Rogers Stirk Harbour, Buro Happold and Base Structures have created another reason visit the dome in North Greenwich
Project: Up at the O2
Architect: Rogers Stirk Harbour + Partners (concept design); bblur (post concept detailed design)
Location: Greenwich, London
Completion: May 2012
Bon Jovi has played on top of it, Justin Rose has teed off from it, James Bond has slid down it — and now you too can climb it.
Up at the O2, as the dome’s new walkway has been branded, is the latest attraction to open on the surreal festival peninsular of North Greenwich, site of that failed New Labour jamboree and more recent attempts to leverage private sponsorship — from the Emirates Air Line to the BT Vision spinning tower ride — to cheer the place up.
Designed by the original Millennium Dome concept team of Rogers Stirk Harbour + Partners and Buro Happold, and built by ISG with Base Structures, the walkway forms a tensile spine in a straight line across the 350m equator of the O2, deftly suspended from a minimal number of new cables. With its taught skin of PVC membranes stretched between a multitude of wires, sprockets and nodes, the structure employs the high tech language of its host to great effect. It is as close to the original intention of walking on the roof of the dome itself (impossible due to wear and tear of the 0.7mm-thick fabric), an exhilarating journey through the cat’s cradle of cables and soaring masts.
The thrill is magnified by the minimal nature of the walkway design, which seems delightfully unencumbered by the deadening hand of health and safety regulations. There are no handrails or balustrades, just a central cable to which you are attached, and the fabric remains pleasingly bouncy.
Source: Base Structures Ltd
The view of London from the top, where a 12m diameter viewing deck is suspended above the apex of the roof, is not really the point – far better views can be had from the adjacent cable car for £3.60, a bargain compared to the £28 ticket for the O2 climb. The exciting part is to be on top of and in the structure of the fabled Dome – to “climb an icon” as the billboard trumpets. This is cable-stayed, tensile porn, as close as you will get to dancing on the roof of the Schlumberger Centre.
Early in the process, the design team considered a number of options for what the walkway could be, from a suspended steel gantry to a moving travelator. Richard Rogers was keen that the experience should replicate the experience of walking on the surface of the dome, and so a tensile fabric scheme was developed.
Source: Base Structures Ltd
The first problem to overcome was to ensure it was physically possible to walk up the fabric surface – no small feat given 30° inclines and unpredictable English weather. To help increase traction, manufacturer Mehler created a bespoke PVC-coated polyester fabric with a textured surface. Extra fins have also been welded to the surface on its steepest inclines, perpendicular to the length of the walkway, making it possible to climb even in driving rain – tested against a gushing hosepipe.
“It was important that it was possible, but that it still felt steep and slightly scary,” says David Dropkin, inclusive design consultant at Buro Happold, who worked to ensure that even wheelchair users could have the same experience as able-bodied climbers, with the aid of a chair harness.
Andy Traynor, head of installation at Base, explains how the walkway is suspended from 24 major cable hangers, hung in pairs from the top of each of the existing yellow steel masts. 16 tie-down cables then connect the walkway back to existing wishbone nodes on the roof of the O2.
Before going to site, each of the 75 panels that make up the walkway was fitted with cables and clamps and pre-tensioned on a rig for quality control tests. A total of 3km of cable runs through the edges of the walkway fabric and nearly 4,000 clamps are used to attach the panels to the supporting cables on the O2. With everything in place, the structure was manually tensioned out, a very delicate procedure to prevent uneven loads being applied to the masts.
Source: Buro Happold
A white mesh skirt runs either side of the central walkway, rising up and down along its length to form a sculpted profile. “It’s mainly an aesthetic addition, and it’s also there to provide reassurance,” says Glyn Trippick, project director at Buro Happold, “so you don’t feel quite so high above the roof.”
With the existing dome roof not designed to take extra loading, the entire weight of the viewing platform had to be suspended from the existing yellow masts on support cables. With no hot works allowed on the fabric roof the steelwork was fabricated to simply bolt together like a huge Meccano kit.
Mark Smith, head of projects at Base, explains how, with a construction window of only seven months for the project to open in time for the Olympics, the programme was hugely compressed — leading to schematic and detailed design running parallel with work starting on site.
Over 6,000sq m of fabric and 7km of steel cable had to be transported into place, but there is no crane in existence that can reach the top of the dome. Various possibilities were considered, from cutting a hole in the roof to pass materials through, to using a helicopter. In the end, every single tool, cable and section of walkway was manually transported into place using a sledge system.
Source: Buro Happold
Weight restrictions on the roof meant that instead of using a powered winch, each sledge was pulled up the roof using a tirfor, a manual mechanical cable-pull with a long crank arm. With each crank of the lever pulling the sledge only 50mm at a time, it took over 2,800 cranks to pull a single sledge to the top of the dome – with over 200 sledge trips needed for the whole project. Once at the apex, the materials were hoisted into place using a “skyhook” system, suspended from the yellow masts.
Architects: Rogers Stirk Harbour + Partners (concept design); bblur (post concept detailed design)
Engineer: Buro Happold (client adviser, lead consultant, engineering and inclusivity consultants)
Client: Ansco Roofwalk Limited (AEG)
Specialist contractor — tensiles: Base Structures