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Inverted pylon presents engineering challenge

A dramatic new art installation on London’s Greenwich peninsula posed some unique challenges for the structural engineers.

Sculptor Alex Chinneck is noted for his quirky and challenging creations. One recent highlight was the “Melting house’ in London, officially titled “A Pound of Flesh for 50p.” Chinneck’s latest creation has been dubbed “A Bullet from a Shooting Star” was commissioned for this year’s London Design Festival and is sponsored by Hong Kong developer Knight Dragon.

“I wanted to celebrate the rich industrial history of the site, particularly the power generation”, Chinneck explains. “This is an extremely ambitious project, and I had to work very closely with the structural engineers.”

Bullet from a shooting star

The gravity-defying end result features what appears to be a semi-inverted standard 35m high electricity pylon balancing on its tip at a vertiginous 64.5o from the horizontal. Structural engineer Smith and Wallwork principal Simon Smith says that while the structure is based on standard pylon design, it had to utilise higher grade steel to cope with the non-standard loading case.

“We were also very concerned about icing loads”, he adds. “The site is fairly exposed, and the installation is due to be there for up to five years. We calculated that as much as 26t of ice could form on the steel lattice.

Bullet from a shooting star

“In the event we didn’t have to add much more steel beyond the original design, although we had to beef up the stabilizing cable size slightly to take the extra prestress needed.”

More problematical was the foundation design. Ground conditions were unhelpful, with 4.5m of made ground over the Peninsula’s estuarine deposits. Smith says three foundation options were considered before the most economic was adopted.

Four 300mm diameter by 18m long minipiles underpin a 600mm wide by 750mm deep insitu concrete ‘pile cap’, which is linked asymmetrically by similar sized ground beams to a rectangular ring beam some 750mm deep by 1000mm wide. Overall dimensions of the ring beam are 17m x 16m, total weight of concrete is 175t.

Bullet from a shooting star

Post-tensioned stabilizing cables run from the four corners of the ring beam up to the pylon. Cast-in steel connectors anchor both the cables and the tip of the pylon. These cables provide all the resistance to wind loads on the structure, which could be as high as 7t in total, Smith and Wallwork calculate.

In all, more than 11t of steel angles made up the pylon, with around 900 connections adding another 10%. Final erection, where a massive crane lifted the pylon into position, went well, Smith reports, despite a gusty wind. “Raising the pylon and connecting it to the foundation only took about half an hour,” he says. “Rigging and tensioning the cables, however, took six hours.”

Night and day

Visible day and night from a multitude of locations around the Greenwich Peninsular, Alex Chinneck’s latest and largest installation is a focus for sponsor Knight Dragon’s major development of 15,000 homes on the Greenwich Peninsula. It faces towards Canary Wharf, sits close to the Greenwich Meridian and bears more than a passing resemblance to a giant sundial.

The inspiration for the installation’s title came from Chinneck’s discovery that an artillery works had once been located on the site. His creativity is widely acknowledged: Chinneck himself is generous in his praise for key collaborator Simon Smith.

“Simon solves the problems I create,” he says. “We’ve done several projects together, and I always know when I’m onto a winner when his response to a new idea is laughter.

“On projects as big and complex as this one I have to trust Simon – he’s so good at remaining true to the sculptural vision and coming up with a structural solution with the minimum of compromises.”

Located close to the O2 Arena, A Bullet from a Shooting Star will be accessible to the public during its lifetime.

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