Tower cranes are commonplace on construction sites around the world. They are integral to the construction of large structures such as skyscrapers. With the size and complexity of buildings they need to build, they have to handle very heavy pieces of equipment at great heights. They therefore need to be carefully and cleverly designed to do their jobs. Let’s now see how a tower crane works.
Crane Structure
Tower crane structures are universally recognisable. You have probably seen plenty of these lopsided, T-shaped structures towering over skyscrapers or in the centres of construction sites. The basic structure consists of a large concrete base from which a mast rises which gives cranes their height. Atop this mast rests a slewing unit, which consists of a gear and motor that allows cranes to rotate.
Atop this slewing unit there are 3 parts. A long horizontal working arm which sticks out and acts to carry the loads. A trolley runs along this arm which carries the load and allows it to be repositioned along the working arm. Behind the working arm there is a shorter machinery arm which holds the crane’s motors, electronics and concrete counter weights. Finally there is also the operators cab. Typical cranes can reach a height of 70 meters, able to carry 18 tons at a maximum reach of 70 meters horizontal to the base.
Construction
With the large size, weight and complexity of tower cranes, you might think its construction may take as long as building the skyscraper it would be used for. However, you will rarely see a tower crane construction on the site. In many cases those would be the already formed ones. This is because of the clever design and structure of cranes.
The materials/parts of the crane arrive to the construction site on 10–12 trailers. Another mobile crane is used to put together the parts. First crew and the mobile crane construct the arm and machinery sections which rest atop the slewing unit on the ground. Then two mast sections are connected together which form a vertical height of 12m before the mobile crane adds counter weights. The fully formed arm and machinery section is then placed atop this initial mast.
Now that a mini crane has been formed, it can grow itself. It does this by adding mast section after mast section at a time. To do this the crane makes use of a climbing frame which connects between the slewing unit and a section of mast. The unit uses large hydraulic rams which push the slewing unit up, opening up a 6m gap between the slewing unit and the last section of mast. The crane can then lift up a section of mast and insert it in the vacated space. The hydraulic arms can then extend more space for another section of mast to be inserted, this process repeats until the crane is at its required height.
Stability
Tower cranes have to lift massive weights at quite large distances. It can seem mind boggling at times how the crane manages to keep itself upright under these stresses. To ensure stability, the large concrete base the crane rests on is key. This base can be 10 x 10 meters and weigh 180 tonnes. This gives the crane a solid centre of gravity to stop it toppling. The next element is the counterweights positioned atop the crane. To understand their importance we need to understand the concept of moment.
Moment is a rotational force. It can be best understood by envisioning a seesaw. A typical seesaw will remain flat, balancing upon its base. However if a child sits on one end, the seesaw will rotate and reach the floor where the child is sitting. This is moment, a force applied at a horizontal distance from the base of the structure. However, if a child of equal weight were to sit on the other end of the seesaw, it would rotate and return to its flat position. This is because the moments balance. This is the purpose of the counterweights on the crane. Without them the crane would topple, however with them it remains stable.
The magnitude of the moment is dependent upon its distance from the cranes base. This therefore has implications for maximum loads lifted. For instance, the maximum load cranes could lift is 18 tonnes, but the maximum load positioned at 30 meters from the mast is only 10 tonnes.
SEE ALSO: Tower Cranes Innovations: Lifting Then and Now
Originally published at ilmm.co.uk on January 9, 2017.
