As the tower crane market grew, demand for the much simpler derricks diminished. At the end of the 20th century, the number of derrick manufacturers had dwindled, but crane manufacturers have continued to develop new variations on this older technology. With skylines continuing to grow taller and tower cranes larger, derricks still have a place in the construction industry.
Dismantling tower cranes and working on skyscraper rooftops are the most common jobs for derricks because they provide an economical hoisting solution with their light assembly weight and strong lifting capacities. Outside of urban environments, derricks are still a popular choice for building bridges, working on power plants and in mining.
Larry Howard of DCM Erectors traces the derrick back to just a wooden pole. “You had the ability to take that pole and lean it out. And by putting a bracket on it and putting some sheeves on it you were able to pick up a load.” Or more simply, he explains, it’s a lever situation.
The modern derrick is a central vertical mast, with one of two support configurations, guylines or stiff legs, to form a tripod. At the base of the mast is a luffing boom, and a hoist provides the lifting and booming functions.
Derricks don’t require any counterweight; instead, they are tied down to the structure they work on, such as a rooftop or bridge. The winch package is a separate unit that can be anchored elsewhere, explains Howard.
Two years ago his company, DCM Erectors, installed a new pole for the famous New Year’s Eve ball drop on the top of the 1 Times Square building in New York. The rooftop was extremely crowded so the winch package was left on the ground. They put an assembly off the side of the building and ran the load lines up and over that and into the derrick.
He points out this particular job and building were more complicated than usual. Most derricks are designed to break down into sections small enough to fit in standard elevators, material hoists or through standard doorways.
For this project, the elevator wasn’t large enough to accommodate the dimensions of the derrick’s sections. “We actually had to hang the pieces on the underside of the elevator, take the elevator up and pull them out from underneath,” Howard says.
Derrick manufacturer Timberland Equipment in Ontario, Canada, has about 30 standard designs up to a 350t capacity, says Lawrence Clark, its product manager for construction and mining equipment. About 75% of the applications Timberland’s derricks are used for are on high-rise towers, and dismantling tower cranes is a “fairly significant portion” of that business. “Primarily our customers are people who rent our derricks to people taking down tower cranes,” Clarke explains. “A number of companies that own and rent the tower cranes have our derricks to dismantle the tower cranes.”
The 17t and 35t capacity derricks are the most popular, which he says both break down into sections of about 8ft. “You can actually man-handle them into the elevator to get them up on top of a building,” he explains. For the 17t derrick, the sections weigh at most about 350lb each, and for the 35t, about 650lb a piece.
This light assembly weight is one reason derricks are often used in bridge construction because this allows them to be assembled on the bridge, explains Bob McKenna, executive vice president of Ohio-based F&M Mafco, a dealer for Timberland. F&M Mafco deals almost strictly in rentals, he says, with skyscraper projects being maybe 10% of the workload, and the majority being bridge or power plant work.
F&M Mafco offers a range of derricks from 2.5t to 500t capacities and its most popular derrick for bridge work, says McKenna, is the American S30, which has an 82t capacity and can pick 21t with a 160ft working radius. American Hoist and Derrick was a St. Paul, Minnesota-based manufacturer during the 20th century that is now part of Terex Group.
“By building the bridge from the bridge, it eliminates a lot of contingency during the construction process because you have a stable platform,” says McKenna.
Making lifts from a barge in the water can be challenging because of variables like windy weather or passing boats, which cause waves and bounce. Although, he says, it is acceptable in some cases, “quite honestly, anytime you’re over 35 or 40m, to reach the bridge from the water at that particular time you’ll need at least 60m of boom, and you pick up a hell of a lot of bounce.”
Using one method called balance cantilever, bridges are built starting on each side of a pylon and work across. A derrick can be built by the tower crane once it’s finished building the bridge’s pylons and the pier table. Or a small cherry picker could be set up to build the derrick, explains McKenna.
A sled is tied to the pier table, and the derrick is tied to the sled. This fixture moves the derrick across the bridge on a set of tracks, avoiding the need to take the derrick apart each time. The sled is propped up hydraulically or with screw jacks just enough to clear the tracks.
The idea is to keep the weight on the bridge to a minimum, says McKenna, and the weight of the sled and derrick combined is not very heavy at all because there is no counterweight. No crane that is light enough to be building on the edge of the bridge has the capacity of a derrick.
Another advantage of using a derrick rather than a tower crane, McKenna points out, is very minimal maintenance. “You have to put fuel in the engine, change the oil and grease the bearings. That’s it.
“On a bridge deck, you’re a couple of hundred feet off the water, it’s not like you have unlimited access to a service truck. You pretty well have to be self-sufficient.”
F&M Mafco also sees a lot of derrick rentals for work on power plants because they are typically very congested areas. A derrick can be placed on top of a boiler and lift components from there, avoiding all the interferences on the ground, explains McKenna. For jobs where a derrick is on the ground, it has a smaller footprint than a larger crane with the same capacity that requires space for a boom and counterjib, which derricks don’t have.
Crowding on rooftops and city blocks are other reasons derricks are chosen rather than a tower crane. Morrow Equipment Company, based in Oregon, had three jobs in the Seattle market all at the same time that needed a derrick or a very large tower crane, says Daryl Hicks, one of the project engineers at Morrow. A distributor for Liebherr, the company decided to purchase a 200 DR 5/10 derrick crane, which completed its first job last summer (see p34).
