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.