Bridging the gap at the AA’s Hooke Parke
29 February 2008
Architects and AA tutors Valentin Bontjes van Beek and Natalie Rozencwajg’s web-like footbridge uses trees for its main supports
Architects: Valentin Bontjes van Beek and Natalie Rozencwajg
Site: Hooke Park, Dorset
Looking like an oversized stick insect creeping through the forest, the newly completed Crossings is the first bridge for many years to be built in Dorset’s Hooke Park, which is owned by the Architectural Association.
The pedestrian bridge is held in place by a combination of cables slung around existing beech trees and a spidery network of criss-crossing timber members.
Positioned in a picturesque spot in the 140ha working woodland, the crossing, which spans 19m across a 4m-deep valley, replaces a dilapidated suspension bridge and provides a new link to the public bridleway located nearby.
It was designed by AA tutors Valentin Bontjes van Beek and Natalie Rozencwajg, and won the 2006 Custerton award, an annual £7,500 prize for projects making use of indigenous and sustainable timber, constructed by AA students and tutors in Hooke Park.
“The design” says Bontjes van Beek, “grew out of an early decision to use existing trees as the prime foundation for the bridge, with the intention of minimising the impact on the natural forest environment. As a result, the location of the trees determined the geometry and design of the bridge.”
“The location of the trees determined the geometry and design of the bridge”
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To create the crossing, 41 Norway spruce trees were felled in the forest as part of the thinning process, cut into logs and left untreated. These logs have been connected to the bases of 12 existing beech trees, and are held in place by a combination of concrete and bolts, forming the angled columns that provide the supporting structure to the bridge. Steel cables are wrapped around the strips of rubber tyre which encircle the beech tree trunks.
The untreated Douglas fir and pine walking deck followed, which was cut using CNC technology. Fitting the mild steel balustrade was the final piece of the bridge jigsaw.
The construction of the crossing relied on AA students working on it voluntarily during holiday periods.
Over the two-year construction programme, tutors, students and locals worked on seven different stages. The final cost of the bridge was around £14,500, with additional money coming from local funding organisations.
“Pulling it through and staying with it for the past two years, which has often involved working weekends, has been tough,” says Bontjes van Beek, “but working with so many people has been a brilliant didactic experience.”
The crossing looks as if it could easily be dismantled, packed away and removed without leaving any trace of man’s interference.
In fact, the untreated state of the timber and the lack of maintenance proposed for it mean that over the years, it will disintegrate gradually and will eventually merge back into the forest floor.
Initial concept
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The architects’ original concept used rope for the main structural support, as well as in place of the stainless steel cables.
The idea was to use the geometry of the trees to weave a web-like structure. Through the placement of the knots, a narrow walkable area would emerge.
Using rope also tapped into the now-abandoned ropemaking tradition of the Dorset town of Bridport.
But following a test to assess the deflection of the ropes, it was concluded that these would be too elastic, and that the structure would swing or vibrate too much.
It was then suggested that the ropes should be substituted with a heavier and stiffer alternative — logs.
Balustrade
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Several 6m-long, 32mm-diameter bright mild steel tubes have been bent and joined together to create a continuous handrail loop which weaves over and under the structure.
A further nine vertical 19mm-diameter sections of steel tube were spot-welded to the top rail to stabilise the balustrade. Holes were first drilled into the handrail, with vertical sections pushed through, then spot-welded.
The balustrade, which meanders a total length of 38m, has been painted with a clear varnish so that it will gradually take on a corroded appearance.
Timber framework and joints
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Lengths of timber of between 1.3m and 8.5m form the criss-crossing support framework to the bridge structure.
These are connected to each other and to the existing beech trees using a primitive but tried-and-trusted method of jointing. There are 23 joints overall, of which 12 connect
to the beech trees.
Concrete was used to connect all the joints. The construction team had to ensure the joints were well prepared beforehand. This entailed correctly positioning the logs and fixing screws into the log ends.
Formwork was constructed from locally sourced clay. A bowl was created where the log ends all meet, with a steel reinforcement spiral fixed inside. The next day, concrete was poured into place and the joints left to dry.
The clay formwork was eventually removed, and work then began on the decking.
Steel Cables
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Galvanised steel cables wrap around the existing beech trees, protected by sections of recycled rubber tyres.
These encircle the tree trunks, helping to keep the angled structural logs in place and stiffening the whole structure.
Decking
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Valentin Bontjes van Beek describes the deck as tructurally the most complicated and difficult piece of the bridge, mainly because it has the most human contact and therefore has to bear weight and be made safe.
It comprises 141 different segments cut using Hooke Park’s own CNC machine. Each piece has a different interlocking slot angle, which allows the deck to twist slightly, creating a double curved surface. This forms a deck that slowly rises and falls. As one moves across the structure, it concertinas in and out; at its widest, it is 3.5m across.
The deck had to be made in four sections of similar lengths fixed to metal plates screwed into the deck, with log columns located beneath the bridge.
Two central sections of the deck have been filled with plywood to plug gaps between the planks of timber.
Flying knot
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There is only one true flying knot used in the construction of the bridge.
Concrete is still used to secure this joint, but its point of difference with the others is that the slanting beams attached to it do not connect to the existing beech trees. Instead, it is held up by the steel cables.
Postscript
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Detail photos: Valentin Bontjes Van Beek
Readers' comments
Me impresionó gratamente este proyecto porque tiene como origen la gran preocupación mundial por cuidar el ambiente. En cuanto a la institución educativa, es interesante su preocupación por desarrollar, con estos proyectos, un espíritu creativo y emprendedor en sus alumnos. Felicidades a la revista que pública este artículo y a los maestros y alumnos que lo emprendieron.