Top Equipment: Tower Cranes

Note: This highly informative and elaborate paper was written by the Author many years back and holds valuable information. Readers should update themselves with the latest developments that might have occurred in this field after the presentation of this paper.

A visit to the upcoming areas of metros and developing cities often presents a scene of multiple tower cranes dotting the sky. Almost every sky scraper under construction is found having a tower crane on its top. The reason for it is simple. A tower crane ushers in immense benefits and ease for construction of high rise buildings. That’s why it has
become almost mandatory for all high rise structures to erect a tower crane for their construction.

Not only the high rise structures but the power plants too need a tower crane for erection of steel structures for the station building, erection of structural framework to accommodate boiler and for construction of cooling towers and tall RCC Chimneys. Then, the television towers also need to restrict their working space and use a tower crane. In general, tower cranes are now becoming an universal equipment like hydraulic excavators.

Often, it is the decision of the engineers whether to use a tower crane or not that consumes time. Once the decision is taken to use a tower crane, the job becomes much easier than anticipated. The engineers should therefore analyze the site situation quickly to weigh the benefits of deploying or avoiding a tower crane.

Points to check: Whenever a tower crane is to be used, broadly, the following points should be decided—

Major Components of tower cranes: A tower crane has the slewing unit at the top of the mast. It has a gear and a motor that allows the crane to rotate. Above the slewing head, the longer horizontal arm, called the working arm, lifts the load. Along this arm or jib, a trolley runs to move the load in and out from the crane’s centre. Shorter arm or jib called machinery arm has concrete counter weights. This arm has the cable drum and
control motor also. Main components can be enlisted as under:

Tower crane capacity: A typical tower crane has maximum unsupported height of about 80 metres. For more height, it needs support from the building or structure under construction. Ties with the structure are provided at regular vertical intervals to support the crane. For this, suitable inserts are left in the concrete structure being constructed as it proceeds above. When the concrete is fully set, ties are provided. However for steel structures, the ties can be provided as soon as the members are erected in position and welded. To be on safer side, one shouldn’t wait for unsupported height to be 80 metres. Better is to provide ties at lower levels also to ensure full stability of stationary tower crane. Generally, reach of tower cranes is 70 metres or lesser. This is no doubt a large reach. Lifting power is of such a crane is about 18 to 20 tonnes. Counter weight is thus 20 tonnes. Closer is the member to be lifted to the mast, more is the load that the crane can lift. Therefore, better is to define the capacity of tower cranes in tonne metres. If the capacity of a crane is given as 250 tonne-metres and the load is lying at 25 metre distance from the mast then the maximum load that the crane will lift from this point is 10 tonnes.

Tower Crane erection: Tower cranes consist of truss pieces in suitable lengths of 2 to 3 metres. The pieces are assembled at site one over the other to raise the required length of tower crane. The operator’s cabin is situated at a suitable height to provide full view of the area of operation to him. A ladder is provided inside the vertical truss to reach the cabin and also to go above to the jib. The jib extends to two directions, the smaller length carrying the counter weight and the larger length for site operations. To provide 360 degree rotation to the crane, the slewing head is provided at cabin level. Generally, the tower crane components start arriving at site and its assembly begins when the track is being laid for its movement.

Laying of tower crane track: Whenever the construction of a power plant is started, the first activity that is taken in hand is the laying of a tower crane track. For movable tower cranes, the tracks are kept flexible and rails are laid over them. A maintenance gang keeps a constant vigil over the track to add stone to it and to maintain side slopes as mostly these tracks are laid over and above the ground and any settlement of track may
destabilize the tower crane leading to dangerous situations and fatal accidents. Generally a T shaped track is laid around the area of operation. However, the area and extent of operation decides the length and layout of tower crane track. For an assembled tower crane, it is difficult to change track. Therefore, sharp curves, bends and even right angles are avoided to the maximum possible extent.

Stationary Tower Crane: Tower cranes are stationary inside the cooling towers, chimneys and buildings and are provided with footings for fixed part of the tower. When these are erected at higher levels, again these have stationary feet. Large anchor bolts are embedded deep in the concrete foundations to secure the legs of the crane to the foundation and to provide a strong base to the stationary tower crane.

Provision of Limit Switches: A tower crane should have at least two limit switches. One limit switch should control the maximum load irrespective of the distance that a crane can lift. This is called Maximum Load Switch. Second limit switch should control the maximum tonne-metres that the crane can lift. This is called Load Moment switch. To avoid collapse of the jib due to overloading or to avoid toppling of upper part of crane when the load is lying at quite a distance, limit switches must be provided and kept operational as sometimes, these do not act in time if not maintained.

Principle of assembly and dismantling: Biggest advantage of tower cranes is that these need a restricted space for their assembly and dismantling. One basic principle must be borne in mind while assembling or dismantling a tower crane that like a building, in a tower crane, the assembly begins from bottom upwards and dismantling begins from top downwards. The first piece erected is dismantled last of all.

