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Sun, sea, sky… and steel

Words:
Stephen Cousins

How did Marks Barfield construct the astonishingly slender i360 viewing tower on Brighton’s beachfront?

The huge hollow circular steel 'cans' on their way
The huge hollow circular steel 'cans' on their way

Opinion has been divided on the social, economic and contextual impact of Marks Barfield’s huge i360 tower in Brighton since plans were first submitted in 2006, but few will question the groundbreaking engineering needed to make it a reality.

Located on the seafront near the West Pier and designed by the team behind the London Eye, the 162m tall steel mast has a mere 3.9m diameter, earning it the accolade of the world’s most slender tower in the Guinness World Records. When it opens later this summer, it will support a huge, 90-tonne, 200-person circular glass observation car that rises vertically on just four wheels along tracks in the sides of the tower.

The i360 is constructed of 17 huge hollow circular steel ‘cans’, joined together by 33 giant flanges, each one forged from a single ingot of steel, in a process Julia Barfield compares to chefs spinning dough to create pizza: ‘Structural stiffness is provided by the combination of cans and flanges. The flanges are heated, spun, machined to create a perfectly flat surface, and then bolted inside each end of the steel cans. The tough monolithic flanges are much less likely to crack in use.’

Cans were installed in a ‘vertical production line’ using a specially-engineered 60m high jacking tower to lift the upper section of the tower so new cans could be skidded in underneath and bolted into position.

Unloading what is now in place as the towering structure of the i360
Unloading what is now in place as the towering structure of the i360

Building so high and thin, the principal challenge is wind, which is predictable but comes from all directions. The perforated aluminium cladding around the tower, and the slots the pod wheels travel along, disrupt the flow of the wind and prevent vortexing.

In addition, a series of liquid sloshing dampers, within the shaft and the pod, offset the movement of the tower and ensure a comfortable ride for passengers. As a result the structure is able to move more than 1m if necessary.

Barfield comments: ‘The dampers are essentially boxes of Australian rainwater, shipped to the UK by the Australian designer. Most tall buildings have some kind of dampers: the London Eye has mass dampers, tubes of weighted steel inside cans that moved against the direction of the wind, but here we used water.’

An idea of the scale of the 'cans'
An idea of the scale of the 'cans'

At the top the doughnut-shaped glass viewing pod is 10 times bigger than capsules on the London Eye. It is made from 24 segments of handmade toughened glass from Italy, double glazed and four panes thick. Rather than be cut to size, each piece had to be precisely formed so they could fit together accurately in layers before being attached to the steel ‘ribs’ forming the frame for each superstructure.

The pod has two types of backup power, but in the event of a complete power outage, an automatic braking system kicks in. The pod and its chassis weigh more than the counter weight, inside the shaft, making it possible to lower the pod using manual machinery in an emergency.

The last element of the pod being craned in
The last element of the pod being craned in

The architect's original concept design for the i360 was ‘somewhat thinner and higher’ than the version being built, says Barfield. A plan to include a wind turbine on the summit was also dropped due to concerns over the impact from wind and the likelihood of frequent blade replacement.

‘Losing the wind turbine was a disappointment, but it enabled us to push for an energy generation system, similar to that used in modern lifts, which generates enough power, as the pod descends, to provide 50% of energy needed to raise it to the top,’ says Barfield.

Just a few things to think about later this summer as you fly with the seagulls in a futuristic glass bubble.

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