Tower cranes are used for lifting heavy building materials like concrete slabs, steel structures, bulk sand bags; machinery equipment like power generators, cement mixing machines; and other objects. They are usually located on top of buildings with exact positioning to handle these heavy loads.
It is evident from current building trends that civil engineers and architects plan and compete with each other to build huge skyscrapers. As an average person in a city somewhere in the world, you can probably find nearly twenty sky-scrapers in any given direction. But have you ever wondered about tall structures which look like they have a crane on top?
These are called “tower cranes," and they are very important for any new building built beyond certain heights. During the initial phase of construction, these tower cranes become an integral part of the building and are very important in aiding in the construction of the building, and in keeping construction workers, as well as city inhabitants, safe. These cranes are suitably fixed on top of the building, so that they can reach any part of the building with ease to lift and drop materials.
Basic Parts of Tower Crane:
The tower crane usually has the following parts.
Base: This forms the base of the tower crane and it is very important component as it forms the interface between the tall steel structure and the concrete building top. It can be compared to the spine supporting a human body. The steel structure of the tower crane is bolted to the concrete pad.
The Mast or the Tower: The base is connected to the mast or the tower, which gives the height to the crane. This mast or the tower has guard rails and guide rails which guides the elevator. This elevator facilitates the operator and the maintenance technician to reach the operator’s cab and the machinery arm from the base.
The Slewing Unit: This is the mobile unit of the tower crane, which is capable of rotating almost 360 degrees. The rotation of the crane around its own axis is called slewing. It is a set of combination of huge gears and motors which enables the crane to slew.
On top of the slewing unit, the tower crane has three more parts.
Long horizontal jib: We know that the jib is one of the most important parts of a crane and in a tower crane it extends horizontally carrying a trolley which runs in and out of the crane’s center carrying the load. So it can be called as the “working arm" of the crane.
The machine house: It is also located horizontally along the line of the jib, usually extending behind the crane’s center. It houses the crane’s motor and other electronic components which are necessary for the operation of the crane. It also incorporates all safety devices for the safe operation of the crane. It is designed to eliminate water ingress and withstand heavy winds. These also incorporate huge concrete counterweights. The main purpose of these counterweights is to provide balance while lifting a load on the other side of the crane through the jib.
Operator’s cabin: The operator’s cabin is comfortably located in such a way that none of the crane operation would hinder his visibility. The cabin is provided with almost all facilities which are required and sometimes even comforts are provided.
How much Weight can a Tower Crane lift?
Tower cranes cannot handle heavy weights as handled by gantry cranes at ship yards. The limitation for lifting weight starts from the base of the tower crane. Each every component of the tower crane affects the load lifting capacity of the tower crane. For a typical tower crane,
Maximum unsupported height – 80 meters (265 feet),
If the crane height is more than 265 feet, then some means of sturdy support is required. It may be some steel wires fixed from the crane mast to the different parts of the building.
Maximum Reach – 70 meters (230 feet),
Maximum Lifting Power – 19.8 tons (18 Metric Tons), 300 tonne-meters.
Counter weights – 20 tons (16.3 Metric Tons).
The maximum weight that the tower crane can lift is 18 Metric Tons. However, it has to be understood that the quantity of weight to be lifted keeps on decreasing as the distance from crane center increases. So, it is always better to lift loads keeping them as close as possible to the crane’s center.
The factor 300 tonne-meter is instrumental in understanding the load limitation. It is the moment of the load lifted about the crane’s center. So moment = load * distance.
A 300 tonne-meter limit can be inferred as a load of 10 tons being lifted at a distance of 30 meters from the center of the crane. So moment about the crane’s center is 10 tons * 30 meters = 300 tonne-meter.
Basic Safeties for a Tower Crane with respect to load being lifted:
Maximum load limit switch: This switch along with its accessories monitors the pull (strain) on the cable and thus does not allow the maximum load limit to cross 18 tons, after which it trips the motor and gives audible and visual alarms.
Load moment limit switch:This limit switch along with its accessories ensures the load-moment (tonne-meter) rating does not exceed beyond the given limit, after which the hoist motor and traversing motors are tripped with audible and visual alarms.
These towers are held at the base by concrete structures/pads, with the help of anchor bolts. These concrete slabs are made ready several weeks before the arrival of the tower cranes. These concrete pads weigh about 2 tons.
The erection sequence is to be strictly followed as even a small error may create huge devastation. For erecting and before put into use, surveyors are designated for inspections and checks to be carried out on the crane structure and the load limits. Well trained operators are licensed and examined before operating these cranes. Their education includes various levels of examinations and fitness check for operating at huge heights. These operators are well-versed with the load distribution systems and they keep monitoring the stresses and abnormalities when lifting through a computer which is programmed to calculate the bending and load moment calculations.
Image and Content Credits:
www.howstuffworks.com (moment calculation values)