Andersen Construction of Seattle started to erect this Liebherr 200 DR 5/10 in early March to install a 124,000lb building maintenance unit (BMU) at the top of a 63-storey skyscraper. Project superintendent Kevin Larkin calls it “not your everyday derrick install.”
The building, completed in 1986, has a glass roof with a steep slope and a rounded top. Several months in advance, Andersen added 24,000-30,000lb of temporary reinforcing steel to support the load of the derrick, which weighs about 65,000lb. This was in addition to installing 1,000lb of steel running through the top four stories of the building for permanent support of the BMU.
Andersen spoke with three helicopter companies while planning the job, explains Larkin. “Part of the logistics of a helicopter was blocking off six square blocks downtown so we wouldn’t be flying over any occupied buildings with a 16,600lb load”—the heaviest pick for assembling the BMU, and also the largest at 37ft-long by 3ft-wide by 4ft-tall. There were other concerns, he says, such as having a lay down area and not causing rotor wash to break out any of the glazing in the surrounding buildings. “We also looked at installing a tower crane on the east elevation of the building. It would have been four tie-ins to the existing building and an 840ft under hook height,” says Larkin.
Unfortunately the building is directly adjacent to Interstate 5, a main highway on the west coast that connects Canada to Mexico, leaving “very, very tight quarters,” he explains. “We walked though a cost analysis and a feasibility analysis on all three and came up with the derrick.”
Cost is a factor for end users when choosing a derrick, but mainly they look for capacity, ease of assembly and light components. “Derricks are usually your last alternative when a conventional crane cannot reach,” says Larkin.
More frequent users are also looking for multi-use (guyline and stiff-leg) options and flexibility. DCM Erectors purchased a 35t capacity adjustable derrick from Timberland to take down tower cranes.
It’s an important piece in the fleet, Howard says, because the stiff legs and sills can be set anywhere from a 45° angle and the other way to around 110°. The actual length of the sill can be adjusted, which is important for the configuration of the derrick on the roof. “You try to get on top of columns because the uplifts are heavy. So you want to get something that takes the load back down into the structure.
“The main concern is to be able to locate the derrick in a position it can not only pick the pieces, but also that it still has enough capacity and reach to be able to put the pieces over the side and down to the ground,” says Howard.
DCM is building the new World Trade Center Freedom Tower in New York and has already made plans over the past three years to use a derrick to take down the tower cranes once the project is finished. “We’re actually, in conjunction with another company, building a new winch for the derrick so that we can reduce the number parts of line because we’re talking 1,300-1,400ft that you have to come down with the derrick.”
In 2006 Liebherr introduced its 200 DR 5/10 to meet the demand for derricks during the high-rise construction boom in the Middle East, says Christian Ogger, product manager for derrick cranes at Liebherr. “No other crane is needed to disassemble the Liebherr derrick crane,” he says, “just a hoist.”
This derrick can be used with or without stiff legs, and can be configured with load moments of 100t or 200t. Ogger says the hoist winch capacity is designed to lower components down from very high buildings. Using a jib for a maximum radius of 20m, the 200 DR 5/10 lifts 10t, in double-reeved operation across the whole radial area. A jib for a maximum of 25m working radius can hoist 7.5t at the tip.
It is this same growing high-rise market that appealed to crane manufacturer Wilbert, which is debuting its WT 175L e.tronic this month at Bauma. While technically not a derrick, this luffing jib crane can be assembled with or without the tower or base.
“We assume that there will be a high demand for the WT 175L e.tronic for assembly and disassembly situations, skyscrapers and sites where other assembly cranes are too small or too weak,” says Günter Krone witter, sales manager at Wilbert.
The new crane can be assembled using any lifting equipment capable of at least 1.5t—the weight of the WT 175L’s heaviest component. The jib length ranges from 12-36m at 6m increments, and the crane has a 16t capacity. It is capable of lifting 10t to 13.5m or 5t to 20m.
Unlike a derrick it has a counterjib, though it’s only 4m long, explains general manager Franz-Rudolf Wilbert. He says Wilbert has several products for sophisticated assemblies but, “the assembly crane that we were using so far is too small; the decision for the WT 175L e.tronic was very easy to make.”
Another new variation on a derrick has been developed by Australia-based GTC Engineering, which specialises in tower crane erection, dismantling, repairs and manufacturing other solutions. It recently designed a modular crane for what was thought to be a one-off prototype.
Total Machine Relocations (TMR), had a job installing new chiller units on the roof of a 38-storey building in Melbourne. The units were too large and heavy to be transported in the service elevator. It needed a crane with the capacity to lift 5t at a 20m radius, and that would easily break down to components weighing no more than 1t, and small enough to fit in the elevator.
GTC designed an electrically powered stiff-leg derrick to TMR’s specifications. Choosing electric rather than hydraulic operation made the crane lighter and more versatile, says Drazen Vibovic, general manager at GTC. Unlike other derricks, GTC used tube sections with wire rope running inside. This is for strength and simplicity, he explains.
GTC considered the derrick a niche crane application, but the prototype can be fixed to a variety of roofs and accommodate a range of layouts, giving it “huge potential for widespread applications,” says Vibovic.