Assembly of tower cranes: For assembly of tower crane, the parts are brought at site on tractor-trailers and unloaded at site. A mobile crane is utilized to assemble the long or working arm and short or machinery arm. A climber or climbing frame is used to build the mast. Generally the mast has around 10 sq. ft. area at the bottom. The climber builds one mast section over the other and continues till it fits between the top of the mast and
the slewing head. During the raising of mast, large hydraulic rams in the climber push the slewing head detached from the top of mast by 6 metres at a time while the crane operator fits the mast pieces in 6 metre high gaps thus created. The newly fitted piece of mast is well bolted in place and tower crane becomes taller by 6 metres. The process continues till all mast pieces are fitted in position.

Unique case: In October, 2004, a Liebherr 30 LC tower crane was assembled by using a helicopter at an altitude of 3000 metres. A helicopter was used as it was very difficult to transport tower crane parts 3 km vertically above the ground and then to erect them by use of cranes. More interesting part of the project was that the crane was not stationary but mobile. A 3 metre long railway track was provided for it. The Tower crane was
provided with guys to counter the wind speed at such a height. By using helicopter, the Tower crane was assembled in just 4 hours. The helicopter had to make 28 trips to collect the tower crane parts, the rail track ballast and the counter weights.

Dismantling of cranes over buildings: For dismantling of tower cranes erected over the buildings, a suitable roof mount crane is first erected by using the tower crane. When the crane is erected in position, it dismantles the tower crane from top downwards. The tower crane parts are lowered to the ground by use of winches.
When the tower crane is fully dismantled and lowered, a derrick is erected on the roof.The derrick is now used to dismantle the erected crane. The parts of the crane are lowered to the ground.

When the crane has been dismantled and lowered, the derrick is dismantled manually and lowered to the ground.

Dismantling of cranes on ground: For tower cranes erected on the ground, it is a much easier job to dismantle them. A crane is used to dismantle them the way it was used to erect them.

The most important activity is dismantling of the counter weights and the long or working arm. The counter weights are in parts and removed one by one while the long arm is kept in balance by removing inner and outer trolleys and other accessories on it and later by locking it in position. After removal of all the counter weights, the long and
short arms are brought down by use of a crane.

Some tower crane suppliers advise for lowering the height of the crane before removal of counterweights and long and short arms. In that case, the counterweights and arms are kept balanced and in position while the vertical mast pieces are removed one by one till all these pieces are removed. Great care has to be taken in this type of dismantling against imbalance of counterweights and jib arms. However it is considered better to move as per
supplier’s advice.

It is always advisable for the site engineer to maintain a date wise and component wise record of erection of every small and big component of a tower crane. This record proves of great help during the dismantling of tower cranes.

Precautions during dismantling: It is very important to take full precautions during dismantling of tower cranes to avoid accidents at sites. Operators should be fully alert and should never exceed the load and distance limits. They should keep an eye on the jib balancing. Dismantling should be suspended on windy days. Operator’s judgement in rotating the loaded crane plays a significant role. While using a tower crane in crowded
or restricted areas, an operator must be an experienced one. Most of tower crane accidents occur during their erection and dismantling because of falling objects or parts rather than during their use.

Production scene: Kroll Giant Tower Cranes, Denmark has so far produced world’s largest tower crane Kroll K-10000. Its operating radius is 100 metres and maximum load capacity is 94 tons. Many Indian companies are now working either for production of tower cranes or for having tie ups with foreign companies to market their products in India. Sometime back, Indian crane manufacturer, Escorts Construction Equipment Limited has tied up with Huata, China for marketing their Tower Cranes. Escorts’s top slew tower cranes with mast height of 25 m to 230 m, jib length of 30 m to 70 m, tip lifting capacity between 0.6 t to 2 t and maximum lifting capacity of 2 t to 16 t are getting
popular. These cranes are available with flat top and tower top options. Escorts’s mobile cranes have a mast height up to 39 metres, jib length of 30 m, tip load capacity of 0.8 t and max load capacity of 2.0 t. These are self folding and unfolding type with variable height and drive motors.

Metros are focusing on real estate sector and tower cranes are seen all around on top of multi-storey apartment buildings under construction. ECEL has already sold more than 50 tower cranes in less than a year. Sanyo heavy industry group, Omega construction equipment, Spartan Engineering Industries and Alpha services are providing tower cranes in India. Omega’s MTC 2518 is a self folding, self erecting and remote controlled tower crane. Its MTC 3725 tower crane is mounted on a trailer that can be towed away. India holds bright future prospects for Tower Crane producers and suppliers. Spartan India is supplying cranes tower cranes with split type mast with pin joints. Buyers look for after sales support as a major factor besides timely supply of the equipment, competitive price, ease of erection and dismantling, low maintenance cost, easy availability of spares, quality and reliability while making purchases. Paying attention to these points acts towards building a supplier’s credibility.

Typical examples: As an example for crane data, let’s consider a tower crane TC6516B offered by Zhejiang Deying, a Chinese company. This crane is claimed to be quite popular in Middle East.

A. Stationary type:

The height of tower crane can be extended to 176m and the big size of tower mast can support the whole weight of crane, wall tie with the building to stable the crane.

B. Traveling type:

C. Self erecting mobile tower crane: Koenig equipment india claims supplying one of the world largest self-erecting mobile tower cranes. These self-erecting mobile tower cranes are available in jib 20m, 24m, 28m, 31m, 33m, 34, 37m, 40m, 45m, jib-end load lifting 750 kg to 1,500 kg, maximum load lifting 2,500 kg to 5,000 kg and hook heights 18.0m to 53.3m.