About cranes
A crane is one of the many machines used in the construction, manufacturing, and transport industry to move heavy materials or equipment vertically or horizontally from one point to another. A crane machine is made from smaller machines as well as wires, ropes, winders, and sheaves that are assembled so as to create a motorized advantage. The motorized advantage enables the movement of a variety of heavy loads that goes beyond the normal physical capability of humans.
The crane uses simple machines such as the lever and pulley to create its motorized advantage (2). Cranes are a common sighting in the transport industry, particularly in ship harbors, because they are used to load and unload consignment from ships that are otherwise impossible to move. Cranes are also a common sighting in the construction industry as they assist construction workers to move heavy building materials like concrete slabs and steel bars from the ground to the places they need to be installed. In the manufacturing industry, for example, the motor assembly industry, cranes are used to lift heavy equipment and through automation processes, assemble them with minimal assistance from humans (2).
There are two major factors that are considered when designing a crane: the first consideration is the weight the crane is expected to lift, and the second consideration is the crane stability. For example, a crane used in the manufacturing industry cannot be expected to lift consignment from a ship due to the weight differences. In regards to stability, a crane should be built to effectively handle heavy loads without the remote possibility of it toppling over; thus, being a hazard to everyone in the area.
In the old ages, cranes could only lift and lower heavy materials, but with the passing of time and major technological advancements, the modern cranes have the ability to move vertically as well as horizontally (2).
Reasons why cranes are used in construction
The aspects of modern construction projects are and are continuing to become more highly automated. As industrialization in the construction industry continues to grow, more companies prefer to use building structures and elements that are assembled elsewhere as opposed to doing the actual assembly on site. Concrete, for example, is only produced on-site when the construction project requires very high volumes of concrete; thus, it would not be prudent to transport the concrete (4).
If the project is smaller, it would be cheaper and more practical to transport ready-mixed concrete to the site. It means that someone is likely to see transportation equipment on-site as opposed to production equipment. In modern building construction sites, there is usually clear domination of lifting equipment, and these types of equipments are considered as critical elements in attaining productivity. Concrete pumps, earthmovers, cranes, lifts, and hoists, and material handlers are some of the most notable equipment types in a construction site (4). Though earth moving types of equipments are seen during the startup of the project, cranes are the only conspicuous equipment during the actual construction process. This is due to the important role they play in the vertical and horizontal transportation processes.
During building construction in urban spaces, there usually is not much space to place production equipment. Cranes in this situation provide the ability and flexibility of accessing smaller sites and only occupy a limited stationary area. Cranes depending on their size, also reduce the time it takes to transport materials from one point to another. It is because a crane can lift materials and deposit them on the other side without wasting time trying to navigate around barriers (4).
Cranes are used in construction because they are relatively easy to install, they can be installed in high places, and they also have the ability to lift quickly heavy material and equipment to great heights i.e. skyscrapers.
Types of Cranes
There are several crane types, each depending on the work they are meant to do. Below is a list of the several crane types and when they are used.
Crawler Crane
A crawler crane is a full revolving crane that is mounted on an undercarriage. It has a set of continuous parallel tracks also known as crawlers that provide the crane with mobility and stability. Crawler cranes configurations can be changed depending on the manufacturer specification (3). These configurations include its application type, the tower unit and the duty cycle. Crawler cranes have good lifting abilities and handle work such as lifting concrete buckets to great heights. The cranes advantage is it can move on site thus, it requires little setup. The crawlers on the crane also reinforce its stability. The crane can also travel as its lifting a load. The disadvantages include its heavy weight thus, it is expensive to transport. Also disassembling the crane for transport requires a lot of time, energy and expertise (3).
Hydraulic Truck Crane
A hydraulic truck crane is a type of vehicle mounted crane. The crane has three control and power arrangements. Both the truck and hydraulic can be controlled and powered by a single engine. The truck and hydraulic can have different power and control units. Bothe the truck and engine can the same power source but different control panels. This type of crane is always ready for a move (3). The main benefit of this crane is that it is mobile and is appropriate for use for short periods of time. The main disadvantage is that since the crane is mobile, it means it may have stability issues.
Lattice-Boom Crane
A lattice boom crane is similar to a hydraulic crane. It is a fully revolving crane that is mounted on a truck. It has a suspended cable thus, acts as a compression component as opposed to a bending component. The main benefit is that it is lightweight thus, has an additional capacity for lifting. It is also mobile thus, can be used for short time periods. The main disadvantage is that the crane is that it takes a longer time to assemble and disassemble for transport (3). It is also expensive to assemble and disassemble since it needs the aid of another crane.
Rough-Terrain Crane
A rough terrain crane is mounted on an undercarriage, and that has four unusually large rubber tires which allow the operator room for maneuvering in the job site. It is designed to pick and carry a load on rough terrain. The operator cabs can be attached to the upper works thus, allowing the operator cab to swing in the same direction as the load. Both the crane and truck draw power from the same engine which is mounted in the truck. The main benefit of this crane is that it operates at a lower cost. The main disadvantage is that it is slow due to its large size (3).
