Tower Cranes

There are two main types of tower crane in general use;

1. Self Erect Tower Crane

This is a tower crane that is designed to be easily and rapidly transported and erected. Some of them carry their own generator and are therefore completely self contained.

2. Assisted Erect Tower Crane

This type of tower crane is usually larger with a greater lifting capacity than the Self Erect. Its design calls for it to be builton site, piece by piece, often with the assistance of a mobile crane, hence its name ‘Assisted Erect’.

Both yypes of tower crane can be found in operation with a variety of jib configurations, all designed to provide a specific type of lifting service to sites where the lifting requirements and the environment produce different demands.

The two types of basic jib design are:

1. Horizontal Jib

This jib takes the form of a simple structure extending from the tower, along which a trolley can travel, carrying the hoist rope and hook assembly to vary radii.

2. Luffing Jib

The luffing jib has no trolley, the variation of hook radii is achieved by altering the jib angle, the same as with a mobile crane.

More detailed information on these crane and jib types can be found by reading the following pages.

Horizontal Jib Tower Crane Self Erect

This configuration of crane is not found as frequently in this country as on the continent, where it is common use as a small builders crane, employed in lifting activities more usually carried out by telescopic forklifts in the UK .

The true self erect tower crane is smaller and of lower capacity than its assisted erect counterpart, but has the great advantage of being quick and easy to erect. The erection procedure is well within the capability of a trained fitter or competent driver and the newly erected crane does not have to undergo the sort of comprehensive commissioning procedures required by an assisted erect crane.

In recent years there has been a move toward tower cranes that are quick to erect, but the small size and capacity of the self erect crane has always proved a barrier to its usefulness on most construction sites. Tower crane manufacturers are now producing a design of crane which embodies the best features of both the self erect and assisted erect tower cranes. Called the ‘City Crane’, these machines can be erected quickly and have a size and capacity which makes them an attractive proposition for use on general construction sites.

Some self erect tower cranes are mounted on a crawler track base and with the provision of an on-board generator and independent of an external power supply. These cranes have the abilty to be extremely mobile around a site, travelling fully erected without a load. Whatever their base type, self erect cranes usually require the use of outriggers which are used to both support and level the crane during lifting operations. Because of their importance to crane stability, outriggers should be checked by the driver frequently for security and level.

Horizontal jib tower crane assisted erect

This configuration of crane is widely used in the construction industry. It requires assistance in erection and this is usually provided by a suitable sized mobile crane, erection times vary but on average a time of two to three days can be expected for a crane of thirty metres square.

Erection of this type of crane is normally carried out by a crew and an engineer, trained in the specific erection procedures required. At the conclusion of the erection the crane will be subjected to a thorough examination and an application of a 25% overload followed by the setting of the safety devices.

Jibs of 50m and longer are available for applications where greater area coverage is required, and tower heights of up to 180m can be readily found for the project that needs this sort of lift height. A range of hoist and trolley winches are offered by the crane manufacturers, providing the crane user with a wide choice of hook speeds and pull combinations. Rail-mounted bases are often available for this type of crane.

Cranes of this type are excellent in the role of rapid light weight material handling and when correctly sited and driven will provide comprehensive and efficient lifting services for a large area of a site. Though not usually of very large maximum lifting capacity, average construction industry capacities being between 5000kg and 10000kg, the horizontal jib tower crane can lift its maximum capacity out to a radius in excess of 25% of its total, and at maximum radius a healthy capacity still remains.

Lifting jib tower crane assisted erect

This configuration of crane tends to be used where special circumstances exist that would restrict the use of a horizontal jib tower crane. The main advantage of a luffing jib is that both jib and suspended load can be kept from over areas where there intrusion is not wanted.

As far as its erection is concerned it follows the same lines as the assisted erect horizontal jib tower crane discussed previously, each erection being followed by a thorough examination and test.

Jib lengths, tower heights and winch types, as with horizontal jib cranes, are available for selection by the user, so as to be able to customise the crane to the lifting environment and requirements. This configuration is not as quick as the horizontal jib crane, particularly when the lifting operation requires frequent radius alterations, the luffing winch having to move the weight of both the load and the jib; also alteration of the load radius is accompanied by a change in load height which must be compensated for by the operator if level luffing is required. Some winch units can do this automatically but of course they are more expensive.

Unlike the horizontal jib crane the capacity of a luffing jib drops immediately radius is increased, therefore confining maximum capacity to minimum radius.

The asset with this type of tower crane is that it can travel over any length of straight or curved track to cover greater areas:

Its limitations however are quite severe:

It can travel freely only when its working height is within certain limits specified by the manufacturer. (Maximum free standing height.)

