Xstrata Walkway_Blueprint spread

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XSTRATAWALKWAYLONDON EYE DESIGNERS,MARKS BARFIELD AND JANEWERNICK HAVE A THING ABOUTHEIGHTS. UP IN THE TREE-TOPSAT LONDON’S KEW GARDENSTHREADS THE XSTRATA WALKWAY,THE DELIGHTFUL PROJECTTHAT MARRIES ARCHITECTAND ENGINEER, THE MAN-MADEAND NATURE. TIM ABRAHAMSADMIRES THE VIEWPHOTOGRAPHY BY DAVID COWLARD

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67Standing 18m abovethe woodland floor,the Xstrata Walkway,engineered by JaneWernick and designedby Marks Barfield,provides privilegedaccess to the canopy ofbroadleaf deciduous trees

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up to 1.9m. Crane changed admissionprices, invited the BBC in to filma documentary, and commissionedone-off exhibitions.

Crane also commissioned UKarchitects Wilkinson Eyre to createa master plan in order to addressthe areas of the Gardens that were beingneglected by visitors. Their solutionwas the arc that starts at the MarianneNorth Gallery and then sweeps westand north to the river. The SacklerCrossing, a sinuous low-level bridgewith granite paving and cast bronzebalustrades, by John Pawson wascommissioned as part of this project,as was the recently completed ShirleySherwood Gallery of Botanical Art,which sits next to the Marianne Northby Walters and Cohen.

Indeed, the arc reminds the visitorthat in many ways Kew is an architecturalcuriosity shop masquerading as theworld’s most famous botanical gardens.The Marianne North Gallery that openedin 1882, is a case in point. It is the onlybuilding designed by the architectural

In the heart of London’s Kew Gardens, theelegant new Xstrata Walkway rises 18minto the canopy of sweet chestnuts andoak trees, originally planted by CapabilityBrown. It is the brainchild of architectsMarks Barfield and engineer Jane Wernick –the team who worked on the LondonEye. In contrast to that landmark project,it is designed to be unobtrusive: thewalkway, suspended on eight Cortensteel pylons, is more subtle in its brilliancethan the Eye. Though it won’t matchthe giant ferris wheel in terms of visitorvolume, it is the most dramatic momentin a recent spate of buildings at Kew.

Chris Smiles, project architecton the walkway, counted 24 capitalprojects on site during his time at Kew.This is partly because the Gardens is anexpanding research institute. The recentlycompleted Wolfson Wing of the JoddrellLaboratory by Wilkinson Eyre is partof this. However, Kew is also opento the public. When Peter Crane camein as director in 1999, visitor figures wereat 1.2m a year and declining. By the timehe left the post in 2006, they were back

Above left: Approachingfrom the TemperateHouse, the Walkwaymade from Corten,emerges from woodlandlandscaped by Lancelot‘Capability’ Brown

Above: The 18mpylons stand alongsidesweet chestnut, horsechestnut, limes andthree kinds of oak

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node points 3m in diameter were the bestmeans to create a 200m-long loop whichcould thread through the complex rootstructures of the site.

The triangular tree-form designfor the pylons emerged from the choiceof Corten as the basic material for thewalkway. ‘We looked at statues by RichardSerra and thought that the materialhad the right earthiness to blend in,’says Wernick. As Corten comes in plate,the design team chose a triangular sectionfor the trunk with the branches trifurcatingoff that. The height of the pylons – 18m –was decided after several trips upin a cherry-picker to determine wherethe tree canopy could be viewed best.The walkway also provides viewsof the Temperate House, designed byDecimus Burton; architect of the nearbyPalm House, the Giraffe House at LondonZoo and the triumphal arch at Hyde ParkCorner. Inside the Temperate Housea spiral staircase gives access to a raisedviewing platform. The Xstrata Walkwayfits into the gardens’ traditions.

