Modern cranes are masterpieces of mechanical, hydraulic and electronic engineering. Who would have thought, just 20 years ago, that we would be using crawler cranes capable of lifting in excess of 2,000t? Yet there is one company in the US which remains remarkably unimpressed by such advances in crane technology.
Specialist lifting contractor F&M Mafco of Cincinnati, Ohio is a firm devotee of a technology which would have been familiar to those who built the great pyramid of Giza – the stiffleg derrick. These machines are simplicity itself, consisting of a central vertical mast supported by two stifflegs to form a solid tripod. Pivoted to the base of the mast is the lifting boom which luffs and slews by use of wire ropes and winches. The lifting hoist is mounted behind the stifflegs.
Needless to say, cost is one of the biggest advantages of such venerable methods. Marketing manager Tim McKenna estimates that a stiffleg derrick can do many jobs for just a quarter of the cost of using a modern crawler or truck mounted crane.
But there are other advantages besides cost. “A stiffleg derrick is inherently more efficient than a crane”, says McKenna. “It all comes down to geometry – a derrick is all triangles. It can get right over the job and direct the thrust reactions straight down into the structure that it’s mounted on. A derrick is an engineered structure powered by a hoist, whereas a crane is basically a hoist with a derrick attached. A crane is just a cantilever… it weighs what it picks”, says McKenna.
F&M Mafco, founded in 1946, has grown to become a major lifting specialist to the power, dockside and heavy civil engineering sectors. Its first clients included major players such as Babcock & Wilcox and Foster Wheeler.
Today the company has a fleet of about 50 stiffleg derricks, mainly American Hoist models. It also has a 75-strong crane fleet.
“We’ve been involved in crane and derrick operation since 1946 and we understand the advantages and disadvantages of both”, McKenna says.
Two recent jobs completed by F&M Mafco illustrate the capabilities of these simple but versatile machines. In Hong Kong, the firm supplied derricks to work on construction of the Ting Kau Bridge. Closer to home, the firm supplied derricks for assembly of the National Radio Astronomy Observatory’s massive new antenna in West Virginia.
The Hong Kong project illustrates the stiffleg derrick’s power to weight ratio. Its low gross weight allows it to be mounted directly on the bridge deck which is not only safer than working off water with a barge mounted crane, but also more economical.
The weight of the derrick is distributed throughout the bridge, so much bigger pieces can be picked up and assembled. Hence, large components can be assembled on the ground and lifted into place in a single lift. This is both safer and quicker, argues McKenna.
On the Ting Kau bridge, derricks were placed on each end of the balance cantilever of the bridge for simultaneous construction in both directions. F&M Mafco also mounted derricks on a movable sled so that the whole assembly could be progressed with the bridge.
Several cable stayed bridges have been erected this way, McKenna says. The method is currently being employed in Uddevalla, Sweden, for erection of the Sunningeleden Bridge.
The new National Radio Astronomy Observatory telescope is also making good use of Mafco’s derricks. This huge structure, which will weigh over 7,200t, has a dish measuring 100m x 110m. F&M Mafco’s 500 US ton capacity S-70 derrick, mounted on a 60m high tower, is assembling the telescope. So far, the heaviest lift was a 113.5t component, lifted at 67m radius to a height of 61m.
The only other machine available to carry out a lift of this magnitude was a Lampson Transilift which would have been considerably more expensive than the S-70 derrick.
Stiffleg derricks are ideal for jobs like these, where the machine is going to be on site for an extended period, especially if it is to carry out a whole series of lifts from one location.
F&M Mafco director Bob McKenna – Tim’s father – says the relatively limited slewing range of a stiffleg derrick (about 250O) is hardly ever a disadvantage on such jobs. “On a bridge or a power plant, you set up the derrick where you don’t need to swing too far”, he says. This is simply good practice. You would also position a crane, which has unlimited slew, so as to minimise the amount of crane movement needed to pick and lower the load.
And once the derrick is set up, it needs hardly any maintenance, he adds: “It’s such a simple piece of equipment – if something goes badly wrong, then maybe you have to replace a sheave!” “Anybody could fix a derrick. Your kids could do it. Modern cranes are all bells and whistles and you need to call in the specialists if they go wrong.” So if these ancient machines are so good, why do you hardly find them on site these days? “There’s a bit more engineering involved with derricks, and cranes are more glamorous”, is Bob McKenna’s verdict. “There’s an attitude out there that ‘if it ain’t a Manitowoc, it ain’t a real job’.
“Derricks take more skill and knowledge to erect and operate. Crane manufacturers do a lot of the planning for you… rigging is becoming a lost art”, says Bob McKenna.
Despite that, F&M Mafco is still finding plenty of work for its stiffleg derricks. “It’s a dying breed thing, and we’re keeping it alive. We ship these machines all over the world, and, believe me, once you see what these things can do you don’t go back to using a crane.”
