the water.

 

Sustainability

The main structure was of plantation timber from a site registered by the Forestry Stewardship Council or equivalent. The steel was a very small element of the structure and was mostly recycled. Concrete in the foundations was kept to a minimum by using special structural connections to transfer load. The pressure treatment for the timbers uses Tanalith E, the least toxic option and the most sustainable available. With proper maintenance the bridge will have a minimum life of 40 years.

It is also possible to dismantle the bridge for re-use by de-stressing the tension bars and taking apart the deck and arches, one laminate at a time. It could also be lifted from its supports and dismantled on the bank.

 

Cost

A previous tender, of similar size but of a different design, had indicated a price of around £500,000. Far Moor Bridge cost £130,000 - about £1,000 per square metre of deck.

Completion Date:

January 2011

Year Published:

December 2011

Building Type:

Bridleway bridge

Location:

Far Moor, near Selside, Yorkshire

Client:

Yorkshire Dales National Park Authority

Structural Engineer:

Civil Engineering Design Services (formerly Forest Civil Engineering)

Builder/Main Contractor:

Houseman and Falshaw Ltd

Joinery and Construction:

CTS Bridges Ltd

Landscaping / Holding Areas:

Marsden AES Ltd

Timber Supplier:

James Jones (timber cut, sized and drilled by Forestry Commission)

Timber Element(s):

Bridge structure, deck, balustrade posts and rails

Timber Specie(s):

Scottish larch, oak

Awards:

Wood Awards 2011 Structural category: Highly commended British Construction Industry Awards 2011: Judges Special Award

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