What makes this project so delightful

Above left: The Cortenstructure supportsa steel mesh walkway1.5m in width

Above right: The Fibonaccisequence was usedto create node pointsfor the structural memberson the walkway

historian James Fergusson – its verandahbetraying North’s interest in Indianarchitecture. Inside, beneath a clerestory,the walls of two galleries are completelycovered with 832 vivid oil paintingsfrom all over the world by the remarkablebotanical painter who gives the galleryits name. Next to it, you have a 50m-tall Chinese pagoda built by WilliamChambers in 1761. During the SecondWorld War, a munitions company drilledholes in each of its 10 floors and usedit to test bombs.

The Xstrata Walkway is more thancapable of living with this rich architecturaltradition. Its plan emerged from a groundradar survey done specifically for theproject at the proposed pylon locations.When the final plan of the route wasdecided upon, a second survey was doneto optimise proximity to the trees. The sitecontains several champion trees,so named as they are the largest knownexamples of the species in the country.The architects, together with engineer JaneWernick, determined that eight walkways12m in length and 1.5m wide, attached to

is the subtlety and slenderness of thepylons and the way the walkway remainssimultaneously distinct and a partof the woodland surrounding it. This waspermitted by Wernick’s first foray intocapping piles with a frame of steel beams360mm deep, rather than concrete.‘A concrete pile would have reachedup above the ground level or requireda mound of earth to cover it, whichwe felt would’ve ruined the effect of thepylons,’ she says. Also, a solid cap wouldhave interfered with the tree-root networkwhich is at its thickest in the top 600mmof soil. While each pylon and walkwayis largely regular, the pile caps are irregular;pulled by the engineer into different shapesto avoid the arterial roots.

It is frequently impossible to tellwhere the engineering ends and thearchitecture begins on this project, a signof quite how purist Marks Barfield’s attitudeto the technology of construction is andthe long working relationship Jane Wernickhas with the husband-and-wife team.

Wernick first worked with DavidMarks and Julia Barfield as part of Team X, !

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on a competition before they had left theirposts at Richard Rogers and Norman Foster’srespectively. The architects were forcedto smuggle Wernick onto the Kew projectas an existing framework document hadalready proscribed a number of engineers.

One of the most illustrative momentsof this close collaboration is the useof the Fibonacci sequence in the structuralsupports for the walkway sections. TheFibonacci sequence is a numerical patternwhich occurs regularly in nature, andbegins with 0 and 1, and follows 1,2,3,5,8.Each successive number is the sum ofthe previous two. The sequence describesthe relationship of bones in the humanhand and the way the spiral of a nautilusshell is formed. It also relates to the GoldenRatio in mathematics. Marks Barfieldand Wernick used this as a meansof creating the points at which the cross-bracing would meet on the walkways. Froman engineering point of view, the bracingis most dense where the shear loadsare highest. From a design point of viewthe apparent irregularity of the patternallows the structure to sit within thenatural rhythm of tree branches whileremaining inevitably apart from it.

It is a beautiful solution. The effect

of which is only minutely diminishedby the fact that where the cross-bracingwould have been at its most dense, thecontractor has realised that it is easier andcheaper to cut plate steel into a lattice ratherthan weld structural members. It is a tinyniggle in a fantastic project, but when youhave a team that aims for such structuralpurity, the slightest deviation can catchin the mind, even if it doesn’t catch the eye.

Yet the closeness between architect,engineer, constructor and client on thisproject is clearly the reason why suchgreat work was produced. W S Britlandwas chosen as contractor because it hadused similar parametric software – wherea computer model reacts if one memberis changed – as the architects. Britlandfabricated all the main members on site,including the classroom node – a largeplatform for up to 35 children – whichstands halfway round the walkway.

Barfield laughs when she says that‘it’s great having your client as the landscapecontractor’. This doesn’t give creditto the ease with which it is assimilatedinto the landscape, making it hard tobelieve that it weighs around 400 tonnes.Not only that, it also fits perfectly into thelong tradition of great architecture at Kew.

Above: For financialreasons, the architectschose not to remove themill scale from the Cortensteel. The positive resultis that the steel echoesthe trees bark

Top right: The pylonstrifurcate. A concreteelement will be added toencourage water run-off

Bottom right: A rootsurvey was used to plotthe course of the walkwaythrough the wooded area

Right: The walkwaywill allow visitorsto appreciateKew Gardens’ plantings