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The wheel thing

Waterways Falkirk wheel

This time next year thousands of tourists could be flocking to the Scottish lowlands to take a turn on the most unusual ride of the 21st Century. David Hayward reports.

In a remote Scottish field near Falkirk a multi-disciplinary engineering team is assembling a structure destined to become one of construction's most distinctive 21st Century icons.

Its builder and structural designer see it as an innovative yet simple machine. To its architect it suggests a Viking ship with a propeller or the rib cage of a fish. In fact the £20M Falkirk wheel is the world's first rotating boat lift, designed to raise and lower canal boats 34m between the Union and Forth & Clyde canals.

To client British Waterways the dramatic 14,000t structure is much more than the final link in a rebuilt 110km long canal network linking Glasgow and Edinburgh. It is both a business venture attracting an estimated 200,000 tourists a year, and the catalyst to a ribbon of canalside redevelopment stretching right across southern Scotland.

So the primary role of the structure now slowly taking shape seems curiously secondary on the team's dream list.

Yet the complex of revolving steelwork enclosing two vast 30m long boat-carrying gondolas, is the most visible component in the otherwise complete £78M Millennium Link.

Over 120 structures, including locks, bridges and buried sections of waterway, have been reinstated or rebuilt on both the 1822 Union Canal, stretching 51km west from Edinburgh, and the 32 year older Forth & Clyde, linking Dumbarton on the Scotland's west coast with Grangemouth on the Firth of Forth.

Only where the two canals converge, mid country at Falkirk, with the Forth & Clyde lying 34m lower than the Union, is the crucial connecting link still being built. And it will be early next year before the wheel starts transporting up to eight boats at a time between the two waterways - completing in 15 minutes what took half a day to achieve in the nearby Victorian flight of 11 still existing but long buried locks.

A saving of not half but a whole day in construction terms was Paul Hudson's boast late last month as he supervised the last of five, up to 270t, crane lifts to complete the wheel's main assembly on site.

'By lifting two of the 180t gondolas on the same day, saving a planned extra 10 hours' crane time, we cut our crane hire charge by £10,000, ' says the contracts manager for Butterley Engineering, the wheel's designer, fabricator and erector.

'It was an appropriate finale to a series of delicate but trouble free lifts, ' adds Hudson, displaying the same smile he had worn throughout the week long operation.

The double walled, trough shaped gondolas - each designed to hold 250t of water plus boats - are now supported at their ends by two propeller shaped arms that rotate about a central 3.8m diameter steel axle.

As the wheel turns through 180infinity, transferring boats between an upper 100m long aqueduct extending from the Union Canal and a lower basin linked to the Forth & Clyde, the same set of hydraulic motors rotating the axle ensures, through a series of cogs, that the gondolas always remain horizontal (see box).

Over this summer, as the project's main contracting joint venture Morrison-Bachy Soletanche completed the aqueduct and canal links, some 35 lorry loads of gondola, axle, cog and arm were arriving on site from subcontractor Butterley's Derbyshire factory. That the numerous sections were then assembled on the ground into five giant lifts, and raised by a 1,000t Demag TC3300 crane to a positioning accuracy of just 10mm, owes much to a couple of factors that had occurred months earlier.

'We repeatedly brainstormed what-if scenarios during numerous planning meetings so the lifts themselves were somewhat uneventful, ' Hudson recalls.

The second aid to spot-on erection was more design related. As the balanced but heavily loaded wheel rotates, many of its moving parts will face 100% stress reversal as forces change repeatedly from total tension to total compression. To counteract the resulting high fatigue stresses, Butterley designed bolted joints rather than welded connections.

The need for 15,000 bolts, plus an amazing 45,000 bolt holes - vividly described in terms of punched out steel as 7.5t of holes - had a plus side, counteracting the obvious challenge on high accuracy requirements. It meant that all major components, including the 35m long, 12 section axle arms, could be temporarily preassembled in Butterley's factory. Arms could also be checked dimensionally to assembled gondolas lying nearby.

Dismantling for transport to site, and then reassembling these key sections on the ground at Falkirk, meant that by hook up time confidence was high that the jigsaw would fit. The positioned wheel now sits on a dozen hydraulic jack-equipped temporary support towers, which can adjust the entire structure by up to 20mm if necessary while grouted joints with the adjacent concrete aqueduct are completed.

The first unofficial turn of the wheel is scheduled for late November to help place gondola gates. But Butterley's £5M subcontract - which includes £400,000 of computerised controls - lasts until four days before Christmas. Although most surrounding infrastructure, including the visitor centre, canal basin, three locks and a tunnel is nearing completion, the joint venture still faces construction tolerances just as tight as Butterley's.

'Water levels in both the upper aqueduct and lower basin must always be accurate to 37.5mm, ' explains JV project manager Jim Steele. 'Otherwise the computers trip a warning and the wheel will not turn.'

The reason for such exacting tolerances is to ensure equal water levels in the two gondolas as the wheel rotates. But to the JV it creates the need for a complex network of sluices, pumps and chambers to ensure the locks at the top and bottom of the wheel always perform exactly as designed.

'The water management regime needs to predict and counteract in real time every possible event, including lock gates being mishandled, ' says Steele.

In with the new The comment from the Morrison-Bachy Soletanche joint venture (JV) project manager Jim Steele, that 'the end product closely resembles its engineering drawings' is ironic praise - as the wheel could not look less like the structure originally tendered for. The JV, plus its design team of Butterley Engineering, architect RMJM and structural consultant Arup - became preferred bidder of the design and build contract based on pricing a 'suggested' open Ferris wheel design with free hanging gondolas.

When it emerged that no one, client British Waterways included, really liked this old fashioned Victorian style design, the entire team, plus Butterley's own structural designer Tony Gee and mechanical consultant Bennetts, repeatedly brainstormed new options.

What emerged is regarded as a moving sculpture, set to be the construction industry's most exciting structure so far this century, and potentially a major Scottish tourist attraction to rival Edinburgh Castle.

Level pegging Clever but simple is how the single action wheel and gondola turning mechanism is best described. The 180t gondolas, plus 250t of water, could have been left free hanging, but the latter could have 'slopped around', upsetting boats and passengers. Instead a set of 12 hydraulic motors, connected to the central axle's fixed end bearing, turns both shiplift arms and, through a series of cogs, also the ends of each gondola contained within them.

The cogs rotate the gondolas at the same rate but in the opposite direction to the wheel, ensuring the water and boats always remain horizontal.

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