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Tower Crane
Take a  good look out over any major city skyline and you’re likely to see tower cranes erected beside tall buildings and skyscrapers. These eye-catching lattice-woven like structures are best described as a central shaft with a long boom and counterweights used to balance the crane’s load. Different types of tower cranes include top-slewing, bottom-slewing, self-erecting, and special application. Slewing refers to turning about a fixed point.[1] The most common top-slewing tower crane used in construction today is the horizontal boom.

Contents

[edit] History

[edit] Early Crane Evolution

The earliest recorded version or concept of a crane was called a shaduf and used over 4,000 years by the Egyptians to transport water.[2] The crane consisted of a long pivoting beam balanced on a vertical support with a heavy weight attached at one end of the beam and a bucket on the other.

In the first century, cranes were built powered by animals or humans moving on a wheel or treadmill. The crane had a long wooden beam known as a boom and was connected to a base that rotated. The wheel or treadmill was power-driven by a drum with a rope that wound around it. This rope, connected to a pulley at the top of the boom, also had a hook that lifted the weight.[3]

During the Middle Ages the use of cranes to build Europe’s massive cathedrals and to load and unload ships in major ports led to advancements in crane design. A horizontal arm was added to the boom. This arm would become known as the jib. This addition to the boom provided cranes with the ability to pivot, thereby increasing the crane’s range of motion. By the 16th century, cranes had incorporated two treadmills on each side of a rotating housing holding the boom.[4]

Up until the mid-19th century, cranes continued to be dependant on human and animal power; however, this quickly changed with the development of steam engines. At the turn of the century, internal combustion engines and electric motors powered cranes. Cranes were also being built with cast iron or steel instead of wood.

[edit] The First Tower Cranes

Tower cranes first started appearing in Europe in the first half of the 20th century. Streets in European cities were narrow and tall cranes with booms and the operator on the top proved to be more advantageous. As a result, some of the earliest manufacturers of tower cranes originated within Europe. For example, in 1908, Maschinenfabrik Julius Wolff & Co. introduced the first series of tower cranes specifically designed for the construction industry. These first-generation tower cranes mostly appealed to shipbuilders who bought and installed them in shipyards and on docks. Over 10,000 units were built and the basic design of these tower cranes remained in production up until the late 1960s.[5] Other manufacturers like Kaiser and Potain were also developing tower cranes in the early 1930s. Their designs were largely based on dockside and harbor cranes. These cranes were exceptionally heavy made of steel and iron and difficult to set up, dismantle and transport. Even though Wolff’s cranes began to be used by the construction industry, in 1948 there still existed a need for a fast assembly tower crane.

[edit] Hans Liebherr

In 1949, Hans Liebherr realized that fast assembly tower cranes were virtually non-existent on the market. He took it upon himself to build a bottom-slewing tower crane with a horizontal jib on top. The crane could also lift material from the ground up, by hoisting it, swinging over, and then dropping the material directly onto the new structure—a feature uncharacteristic of cranes up until this point as materials had to be manually carried from the drop off point. Another defining feature was that the crane could be transported partly assembled and could fully assemble itself. Liebherr presented his first crane, the TK-10, at the Frankfurt Trade Fair in Germany in the fall of 1949. At first, the industry was cautious about Liebherr’s new crane but eventually the design caught on and Liebherr took the TK-10 into mass production.[6] A whole series of construction cranes based on the TK-10 design concept came out over the following year. With his design, Liebherr managed to revolutionize the tower crane industry. As the 1950s approached, new ways of using cranes were developing and there existed very advanced slewing tower cranes on the market.[7]

With a worldwide construction boom occurring during the 1940s and into the 1950s another trend that emerged with tower crane manufacturers was to complement the manufacturing of tower cranes with a combination of other construction equipment like concrete mixing machines. For example, Reich in Ulm, Germany manufactured slewing cranes and advertised concrete mixers as an add-on by matching lifting capacity and working speed.

By the 1940s, important slewing tower crane manufacturers included Peschke, Peiner, Wolffe, Weitz, Liebherr, Potain, Boilot, Braud & Faucheux, Campistou, Favellle-Favco from Australia, Ferro, Fiorentini, Fives Lille, Fuochi-Milanesi, and Haulotte. [8]

Tower cranes working on top of building

[edit] Tower Cranes Grow to New Heights

The 1950s marked a number of monumental milestones in tower crane design and development. First, multiple manufacturers were beginning to produce more bottom slewing cranes that had telescoping masts and came to dominate the office and apartment block construction market. The use of cantilever jib designs were abandoned by a number of leading tower crane manufacturers who switched to luffing jibs. The use of luffing jibs eventually became the norm.[9]

In Europe, other substantial inroads were being made in the design and development of tower cranes. Construction sites were often constricted and using rail systems to move large tower cranes was proving to be too costly and inconvenient. At the same time, a number of manufacturers were offering saddle jib cranes with hook heights of 262 feet (80 m). These cranes were outfitted with self-climbing mechanisms that enabled sections of mast to be inserted into the crane so it could grow as structures were built upwards. The long jibs on these cranes also covered a larger work area. These developments precipitated the practice of anchoring and erecting cranes inside the building’s lift shaft and eventually this method became commonplace in the industry.[10]

