One of the most extraordinary engineering structures you can see in Britain is the Falkirk Wheel. To many of us living in other parts of the world, its function is odd: it’s designed to transport canal boats from one canal to another. But it’s shape is also weird: with its huge curved axe-like arms, it’s like nothing else that you’ll find. And then there’s the mechanism – massive, exposed cogs that obviously perform an engineering function – but what?
So let’s take a look at a marvel that mixes artistry, engineering and function.
The Canals
Before you can understand the function that the Falkirk Wheel performs, you need to know about the canals it links.
The Forth & Clyde canal was constructed in the period 1768 – 1773 and fully opened in 1790. The 56km long canal connected Grangemouth (on the Firth of Forth) to Bowling (on the Firth of Clyde). It therefore connected the east and west coasts of Scotland. In addition, the canal had a short spur to Glasgow.
The canal boasted no less than 40 locks, 25 aqueducts and 32 opening or swing bridges. It closed to navigation in 1963 when a highway was routed across the canal at low level.
The Forth & Clyde canal might have needed to climb mountains, but the Union Canal was quite different. For most of its length it was level – running along a single contour. This canal connected Edinburgh to Falkirk. It was 50km long and was constructed between 1818 and 1822. It closed in 1965 – again, the final nail being the routing of a highway across it.
At Falkirk the difference in level between the two canals was 34 metres. Canal vessels were able to move from one canal to the other via a flight of eleven(!) locks. To do this took most of the day and used 3500 tonnes of water.
In their day, the canals were a major transport link, carrying up to 200,000 passengers per year. The boats were drawn along the canal by horses walking alongside. But not all vessels were small: the Forth & Clyde canal was the world’s first sea-to-sea ship canal and had to accommodate tall-masted vessels. This meant the bridges over the canals had to open or swing to allow the ships passage.
For decades after closing, the canals decayed. Locks were filled-in, bridges crumbled and the canals were maintained only for safety and to act as drains.
Then, in the mid 1990s, British Waterways – the canal owners – decided to revitalise the full length of the two canals. Public interest and support were high, and government made funding available. Locks were restored, new sections of canal dug to replace those covered by urban development, rubbish was removed and bridges built.
But what was going to happen at Falkirk, where the canals differed in height by 34 metres – and the original locks, that had not been used since 1933, were long buried?
The Wheel
To re-join the canals, an approach had to be decided upon. Building new locks was not considered a viable option, so the device that would transport boats from one canal to the other needed to be mechanical.
All sorts of options were considered, including an overhead mono-rail that transported basins of water containing boats; a vast spoon-shaped see-saw; and even a vertical lift concealed within a cylindrical waterfall.
However, over the 5-year design process, the concept of a turning wheel was decided upon. In its initial stages, this looked rather like a huge Ferris wheel containing two basins for holding boats and water. By 1999 the design used four hanging gondolas.
However, even with the construction company by then chosen, British Waterways had a change of heart. In what must have been an intense month of meetings, 20 engineers, architects and steel fabricators devised the final design.
And what was that design?
Opened in 2002, the Falkirk wheel is 35 metres high and 27 metres long. It comprises two enormous steel arms, for aesthetic reasons shaped into axe-like curves at each end. The arms support two gondolas, each capable of holding 250 tonnes of water and boats.
The arms were fabricated off-site in a factory, assembled in that factory, and then disassembled for transport to the site. There they were bolted (not welded) together, with over 15,000 bolts used. The steel assembly has a mass of 1200 tonnes.
The 23.7 metre long gondolas, each of which has a mass of 50 tonnes empty, ride on wheels mounted within a circular opening at each end of the arms. To ensure that the gondolas remain level as the Wheel rotates, each gondola is geared by means of large (very large!) timing cogs to the central part of the axle.
This gearing is passive – it needs no extra motors, as the motive force is provided by the turning of the wheel as a whole. The wheel itself is driven by ten 7kW hydraulic motors working on the central axle.
Because each gondola has the same mass, irrespective of how many boats it contains, the wheel remains balanced. (Each boat displaces its own weight in water – so it doesn’t matter if the gondola is full of boats or is empty… it weighs the same.) It takes only 1.5 kW/hr to turn the wheel through a boat transfer operation. To retain balance, the water levels must be the same in both gondolas. In fact, computer control of the canal levels keeps the levels in the gondolas within 75mm of each other.
Steel gates are used at each end of the gondolas, and rubber seals shut off the 50mm gap between the end of the gondola and the canal.
There were significant design challenges in developing the wheel. There are 600 tonne, 100 per cent stress reversals during the turning of the wheel, and the design had to cope with potential boat impacts (quantified as a 40 tonne load applied at aqueduct level) and a wide temperature range.
The Wheel is located at the end of a 2km extension to the Union Canal. This extension runs through a new tunnel and then across a 104 metre long aqueduct that leads directly to the wheel’s upper gondola.
To aesthetically match the profile of the Wheel, the aqueduct had to appear to be floating. This resulted in very high loads being transferred through only small areas of reinforced concrete. The upper parts of the aqueduct hoops are actually not concrete, but are instead reinforced plastic.
In the metal
Seeing the Falkirk Wheel in the metal, you’re torn – is this a whimsical exercise in silliness, or an energy-effective engineering structure? The answer is that perhaps it is both – artistry integrated with engineering.
An amazing structure.
|