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Tension in the treetops

Lough Key walkway - An unusual tensioned deck cable stay structure forms a new walkway across Ireland's Lough Key park, as Ed Owen discovers.

Lough Key in Ireland was popular in the 1970s for families coming on trips from Dublin, but died during the 1980s, ' says Richard Nicholl of consultant Ferguson McIlveen.

Now the 350ha forest park is being redeveloped.

As well as adding a 250m long treetop walkway, reminiscent of the Ewok village in the Star Wars lms, the redevelopment will include an adventure house and an imposing concrete lookout.

Design proposals for revitalising the park were sought by client Moylurg in 2003.

Moylurg is a partnership between Roscommon County Council and Coillte Teoranta - the Republic of Ireland Forestry Department.

A team led by Ferguson McIlveen was appointed to manage the £4M scheme. The major feature, though, is the tree top walkway.

'Canopy walkways have been used around the world.

Our brief was to use timber, to demonstrate its structural uses, ' says Nicholl.

One major difference between the Lough Key walk and others around the world is accessibility - this walk had to be open to visitors with impaired mobility, including those using wheelchairs.

The client also wanted a suspended structure whose masts would resemble those of the sailing boats that are seen in numbers on the nearby Lough.

What has emerged is a cable stayed structure with graceful glulam towers and piers.

For extra stablity, the deck is held in tension by an arrangement of V-shaped trusses underneath, connected by MacAlloy stainless steel tie bars.

'One thing we realised early on was a need for vibration control - it is designed for wheelchairs and families. From a purist structural engineering point of view, it appears overengineered, but looking at what happened at the Millennium Footbridge, we were very careful.

Nicholl and his team had found during detailed design that a cable stay structure was too lively for disabled and less able users.

'The client's design requirements were for the structure to ensure accessibility for all, so we had to compromise by introducing a basic beam and post system, with the masts and cable stays retained primarily for aesthetics, but also to assist with damping the suspended decks, ' Nicholl explains.

Contractor Lagan won the tender to construct the walkway, submitting a £1.2M bid which it whittled down by £400,000 through some practical value engineering done with timber subcontractor Donaldson & McConnell and steelworks engineer, Woodburn Engineering.

The circular hollow section glulam masts could have been something of a weak spot.

'Some were nervous and felt it was too risky, but Lagan brought tougher than normal glulam from northern Sweden, ' says Nicholl.

Masts were supplied in 12 segments with a jointing system which worked well with the design.

For the cable stay system 'we looked at semi-harp string designs, with multi wire cables.

Ferguson McIlveen found that these cables would be expensive and the galvanising breaks down over time, which introduces maintenance issues.

'So we used stainless steel.

By December 2003 the costs were down to £800,000, ' says Coates.

During construction, the contractor had to work within strict environmental controls to preserve the ecology of the forest and waterways.

'We had to ensure that any use of concrete would not contaminate the area, so we used small diameter micro-piles.

'The pile caps all had to be above ground level. These limitations meant we used simple forms of construction, ' says Nicholl.

'Everything was designed to be man handled, and the construction used scaffolding, working its way through the forest.

'This was really the only practical way because of the tree canopy. We did use one mini-crane which could lift 3t up to 20m high.

'To gain access we had to create a stone path through the forest oor and pull it out again afterwards, ' explains Coates.

The walkway will not open until spring 2007, but there is already talk of extending it by a further 600m to accommodate the 100,000 to 150,000 visitors expected each year.

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