All-Terrain Crane
An all terrain crane has an undercarriage that allows it to effectively travel on all terrains and at a higher speed. The crane has four wheel-drive, steer and its ground clearance is higher. The crane has two cabs each with a different function. It is best for use when projects are at different locations. Its main advantage is that it can work on all terrains. Its main drawback is that it is more expensive than a rough terrain crane since it is a combination of two distinct features (3).
Floating Crane
A floating crane is mounted on a barge or pontoon. It bears similarities to cranes mounted on undercarriages such as the Lattice-Boom, Hydraulic Truck and the crawler crane. It is mostly used in the construction of bridges and ports. It is used to transport bridge sections from one point to another as well as recover sunken ships. Its main disadvantage is that since it operates on water, it can only be used in offshore projects. The main advantage is its high lifting capacity which is 10,000 Tons (3).
Railroad Crane
A railroad crane can travel along tracks as they carry their load. This is because they are mounted on an undercarriage car that has rail wheels. It is mostly used for train recovery, loading goods and maintenance work. The advantage is that the trainâs wheels can be removed, and the crane mounted on a static crane as opposed to all the other cranes. Its disadvantages are that there is a free standing height requirement and that the crane cannot be used if there is maintenance work on a particular track (3).
Tower Crane
A tower crane is well known as a balance crane and is used in the construction of very tall buildings. It is fixed to the ground thus, giving it great stability and because of the stability; it gives a great combination of lifting power and height. The tower crane can also be attached on the side of structures during construction. In major construction sites, a tower crane is one of the most predominant equipment (4). A tower crane is used to lift a variety of objects such as large tools, concrete buckets, steel bars, generators and other construction building materials. A tower crane is best used where mobility is not required; a great height wants to be achieved and when there is little space to work in. Tower cranes can be categorized into two groups: top slewing and bottom slewing (1).
The major difference between the two is how they are assembled and disassembled. It is easier and faster to erect and take apart a bottom slewing crane because there are a lower number of masts between the base and the slew. Bottom slewing cranes are also used for short-term service. Top slewing cranes take a longer time to erect and dismantle because of their height. Each mast must be taken down and hydraulics used to lower the slew before the next mast is taken down. Top slewing cranes are preferred for long-term service. The main disadvantage of tower cranes are the costly prices associated with the assembly.
A company must pay a charge for the crane to be delivered and set up by the crane company crew. After that, the company is charged a monthly fee for each month they use the crane as well as a maintenance fee. The main disadvantage is that an operator has to climb the mast to get to the operator cab which is a risky venture and a challenge, especially when dealing with the top- slewing cranes (3).
Why it is used in construction
Tower cranes are used in construction because they are quick and easy to assemble and disassemble, and they do not require many parts. They are also used because their height can be increased depending on the project requirement. A tower crane is known for lifting large loads and well as its precision aspect. They are also known to take up little space and provide a good working radius. Tower cranes also provide high speed and high concrete volume placements (4).
Size
Once the crane arrives at the site, a mobile crane is used to assemble the mast. The slewing gear and machinery arm are first assembled followed by the operators cab. To achieve the desired height, the mast can be added one mast at a time and firmly bolted to the lower mast. As the mast rises a hydraulic lift is used to lift the slew and another craned is used to lift the mast in place. Each mast is 20 feet long (3).
Tower crane parts
To achieve its stability and height, a tower crane requires essential parts. To support the tower crane, concrete is poured to create a pad onto which the crane base is bolted. The steel base is in turn attached using bolts to a tower which can be of varying heights as per the crane use. A slewing unit which is a combination of a motor and ring-gear is attached to the tower which gives the crane the ability to fully revolve. The counterweights and machinery arm are attached to the slewing unit. Finally the operators cab is also attached to the slewing unit. The operators cab is where the operators sits while controlling the tower crane. The cranes electronic motors are contained within the machinery arm (3).
How it is prevented from tipping
A tower crane is prevented from tipping over by ensuring it has a strong concrete pad supporting it. Stability is of utmost importance thus, construction companies ensure the concrete base is ready weeks before the crane arrival (4). Anchor bolts are deeply rooted in the concrete pad to support the crane base. The mast is has the strength to remain upright because of a three cross-member structure installed to the base.
Criteria for choosing a crane for construction work
- Construction type â A crane should be chosen according to the construction type. For example, tower cranes are more appropriate in the construction of tall buildings while a floating crane is more suitable for offshore projects.
- Time â A crane which will carry a heavier load at a faster time rate.
- Stability â Since the crane will be moving large loads constantly, a stable crane should be chosen.
- Multiple uses – Cranes that can be sued for multiple purposes should be selected so as to reduce the cost of hiring more cranes.
- Cost â Cost of cranes should be considered. For example, if a company has multiple projects in different terrains they should employ the use of an all terrain crane instead of a rough terrain crane (4).
References
- Aviad Shapira, Gunnar Lucko, and Clifford J. Schexnayder. âCranes for Building Construction Projects,â Journal of Construction Engineering And Management, pp. 690-698. 2007.
- Matthies, Andrea. 1992. âMedivial treadwheels,â Artists’ Views of Building Construction. Technology and Culture. 33 (3): 510 – 547.
- âTypes of Cranes,â 2003. Norman Spencer Insurance Agency. Web.
- âUses and Benefits of Tower Crane,âFeb. 20, 2013. Xinxiang Kerui Heavy Machinery Science and Technology. Web.