In addition the rail track must be kept clear at all times, and this requires a certain amount of responsible supervision.

The area occupied by the rail track cannot be utilised as a site storage area, and on the some sites this alone can be a serious handicap.

This is comparatively expensive form of crane mounting.

This type of Tower Crane is generally self erecting, and usually also generates its own power.

The crew of two men, working as a team, are usually competent in both crane operating and banksman (slinger) duties.

This crane offers greater mobility, i.e. it can be easily transported from one site to another when folded, but when it has been erected to its full operating height this extreme mobility is lost, it can be moved from one part of the site to another only after careful levelling of the ground.

Its working height is restricted to within limits specified by the manufacturer.

This is similar in principle to the lorry mounted tower crane, but with less restriction to working height and freedom of movement.

The crawler mounted tower crane usually has some ability to travel unloaded around the site. Great care should be taken to provide a suitable surface to travel on in terms of support and level.

Transportation by low loader is required for moving from site to site.

Static-mounted

This is the most popular type in general use.

Its lack of mobility is offset by an almost limitless working weight.

When the crane exceeds its free-standing height it must be secured to the building at regular intervals by ties, in accordance with the manufacturers instructions and specifications. (Tied in.)

The cost o f the ties and the time involved in their installation would have to be taken into consideration during the planning stage.

A static tower crane usually creates very few problems regarding erection and dismantling.

This is another popular variation offering an almost limitless working height.

It is usually erected centrally inside the building (lift shaft etc.), and increases its height without the addition of sections to the tower, by climbing from floor to floor as the building increases in height.

Its original base is usually incorporated within the foundations of the building and therefore offers no disposal problems at all.

Dismantling a climbing tower crane can be a slow and costly business, as great care must be taken to prevent damage to the finished building when the sections are lowered to the ground. Only very experienced crane erectors can be entrusted with this task.

A most critical aspect of safety in tower crane operation is stability which can be defined as ‘being firmly fixed or not easily moved’.

The stability of a tower crane depends on:

1. Ground conditions
2. Track condition
3. Central ballast
4. Counter ballast
5. Height of crane
6. Length of jib
7. Weather conditions
8. Operator competence
9. Correct handling

All tower cranes have a ‘free standing height’, this is the maximum height to which the tower of the crane can be raised with the crane in a mobile condition i.e. not fixed to the building etc.

A tower which is raised above the cranes free-standing height must be tied in to the building at regular intervals laid down by the designer.

The track of a rail mounted tower crane must be checked at regular intervals for condition and level by a competent person.

As with most other cranes, the tower crane’s radius is a horizontal measurement between the centre of slew and a vertical line under the hook. An increase in radius will lead to a decrease in load capacity and vice versa. The only exception to this within the constant load area of a horizontal jib tower crane, where radius can be increased without capacity loss.

The tower of a tower crane is stabilised by the central ballast which is positioned at the base of the tower and has the effect of lowering the centre of gravity.

As the height of the tower increases, the amount of central ballast must be increased in accordance with the manufacturers instructions, to ensure that the crane is always stable at its working height.

This principle does not apply to cranes which are mounted on a fixed concrete base (disposable base) or tied in to the building.

The stability of the working of a tower crane, is achieved by counterbalance or moment. The counterbalance force is generated by the positioning of weights (counterweights) at the end of the counter jib, which is usually situated opposite the working jib.

The amount of counterweight used will depend upon the length of the working jib. The longer the working jib the greater the counterweight employed. The crane manufacturer will provide a table of jib lengths linked to the required amounts of counterweight.

Some tower crane configurations do not have a counter jib, a self erect for example. In the interests of tower stability the designer will keep as much weight as possible as low as possible, and to this end counterweights are often positioned at the cranes base, lnked to the working jib via a wire rope system.

During the tower crane erection and dismantling procedures, it is usual to reduce or reposition the counterweights to maintain overall crane balance.

The weights and positions of the central and counter ballast must be in accordance with manufacturers instructions, and a ballasting chart must be displayed on the outside of the mast to indicate the correct amounts and disposition.

Load Rating Charts

The tower of a tower crane must be fitted at a suitable (readable) height with a load rating chart indicating briefly the load/radius capacities for that crane.

Tower cranes with horizontal jibs can be fitted with load radius flags to indicate the operating radius and lifting capacity at that particular radius.

Tower cranes with luffing (derricking) jibs may have a scale fitted inside the operators cabin. This scale moves with the variations in the jib angle, indicating the radius and lifting capacity at any time.

With modern tower cranes, load radius information is presented to the operator via a digital readout as part of the electronic safe load indicator system.

Rated Capacity Indicators

LOLER

ACOP. 7. Para 190:

Where there is a significant hazard arising from the use of the machinery it should be provided with appropriate equipment or devices such as rated capacity indicators and rated capacity limiters.

