Since they were designed for London’s Royal Opera House Covent Garden in the late 1990s, Comedil’s CTL-400s have been used on all the big London jobs of the past decade including the Swiss Re building, new skyscrapers in Canary Wharf, and now the St Pancras station Channel Tunnel Rail Link project.
In November 2004, the first of four of Select’s fleet of the 400tm-class cranes was erected on the site, home of the future London terminus of the fast rail link to Paris. The last of the five was scheduled to be erected last month.
What is unusual is that these luffers all use a counterweight rack that moves out to counterbalance a lowering jib.
The moving counterweight system ‘gives a better balanced luffing jib crane,’ says Select Tower Cranes’ technical manager Colin Beddow. Select has 25 luffing tower cranes of that class, 75 luffers in total, and almost 300 tower cranes in all.
‘On most luffing jib cranes, the counterballast is fixed,’ he says. ‘As the jib comes into a short radius with fixed counterweight you have a very high overturning moment.’
In the CTL-400, as the main jib luffs up, a triangular actuating arm pushes the counterweight tray in, reducing the overturning moment. ‘The Comedil design seems to be the simplest,’ Beddow says.
According to tower crane expert Heinz-Gert Kessel, tower cranes with moveable counterweight date back to the 1920s, in Germany at least. Other swinging-counterweight luffing-jib tower crane models include Peiner’s SN series, Liebherr’s 500 ACL, Favelle Favco’s STD series, Potain’s MR 300 – none of which are now made. BKT, now part of Potain, also offered a range of moving-counterweight tower cranes. Potain still offers the 600tm MR 605 B H32, which features a moving counterweight.
According to Kessel, there is now a move toward fixed ballast luffers, which are easier to manufacture and rig.
However, Comedil has sold 20 units of theCTL-400 and about seven of its newer, larger sibling the CTL-630, and is expecting to continue to feature moving counterweight designs on cranes over 300tm capacity, says export manager Mariano Moritsch.
Northern Europe Jost cranes dealer Reinhold Bräuner argues that moving-counterweight cranes can save money on the supporting structure. Fixed counterweight cranes must rest on larger mast sections to cope with increased overturning moment. ‘Fixed-counterweight mast sections can cost as much as double,’ he says.
MTI-Lux has delivered a new 216tm luffing-jib tower crane with moving counterweight from Jost Cranes to UK rental company W.D. Bennett’s Plant & Services in January. Managing director Edward Seager says that the crane’s moving counterweight allows it to hit above its tonne-metre rating, and can lift 3t at 55m radius and a maximum load of 16t. Bennett’s has six Peiner SN86s and eight BKT 105s with moving counterweight in a fleet of some 30 luffing tower cranes, and a total of 85 tower cranes.
There are several advantages of a tower crane with moving counterweight, says Bennett’s managing director Edward Seager. ‘Obviously you get lower base loadings than fixed-counterweight towers. And, because the crane is in balance rather than constantly stressed, there is less wear and tear on the structure.’
Seager says that it is difficult to compare prices of moving-counterweight and fixed-counterweight luffers. Tower crane rental rates depend on many other factors such as brand preference, customers’ attitudes to price, location, and what the market will bear.
Colin Beddow admits that Select chose the Comedils not because of their moving counterweight but because of their lifting capacity – 8t at the end of 50m jib (the luffing platform is 24m high). Railway safety guidelines require the company to add in an additional 25% safety factor to the cranes – bringing maximum capacity down to 6t at 50m.
Select Tower Cranes chose to erect luffing-jib towers rather than flat-top or saddle-jib tower cranes because the site is too small to get a mobile crane in to erect the cranes. The first crane, erected by a Select Terex-Demag AC 200, erected the second crane, which erected the third, which was scheduled to erect the fourth last month. When the cranes are taken down toward the end of next year, they will be dismantled in reverse.
Working on a railway site that is architecturally protected prolonged the planning process, Beddow says.
‘Planning the first crane took a year,’ he says, adding that the other cranes followed the same pattern and so didn’t take as long. ‘We have to get approvals from the Railtrack people, English Heritage, and various other organisations,. Everyone has to pass the method statements for erecting the cranes,’ he says.
Select gave the method statements to its client, Corber, a construction conglomerate made up of Costain, O`Rourke, Bachy and Emcor Rail.
When the project finishes, passengers will be able to take a new railway line from the north London station to Paris in about two hours.