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Walking atop the O2 Dome

March 1st, 2013 / By: / Fabric Structures, Projects

A walkway above a fabric structure offers an adventurous experience—and a spectacular view of London.

This is no walk in the park. The “Up at The O2” adventurer is 53 meters above the ground—safely tethered to the structure—but still, on a walkway only 3 meters wide and 350 meters long, suspended in the air over the roof of the iconic entertainment venue, The O2, in London, England. Climbers can take a thrilling journey across the thin fabric walkway, pausing in the middle on a purpose-built steel central viewing platform on top of the structure to take in the 360-degree views of the surrounding city.

The groundbreaking project, a partnership between AEG (owner and operators of The O2 entertainment venue) and O2, a communications company, is unlike anything constructed in the United Kingdom and required the delivery team’s specialist experience with large-scale tensioned-cable and fabric structures. Base Structures Ltd., Bristol, United Kingdom, was part of that team.

The background

London’s Millennium Dome was completed in 1999 on the Greenwich Peninsula. Originally commissioned to mark the beginning of the new millennium, the landmark structure also formed a key element in the redevelopment of the entire Greenwich Peninsula. Enclosing 2.2 million cubic meters, and with a circumference of one kilometer and a maximum height of 50 meters, it is a fabric structure on an impressive scale and worthy of its iconic stature.

It has since been redeveloped and operated by entertainment and sports presenter AEG and rebranded “The O2.” It’s reported to be the world’s most popular music and entertainment venue.

In July 2011 planning permission was granted for the construction of a roof walkway across the top of the structure, to be designed by Rogers Stirk Harbour + Partners LLP with Buro Happold, the original consulting engineers and co-designers for The O2.

The experience

The climbing experience begins on the south side of The O2, and from there the fabric walkway suspends above the existing fabric structure of The O2 to its apex. Climbers are provided with “roof suits” and harnesses, which enable them to be attached directly to a cable as they climb to the top. The lanyard cable and handrail runs the full length of the walkway.

At 53 meters above ground level, there is a 12-meter-diameter steel viewing platform with a panorama plate to direct climbers to key London landmarks. The roof walk extends to the north side of The O2, where climbers descend back down to ground level. A key driver in its delivery has been to make the experience exciting, fun and safe for everyone within the technical constraints imposed by both equipment and safety. Step-free access means that anyone, including wheelchair users who enjoy the demands of climbing, have the opportunity to experience the challenge.

How they did it

Base Structures was responsible for delivering the detail design, fabrication and construction of the unique cable-tensioned fabric walkway and central viewing platform. Facing a tight schedule, the company conducted extensive prototyping and testing prior to the build to test design assumptions and to ensure quality control and safety on-site.

A custom embossed PVC fabric was manufactured by German manufacturer Mehler Texnologies GmbH to improve grip, with additional ribbing individually welded onto the surface by the company for the steepest sections of the climb. Despite the deceptively minimal appearance, constructing the tensile cable and fabric roof walk was a big undertaking, using more than 6,000 square meters of the custom fabric.

The challenges

The overall challenges of delivering this project were extensive: integrity of the existing structure could not be affected; no hot works could be used on the fabric roof; no crane could reach far enough to fully access the site.

Slipping and sliding. A range of surface modifications on the PVC fabric were manufactured and set up on a custom test rig, replicating a full-size panel of the walkway set at the precise angle of the steepest section. Testing was done with volunteer climbers suitably harnessed and roped up, with a hose pipe to simulate the worst rain conditions possible. The results were clear: the only successful solution emerged to be thick ribbing, applied perpendicularly to the length of the walkway.

Crane conundrum. With no crane large enough to access above the center or the structure, other methods had to be considered. There was discussion about cutting a hole in the roof to pass materials through or using a helicopter. Eventually a unique system was designed to manually convey 30 tons of tools and materials over the roof, pulling custom built sledges over a protective PVC runway laid on the surface of the existing fabric roof to prevent damage to the fragile surface. As far as the company knows, no one has used this type of system before.

Manual labor. Weight restrictions on the roof meant that instead of using a powered winch, each sledge needed to be manually 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 50 mm at a time, it took more than 2,800 cranks to pull a single sledge to the top of The O2—and there were more than 200 sledge trips for the whole project.

Once at the apex, the materials were hoisted into place using another innovation—a skyhook. Suspending a cable and hook system from the yellow masts allowed the team to easily lift everything into place without causing any strain or loading on the existing roof.

Walkway manufacture. Meanwhile, at Base Structures headquarters, manufacture of the 75 fabric panels continued. The panels for the steepest sections of the walkway required that each piece of additional ribbing be positioned and welded by hand, an daunting task with more than 3 kilometers of ribbing to attach.

Before going to the site, each panel was fitted with cables and clamps and pretensioned in the factory on a special rig for quality control tests. Fitting the 4,000 clamps used to attach the panels to the supporting cables on The O2 was done in the factory, so the rigging team on-site could simply lift each section into place and secure it to the previous panel and support cables.

Weather. High winds and the wettest April on record required the crew to work most weekends throughout the project to recover time lost to weather issues.

Increasing the tension. When the walkway panels were in place, the structure could be tensioned out—a delicate procedure requiring perfect coordination. At this stage, the side wing panels were unfurled along the length of the walkway. These are an extra safety precaution, but being made of white mesh means they blend into the surface of The O2 below, to heighten the sense of adventure.

Ultimate success

The London 2012 Olympic Games were obviously going to take place as scheduled. The installation was completed in time for that immovable deadline. The “Up at The O2” roof walk opened to the general public on June 21, 2012, and has proved to be very popular.

The demanding project was a success due to a partnership of experts in their specific areas. “We were extremely impressed by the ingenious problem-solving methods of work that were employed. The hard work ethic is also very prevalent within their organization from director level right through to the work face,” said Jon Clayden, senior construction manager for ISG, a company with expertise in the structuring and provision of financing for the construction of stadia.

The original objective for the roof walk to extend the use of The O2 entertainment venue beyond evening music concerts has been achieved. The roof walk operates daily with climbs departing every 20 minutes.

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