i360 - World's Most Slender Tower

Brighton's i360 'vertical pier' was conceived and designed by Marks Barfield Architects, the same team that was behind the London Eye, also sponsored by British Airways. It is sited at the landward end of the ruined West Pier. Built at a cost of £46m, the civil and structural engineering undertaken by Jacobs UK, it is Britain's tallest moving observation tower. At 531 feet tall, it mirrors the height of nearby Beachy Head. Aluminium wind-diffusing cladding aids a damping system in addressing the inevitable wind shear.

A significant portion of the structure is underground. 2,000 interlocking concrete piles were sunk about 65 feet into the underlying chalk bedrock to enable excavation for the foundations. Over 7,000 tons of shingle was removed, to a depth of about 20 feet, and the concrete base for the tower - over 4,000 tons of concrete reinforced by about 195 tons of steel rebar - built directly upon the bedrock. A precision-engineered 21.6 ton anchor bolt frame was set into the concrete.

The tower is formed of 17 steel tubes, fabricated in the Netherlands by Hollandia Infra BV - the main contractor. Delivered to the beach by barge, along with a jacking frame nearly 200 feet tall, and the counterweight for the pod, the tubes vary from about 15 to 39 feet in length, the shorter ones at the bottom. Their wall thickness reduces with height, to a minimum of just ¾ inch, giving a thickness to diameter ratio less than that of a can of beans. The first three tubes were lifted into place by crane, and bolted to the anchor frame. Four tubes were lifted into place atop the first three, and bolted together, but not to those beneath. The temporary jacking tower then lifted these four tubes, enabling a fifth to be slid in from below and bolted to those above. The final lift, of 13 conjoined tubes, was of about 965 tons. This 'top-down' method of construction obviated the need for a tower crane. Work commenced on site in July 2014, and was completed in July 2016, but the tower of tubes, joined together by 1,336 bolts, reached its full height in just ten weeks.

The tower is just 12.7 feet in diameter. With a height to diameter ratio of 41:1, it is the most slender in the world. It is all the more remarkable, thus, that the pod, suspended from eight steel ropes, is as large as it is. 59 feet in diameter, and weighing over 92 tons, this can carry up to 200 people. The oblate ellipsoid pod was designed and built by Pomagalski SA, the French cable car specialists, and ascends to 453 feet. It consists of trusses that cantilever off a central chassis. 24 solid floor sections sit atop the trusses, and atop those sit 36 glazed sections, 24 facing outwards, and 12 facing the tower. The double-laminated glass, which can't be cut once toughened, had to be first cut to shape before it was double curved at high temperature. The drive mechanism, housed in the basement, is akin to that of a cable car. The descent produces about half the power needed for the next ascent.

As part of the development the Italianate Victorian tollbooths that used to flank the entrance to the pier were reconstructed. The originals were designed by Eugenius Birch as an integral part of his pier of 1866. The western one had been demolished, and the eastern was structurally unsound. The latter was dismantled and detailed measurements were taken. Ductile cast iron mouldings - 24 tons of them - were made anew, by the Swan Foundry, of Banbury, so as to precisely replicate the originals. The western tollbooth is now the i360's ticket office, whilst the eastern operates as the West Beach Cafe & Bar.