Moving Tower Resists Strong Wind Forces

The designers of the London Eye have turned their attention to a slender tower in Brighton, England, with a glass enclosed gondola that will carry passengers more than 500 ft skyward for a 360-degree view—without sensing wind vibrations.

August 28, 2012—The new i360 observation tower, with its vertically traveling glass-enclosed gondola, will take passengers high enough to view Brighton, England’s famous white cliffs as well as the sea for miles into the distance—but designers are ensuring that no one will feel anything resembling motion sickness. Guaranteeing that the roughly 442 ft tall tower does not move in the wind—or at least that any movement is so slight that passengers won’t mind—has been the primary challenge for the engineers planning the slender steel tower, which will be one of the tallest in the world to carry passengers to its top.

The i360 was designed by David Marks, RIBA, the managing director of Marks Barfield Architects, of London, and John Roberts, Ph.D., Ceng, the executive director of operations for Jacobs UK Ltd., who collaborated on the design of the iconic London Eye. The project received funding in July and although some of the tower’s components have already assembled off-site, the primary on-site construction is expected to begin by the end of this year. It is hoped that the sleek, modern structure will help regenerate Brighton’s West Pier on England’s southern coast.

Working with a small site, the architects had to design a very narrow tower. The structure will consist of a steel tube, 13 ft in diameter, and will have an aspect ratio (height to girth) of nearly 40 to 1, roughly five times that of even the tall, slender buildings that grace the skyline of Hong Kong, according to Marks.

The tower will resemble a knitting needle protruding from the ground, Roberts says. “This is probably the most slender tower that you can think of that’s going to carry passengers,” he says. “[It’s] like a communications tower, but we’re taking people up in it.”

But the greatest concern for the designers was neither the height of the tower nor its slender profile—it was wind. To control wind vibrations the engineers have designed two types of dampers to add to the top of the tower that will react at three different wind speeds. The dampers reduce the effects of vortex shedding in different ways, Roberts says.

One type is a tuned liquid damper, essentially a liquid-filled tank that will dissipate energy as the liquid moves from side to side with the tower’s movements. The second type, a mass spring damper, will use a spring to restore the tower to its original position if it strays from vertical. Roberts says the tower will have two tuned liquid dampers and one mass spring damper.

The dampers will account for three different wind speeds: what Roberts calls the “everyday” wind of up to 9 mph; stronger speeds of up to about 44 mph, which may occur several times a month and would cause the operators to close the tower to passengers; and very rare gusts of up to 110 mph, which might occur only once in the tower’s 50-year life span and last just a few seconds. “The three dampers will each be tuned to the frequencies associated with those three wind speeds,” Roberts explains.

The 62 ft diameter glass-enclosed passenger pod, sized to carry up to 200 passengers, was designed with aerodynamics in mind, its double-curved glass panels meant to reduce the wind’s effect on the pod itself. On top, the glass will be fritted, probably through a process similar to sandblasting, to reduce heat and glare from sunlight, Roberts says. The pod will also feature air-conditioning blown up through the floor, a necessity in the hot, humid beachside climate of Brighton.

The glass will be double glazed with a film between the layers to prevent shards from flying should a break occur. In addition to serving as a viewing tower, the i360 will also be a venue for business meetings, and at night the gondola can become a sky bar offering panoramic views.

The 800-ton tower will stand on a concrete slab foundation, almost 10 ft thick and weighing 4,000 tons. Workers will build a jacking frame around the tower’s base to stack the 16 steel segments or “cans” that will comprise the tower itself. By lifting the cans high enough to place new ones beneath, the jacking frame will prevent workers from trying to stack the tower’s components hundreds of feet in the air. Although this method of construction has been used in the past to construct airport control towers, Marks says it has never before been used on a tower this high.

To bring passengers to the top, the i360 will use cable car technology to lift the pod straight up and down—a system that adds an additional 160 tons to the tower’s weight from the pod itself—and also a counterweight that will move inside the tube.

While the dampers will reduce vibration and movement, Roberts says the tower’s height means that passengers will not have a reference point to help them pick up slight movements of the tower. “You’re not living or going to sleep,” in it, he says. “You’re on a theme park ride, so in a sense a little bit of movement is not going to upset you.”

The tower is expected to be completed in 2015.

Read the full article by Dave Reynolds here.