LOLER

Guidance 4.Para 122:

Where there is a significant risk of overturning and/or overloading arising from the use of the equipment it should be provided where appropriate with equipment or devices such as rated capacity indicators and rated capacity limiters. Such devices provide audible and/or visual warning when the safe lifting limits are being approached.

Note: ‘Rated capacity indicator’ were previously known as ‘automatic safe load indicators’ (ASLI’s), and/or ‘moment load indicators’.

The RCI is a safety device, which during the operation of the crane automatically monitors the weight of the load being lifted and the radius at which the lift is being carried out.. If at any time during a lift operation the loading on the crane approaches its maximum safe working load the RCI will give a warning to the driver who can take corrective action. A second warning will take place if the crane is overloaded, then if the overloading continues to increase the RCI will apply a third stage which will cut out critical motions, which are hoist up and trolley out or hoist up and luff down on a luffing jib.

The chart overleaf indicates the sequence and tye of warnings given by the old and new type of RCIs fitted to tower cranes:

The crane driver should remain alert for any warnings given by the RCI, and should be prepared to take appropriate action immediately! Failure to do so could result in serious overloading of the crane leading to possible structural failure or loss of stability.

Rated capacity indicators must be regularly inspected and maintained by a competent person, at intervals prescribed by the competent person.

All necessary information is provided in the manuals supplied by the manufacturers of the tower cranes with each particular model.

These manuals will supply information regarding:

1. base requirements and preparation

2. crane erection and dismantling

3. crane specifications

4. crane operational procedures

5. crane maintenance and fault finding

Legal Requirements

A tower crane comes under the same regulations as other cranes. Refer to LOLER 98 and PUWER 98.

Tower Crane Safety

The following is a brief selection of safety considerations, more information on this subject can be found in the relevant BS Codes of Practice (BS 7121).

• A tower crane should only be erected or dismantled and tested by competent persons, i.e. an engineer and erection crew trained in the erection and dismantling of that type of tower crane.

• A tower crane should only be operated by a trained and competent operator who is physically fit, including eyesight and hearing, conversant with the type of crane, and able to cope with the conditions existing on site.
• Where the driver is required to move loads under the control of another person, a suitable slinger/signaller should be provided who is conversant with the lifting capabilities of the tower crane and able to communicate clearly with the driver via hand or radio signals.

• Any method of lifting other than the recommended vertical lifting of loads should be actively discouraged, as damage may be caused to the crane..

• The tower crane driver must be capable of carrying out a weekly inspection of his crane and should be given sufficient time in which to do this. A report of such an inspection must be made in an appropriate document.

• The tower crane will have a maximum ’in service’ wind speed, usually about 72 kph (45mph), this is a ‘machine’ wind speed and would have to be substantially reduced according to the area and weight of the load being handled by the crane. The crane operator must be given sufficient authority to decide when the crane should be put ‘out of service’ because of high winds and his inability to control the load.

• The tower crane must always be in a position to rotate (slew freely) when it is placed into the ‘out of service’ condition. The slew brake should not be left on under normal conditions.

When the selection of a tower crane is being made against particular environmental considerations, the following are an example of what might have to be taken into account.

1. Overall area to be covered;

2. The height of the building;

3. The required speed of the lift;

4. Weight of critical loads;

5. The type of base or mounting;

6. Existing ground conditions;

7. All proximity hazards;

8. Types of jib;

9. Erection;

10. Dismantling.

The working height of a free-travelling crane is limited (see the manufacturer’s specifications).A free travelling crane can move along its track only at a low speed, and careful planning of the daily work programme may be necessary for obtaining the maximum production.

When two or more tower cranes are employed on a site, each should be erected to a different working height to prevent the possibility of collision between the jibs. Although the jibs may ‘over slew’, the load may still contact the lower crane. Rail tracks should be placed strategically to prevent the jib of one crane hitting the tower of another.

Smaller tower cranes with lighter lifting capacity are usually more suitable on projects where traditional methods of building in brick, concrete etc. are employed. In these situations the lorry or crawler mounted tower crane may be the right one for the job.

The bigger cranes, with their greater lifting capacities, are more suitable for projects where constant off-loading and placing of heavy prefabricated units occur. Here, speed may have to be sacrificed in the interests of precision lowering and placing.

On some projects it may be more economical to use a rail-mounted crane. On others a static or climbing crane may be the answer.

On some sites, the proximity of high obstructions will prevent the use of an horizon-jib tower crane, and a luffing (erricking) jib tower crane will have to be employed. The luffing-jibbed tower cranes have all three base variations; free-travelling, static and climbing