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Revolutionary motion

Moving parts

The world's first rotating boat lift, the Falkirk wheel, is the impressive link between two of central Scotland's canals

As a piece of steel engineering, the Falkirk wheel is revolutionary - in every sense. It is the first-ever rotating boat lift, which lifts boats 35m from one canal to the other and vice versa. Set in the countryside near Falkirk in the Scottish Lowlands, the boat lift is the most exciting and dramatic part of the Millennium Link - a £78 million investment to link the east and west coasts of Scotland with an inland waterway.

The project also includes an extension of the Grand Union Canal, a tunnel underneath the Antonine Wall, a new aqueduct and a lock. Together with the wheel, they connect two long-derelict canals: the Grand Union Canal and the Forth and Clyde Canal, set 25m lower. Formerly, a staircase of 11 locks, dismantled in 1933, connected the canals.

Boats approaching from the higher Grand Union Canal now sail along a new extension before entering a tunnel that takes them underneath the ancient Antonine Wall. When they emerge, they travel along an 80m-long semi-circular concrete aqueduct that passes like a thread through the eyes of a series of five giant concrete needles - the support piers. At the end, a caisson awaits them to take them a half-revolution on the wheel to the basin below.

The wheel consists of two massive steel arms that revolve around a central hub. Each arm contains a caisson - a steel container - that can hold two boats. The caissons have a set of lock gates at both ends to allow boats to enter from one side and leave from the other.When the wheel is stationary the gates open, allowing boats to enter. The gates then shut and the wheel rotates. As it moves, stability gears maintain the horizontality of the caissons. The half-revolution takes 15 minutes, after which the boats leave the caissons and the cycle begins again.

British Waterways conceived the idea for a link to reconnect the two central Scottish canals some years ago but the scheme was delayed by funding problems and a subsequent redesign. After joint-venture contractor Morrison-Bachy Soletanche won the contract in 1999, the company, together with British Waterways, decided to take the opportunity to create a new tourist attraction. RMJM was invited to help with ideas, and architect Tony Kettle produced a concept for a rotating bridge, using a Lego model to demonstrate his plan.

'Our starting point, ' says Kettle, 'was to reinvent the brief, which had produced the original proposals. Previous schemes had been developed on the simple idea of the wheel as a circle, as in a Ferris wheel.

RMJM's concept grew from the design for the connecting aqueduct. Its organic form - the semi-circular canal resting in circular openings - was inspired by the design of a fish skeleton.'

By making two arms with circular openings and using hooked leading ends to add a sense of direction, the wheel was born. Its advantages over the original Ferris wheel proposal are that only two caissons are needed, the structure is simplified and it is made more dramatic.

The wheel is built on a massive scale that is hard to visualise. The 35m lift is equivalent to a nine-storey building, and the weight of four boats plus water is about 400 tonnes.

Manufacturing took place in the workshops of Butterley Engineering in Derby, a firm that specialises in heavy engineering for shipyards.

The rotation of the elements created serious problems in design, as individual elements face repeated 100 per cent stress reversals as they turn, alternating from compression to tension. Specialist equipment was used to carve the huge pieces of steel, based on very accurate steel templates. The curved box-beam arms are made from 12 pieces. Steel thicknesses range from 10mm to 50mm, depending on the stresses involved. Around 15,000 bolts were required, with 45,000 bolt-holes drilled into the steel sections and flange plates.

The canal now has a new purpose: to serve Scotland's leisure and tourist industry.

As well as being practical, the wheel is a major tourist attraction, and even boasts its own visitor centre, also designed by RMJM.

A steel boat lift in the form of a wheel

The wheel is at the head of a semi-circular reinforced concrete aqueduct standing on piers at 25m centres. The joint between the two is made by a section of steel aqueduct that accommodates movement while taking account of the tolerances required to provide a waterproof seal.

The wheel consists of two massive hooked steel arms, each with a central circular aperture which supports a water-filled steel caisson. The arms, carrying up to two canal boats in each caisson, rotate through 180 degrees on a 3.5m diameter axle.

A series of five stability gears is used to transmit the rotation of the central axle to the caissons. The central stability gear is fixed in a static position around the central axle. Two smaller gears revolve around this as the wheel turns, maintaining the horizontality of the final pair of outer stability gears, one around each circular opening in the arms, which are attached to the ends of the caissons. In this way the caissons cannot rock backwards and forwards but are positively located at all times. (The gears do not drive the arms of the wheel - they are 'followers'. ) At the base of the wheel is a dry well that allows the caissons to move without coming into contact with the water in the basin. The dry well includes two deep slots, which allow the hooked arms to rotate freely.

Because the arms are counterweighted and geared, only 95kW of power are used in the operation. The system is driven by one hydraulic powerpack, which pumps oil through 10 hydraulic motors, each the size of a telephone. The motors use synchronised gears to turn the wheel at a controlled pace of one revolution in eight minutes.

CREDITS

ARCHITECT RMJM CIVIL ENGINEERING CONSULTANT Arup

STRUCTURAL AND MECHANICAL ENGINEERING SUB-CONSULTANT MG Bennet and Associates

DETAILED STRUCTURAL ENGINEERING SUB-CONSULTANT Tony Gee & Partners

MAIN CONTRACTOR Morrison-Bachy Soletanche

WHEEL SUPPLIER Butterley Engineering

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