Stihl Treetop Walkway

Introduction

A new high-level walkway at Westonbirt Arboretum gives visitors the chance to travel through the tree canopy at a height previously only experienced by birds and squirrels. This dramatic structure, the Stihl Treetop Walkway, winds through the Grade I-listed historic landscape, presenting a unique view of one of the finest tree collections in the world to its visitors.

The walkway is the latest project for the Arboretum designed by Glenn Howells Architects. The practice was responsible for a new masterplan of the site and has designed an elegant timber pavilion with a curved roof, the Welcome Building, at the main entrance (subject of a TRADA Case Study in June 2015). More recently, in the ‘working’ part of the estate, a pair of innovative new timber buildings designed by Invisible Studio, has just been completed (described in detail in a TRADA Case Study in March 2017). The structural engineer BuroHappold Engineering was part of the design team for all three projects.

The route of the walkway takes advantage of the natural topography of the site. It bridges part of a sunken a valley known as the Silk Wood, maintaining level access over a large dip in the ground below. It starts at ground level, rising very gently at both ends (more than 1:20) to allow access to visitors of all ages and physical abilities; the walkway and most of the interpretation spaces are accessible by wheelchair and mobility scooter.

As the ground dips, visitors reach a viewpoint more than 13.5 metres above the forest floor, among the treetops of the ancient woodlands of the Silk Wood, with further views across to The Downs. At 280 metres in length the walkway is the longest structure of its kind in the UK.

The route takes a sinuous form, snaking above and through the tree canopy, supported by elegant scissoring timber legs. Visitors are encouraged to pause a while at four key observation points where the walkway widens to bring them closer to the trees; at one of these a secondary viewing platform is wrapped round a 36 metre tall pine tree and reached from the mid-point of the walkway by a small flight of steps.

Reflecting the treescape setting of the surrounding natural landscape, timber was the obvious choice of material for the project and was at the heart of the architectural concept developed by the design team. The walkway structure, deck and guardrails are predominantly constructed of timber.

The walkway was designed to be as elegant and sustainable as possible within the constraints of the budget. The walkway deck is supported by a series of timber columns, set in pairs and canted across each other like a pair of scissors, to mimic the lean and sway of surrounding tree trunks. As the project architect explains: ‘The paired scissor columns allowed us more freedom for the foundations because they required smaller foundations and excavations than a single column would have needed. This meant less disruption to surrounding tree roots. The scissors also meant greater stability, which was particularly important due to the uneven terrain.’

The pairs of columns are set apart at 10.75 metre centres and the pairs vary considerably in length, depending on the terrain below.

The walkway structure, which curves to accommodate tree roots, consists of a pair of RHS outer members set at 1500mm centres with secondary steel beams welded between them. The canted balustrades on each side, a series of 15 x 50mm galvanized steel flats set apart at approximately 100mm centres (depending on the curve of the walkway), are welded to steel plates welded back to the RHS members. The balustrade is topped with a solid larch handrail. The deck is of 138 x 75mm C16 larch planks, inset with carborundum strips for slip resistance.

 

The use of timber

Great efforts went into ensuring that when touching the handrail on either side, the deck underfoot, and when viewing the visible columns seen under the walkway – all would be in timber.

Larch was selected as the principal material for the walkway, both for its durability and attractive colour, with Scottish larch chosen for the deck and handrail. The larch, it is anticipated, will weather to silvery tones, which will match the galvanized steel balusters which support it.

The columns are of solid Siberian larch. Because of its harsh growing conditions in the sub-arctic tundra, the Siberian variety of larch produces a tighter grain and higher strength-to-weight ratio than other varieties and so could accommodate the weight of the walkway and the height required.

The architect worked in partnership with the Netherlands ship-mast makers, Ventis & Brasker Masten. The columns taper at both ends, minimising their visual impact. The top and base of each column is notched, pinended and bolted to a steel capping plate, ensuring that it takes only axial load.

After fabrication the columns were oiled to delay the drying out of the timber and to minimise cracking. The larch decking and handrail were treated with micronised copper, a transparent treatment which allows natural weathering while providing a 30 year warranty against rot and fungal infection.

 

The timber structure

Project structural engineer Joe Darcy of BuroHappold Engineering writes:

‘The structural design of the walkway was a close collaboration between BuroHappold Engineering, the client and the architect to create a beautiful structure that has structural engineering at its heart and enhances its beautiful natural setting.

The design required a considered and sensitive solution to the trees, responding to their presence both above and below ground. The route became informed by the journey through the canopy at high level and the necessity of providing foundations within the root zone of mature trees. This in turn led to an increased geometrical complexity of a continually curving deck, supported by crossing legs stabilised by tension rods. Parametric software was used collaboratively on the project to allow quick regeneration of the geometry to suit the developing route defined by the differing demands.

The structural solution was informed by both the architectural vision and the client’s wishes to highlight the use of timber. To support the walkway, significant lengths and diameters of timber were required, but cost restrictions meant engineered timber such as glulam was not a feasible option. To source the wood, we identified a unique collaboration opportunity with an unexpected industry, using the mast-building technology of Dutch shipbuilders to machine solid round timber for the walkway’s cigar shaped legs. While this provided the means to support the structure, careful consideration was required to understand how the timber would behave and change over the course of the structure’s lifetime. The nature of the structure meant that vertical load is taken down into the foundations as compression through the timber legs, which are offset and stabilised by the tension rods in each of their planes. As a result of the offset legs, an inherent twist is induced into the deck structure. The tension rods also proved critical for the natural frequency of the structure, and advanced dynamic analysis was carried out to optimise the stiffness and geometry of the rods to improve the lateral dynamic performance’.

Completion Date:

April 2016

Building Type:

High level walkway

Location:

Westonbirt Arboretum, Gloucestershire

Client:

Forestry Commission

Architect:

Glenn Howells Architects

Structural Engineer:

BuroHappold Engineering

Main Contractor:

Speller Metcalfe, Gloucester

Joinery:

S H Structures, Sherburn-In-Elmet, North Yorkshire

Timber Supplier:

CTS Bridges, Huddersfield;
Russwood, Inverness

Timber Column Fabricator:

Ventis & Brasker Masten, Enkhuizen, Netherlands

Timber Elements:

Structure, deck handrail

Timber Species:

Scottish larch, Siberian larch

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