From the 1960s, the dominant focus on tower crane development and design would be on covering a bigger job radius, higher load moment, new control systems, climbing mechanisms and technology, and faster erection strategies, with the most important developments being made in drive technology.[11] The market was also becoming increasingly crowded with multiple manufacturers producing a range of tower crane models such as luffing cranes and saddle jib cranes, as well as branching off in to the development of smaller cranes that could be assembled on site in a matter of a few hours. Much of the drive behind all the improvements and developments made to tower cranes in the 1960s is attributed to the use of mobile cranes and their ever-increasing operating efficiency during this period.

[edit] The Tallest Crane in the World

As skyscrapers were being built at record-breaking heights, the need for taller cranes existed. In 1975, the Danish company Kroll became manufacturer of the world’s tallest tower crane, the K-10 000. This colossal tower crane has a mast that stands 394 feet (120 m) tall and an extended reach of 295 feet (90 m). Just to grasp the actual size of the K-10000, the crane is five times the size of a standard tower crane and three times as tall as the Statue of Liberty, with an operating range covering 7.5 acres (3 ha).[12] Due to its massive size, only 20 have ever been built.[13]

[edit] Features/How it Works

Crucial support for a tower crane is provided in the base that is usually bolted into a large concrete pad. This base is connected to a mast or tower and stabilizes the crane that is affixed to the inside of the building structure, namely to a concrete lift or elevator shaft. The mast is typically a triangulated lattice structure, 10 feet square (0.9 m2).[14] Attached to the very top of the mast is the slewing unit comprised of a gear and motor that gives the crane the ability to rotate. This slewing unit consists of three parts: a long horizontal jib or working arm is the part of the crane that carries the load; fastened to the jib is a trolley that moves the load in and out from the center of the crane; the machinery arm contains the crane’s motors and electronics as well as large concrete counter weights. The crane operator’s cab is also located on top of the crane just below the horizontal boom. Tower cranes can also be controlled on the ground by remote control.

[edit] Height, Reach, and Lift

Tower cranes can have a maximum unsupported height of 265 feet (80 m). A tower crane’s maximum reach of 230 feet (70 m) and maximum lifting power of 39, 690 pounds (16,642 kg) with counter weights of 20 tons and two limit switches are used to ensure the operator does not overload the crane. A maximum load switch governs the pull on the cable and guarantees the load doesn’t exceed 18 tons.[15] A load moment switch ensures the crane operator does not surpass the ton meter load rating. Something called a cat assembly head in the slewing unit is used to sense when a possible overload may occur.

[edit] Erecting a Crane

Due to there sheer 666 height, there is a definite science to erecting a tower crane. The stationary structure must first be transported to the construction site on large tractor-trailer rigs. A mobile crane is often used to first assemble the jib and machinery section of the crane and then these sections are attached to the mast. The mobile crane than adds counter weights. Other equipment that may be used to erect a crane includes forklifts, and crawler cranes.

As the building rises up, mast extensions are added to the crane so it’s height corresponds to height of the building. Crew use what is called a top climber or climbing frame that fits between the slewing unit and the top of the mast. Work crew hangs a weight on the jib to balance the counterweight and then detach the slewing unit from the top of the mast. Hydraulic rams in the top climber then are used to adjust the slewing unit up an additional 20 feet (6.1 m). [16]The crane operator then uses the crane to insert and bolt into place another piece of mast section.

[edit] Common Manufacturers

[edit] References

  1. Gransberg, Douglas and Popescu, Calin M. and Ryan, Richard. Construction Equipment Management for Engineers, Estimators and Owners. CRC: 2006. 113
  2. Crane. Madehow.com. 2008-09-28.
  3. Crane. Madehow. 2008-09-28.
  4. Crane. Madehow. 2008-09-28.
  5. On the Water. Cranes Today. 2008-09-28.
  6. Cohrs, Oliver Bachman.The History of Cranes. KHL: 2004. 123
  7. Cohrs, Oliver Bachman.The History of Cranes. KHL: 2004. 131
  8. Cohrs, Oliver Bachman.The History of Cranes. KHL: 2004. 131
  9. Cohrs, Oliver Bachman.The History of Cranes. KHL: 2004. 139
  10. Cohrs, Oliver Bachman.The History of Cranes. KHL: 2004. 141
  11. Cohrs, Oliver Bachman.The History of Cranes. KHL: 2004. 141
  12. Machines and Engines. Discovery Channel. 2008-09-28.
  13. Cohrs, Oliver Bachman.The History of Cranes. KHL: 2004. 207
  14. Tower Crane. Howstuffworks.com 2008-09-28.
  15. Tower Crane. Howstuffworks.com. 2008-09-28.
  16. Tower Crane. Howstuffworks.com 2008-09-28.