Singapore flyer

Ferris wheels keep getting bigger. The London Eye was 135m tall; the Singapore Flyer is 165m; the Beijing wheel will be 208m.

Brendon McNiven, associate director of construction contractor and engineer Arup, said that big wheels may get bigger yet. "The upper limit for these things comes when the limits of financing are reached, not the limits of engineering. You will have noticed a lot of these projects are around but only a few make it through-mainly as they are harder to finance than a typical building. Whereas office rentals and returns are well known, the guaranteed number of visitors to a wheel is not.”

Two memorable flops are the 182m Voyager intended for Las Vegas, Nevada and the 200m Shanghai Star proposed for the city of Shanghai, China.

Says McNiven, “The main limit is in the size of the bearings, but everywhere you come up against limits: requirements that might not be available off the shelf or loads that might be too large. Usually an innovative engineering solution can be developed.”

Arup had to find just such a solution for the Singapore Flyer. Its 138m rim stands above a three-storey terminal building. While London’s 122m Eye could be fabricated floating on the Thames and then be pulled upright, the lack of space on the Singapore site made this impossible: It had to be built vertically between two supporting columns.

McNiven recalled doubts expressed about vertical construction. “Early on the client was approached by quite reputable engineers saying the erection could not be done vertically,” he said.

“MHI and Arup knew differently from doing the numbers.”

Arup was involved with the Singapore Flyer project from its conception, while Mitsubishi Heavy Industries (MHI) and Tanenaka Corp had a turnkey contract to execute its construction. Also included were Kisho Kurokawa Architects & Associates, Tokyo, DP Architects Pte Ltd Singapore, and Rider Levett Bucknall project management.

Horizontal construction takes gravity out of the equation, and thus pre-stresses are equal during assembly. But vertical construction means gravity-loads act within the plane of the wheel causing tension to vary at different positions in the cables, making the sequence of cable stressing much more complicated.

“If you have ever tried to tighten the buckle out of a bicycle wheel rim as a kid by adjusting the spokes you will know what I mean,” said McNiven.

To find a solution, McNiven at Arup and Akihiko Nozaka, project manager, and Kei Itou, structural design manager, at MHI oversaw a handful of engineers and designers scattered across offices in three continents testing dozens of methods of erection in a virtual CAD computer environment, analysing the forces at every stage of construction. MHI conducted wind tunnel tests at its labs in Nagasaki.

Eventually, in October 2006, three years after the Singapore Flyer was a sketch on an architect’s pad, and one year after the groundbreaking on September 27, 2005, the time had come for MHI and Tanekana to build the support structure and assemble the wheel. Bulldozers had reformed the grounds, and the tricorne-shaped terminal building was a bare framework of steel girders.

Inside its three-storey perimeter, open on one side to allow entry by heavy machinery, was the platform that would withstand the 2,200t of the wheel, supporting columns, cables and capsules. Beneath it, 38 piles of 1.5m diameter had been bored to 50m, and steel-cased through the top 10 meters. Two clusters of nine piles underlay the positions where the wheel’s two support columns would stand. Four groups of five piles sat beneath the four spots where the cable stays’ anchors would sit.

When erected, the Singapore Flyer’s two support columns would measure 85m high, 2.8m in diameter and weigh a total of 550t. Fabricated offsite, they were delivered in six sections, including their end caps.

During assembly, the lead machine was a 600t Demag CC 2800 crawler with a 112m boom. In its shadow, MHI crews built a temporary structure resembling a sawhorse, its spine a horizontal strut spanning the distance between the not yet erected column bases. Next, the crane lifted the basal sections into place, first one then the other, so workers could bolt them to the foundation, and butt their tops to the temporary horizontal strut.

Then the second sections were positioned, spliced to the section below and braced with another temporary strut.

This procedure was repeated, sometimes without the struts, until the lower five sections of the two support columns were fully assembled, on October 27. Two weeks later, on November 8, the crane lifted the end caps into place. The next phase of construction, the lifting of the hub and spindle, began with the fastening of a temporary gantry across the end caps.

Months earlier, MHI in Japan had fabricated the spindle and hub for the Flyer. With a length of 25m and weighing 180t, it was unwieldy. From the docks of Singapore, it was loaded onto a 120-wheel trailer, and hauled along a carefully considered route. The spindle is believed to be the biggest load ever carried through Singapore.

Lifting the spindle 85m to the top of the Singapore Flyer’s two support columns was not possible by crane. Instead, MHI mounted two 200t strand jacks at flanking positions on the temporary gantry. Early on the morning of December 13 they began hoisting the assembly upwards. By evening, the task was finished, and the assembly was bolted to the column’s end caps. In the following days, MHI secured the structure with four groups of six 100mm locked coil cable stays, lashing the end caps to the four anchors located beyond the perimeter of the terminal building, balancing the forces horizontally at ground level through the floor of the building. The Singapore Flyer’s supporting structure was complete.

Next MHI tackled the construction of the wheel’s rim, that was fabricated in Malaysia from steel pipe, 25.4mm thick and 864mm in diameter. The complete rim would be formed out of 28 sections, weighing 30t each.

The engineers used temporary struts to divide the wheel into seven equal pie slices. Only one of these would be constructed at a time. The method required building two temporary struts extending from the hub to the rim perimeter. Each strut was made of four lengths, winched into position and fastened with high-tension bolts. When two struts were ready, four rim sections would be attached between them, and radial cables (the spokes of the wheel) installed in a slack condition. This finished ‘pie slice’ was strengthened with a lightweight steel chord forming a bowstring truss around the wheel’s perimeter. Fully stabilised, the pie slice would be rotated out of the way so that another strut could be added and work on the rim continued.

MHI worked on the rim and its spokes from January 2007 to mid-July. The crew then removed the struts and stressed the cables in two stages to avoid overloading the rim.

With the wheel itself complete, a 160t mobile crane lifted the 10t passenger capsules one by one to a support platform erected on top of the terminal building. A drive unit on an eye deck near the base of the wheel’s supporting columns rotated the wheel into position, enabling workers to attach the capsules.

By the end of October all that remained was to run some tests and prepare the grounds for opening day in February. On a clear day, visitors will be able to see the borders of Malaysia and Indonesia 45km away.