15. Requirements for mobile cranes used in other circumstances
15.1 Vessel-mounted cranes
15.2 Mobile cranes on suspended slabs
15.3 Use of mobile cranes for demolition
15.4 Use of mobile cranes for tree-lopping
15.1 Vessel-mounted cranes
15.1.1 Characteristics of vessel-mounted cranes
15.1.2 Testing before use
15.1.1 Characteristics of vessel-mounted cranes
Vessel-mounted cranes include cranes that may be operated on a barge, pontoon or vessel. The combined mass is to be considered as a vessel-mounted crane. The vessel should be anchored during crane operation. If this is not possible, then the vessel should be anchored to a craft alongside it.
When positioning the crane on a barge, pontoon or vessel either in loaded or unloaded conditions, the following should be considered:
- the effect on freeboard (i.e. distance between the vessel's deck and the water);
- the strength of the vessel to support the crane structure;
- the installation of stops to prevent the crane driving off the vessel's side; and
- the method of securing the crane when working outside of smooth water limits.
A competent person must determine the list (i.e. deck tilt) and freeboard allowable with the rated capacity and test load conditions of the crane. The competent person must be experienced in crane design and stability of waterborne vessels. The vessel and crane combination is to be certified by a marine surveyor in accordance with Queensland Transport requirements.
As a general guide, the maximum list of the vessel under rated capacity conditions should not exceed ±5°, with one half of the freeboard remaining. Confirmation should be obtained from the crane manufacturer on the de-rating of the crane from land-based ratings when on the barge, pontoon or vessel. Note that generally, the manufacturer's rated capacities for cranes mounted on vessels are frequently not greater than 70% of the land-based rated capacity.
Where the crane can move along the deck of the vessel, lifting should only take place when the crane is secured to the deck (e.g. by chains of adequate strength).
15.1.2 Testing before use
The crane and vessel combination should be inspected after erection and before the application of any loads to ensure:
- all ties, anchorages and ballast are in place and correctly secured;
- the crane configuration is in accordance with the crane manufacturer's or a competent person's specifications; and
- the crane configuration is free from any defects that would preclude the vessel-mounted crane from handling the test load safely.
The crane should undergo testing of its stability, functions and brakes as outlined in AS 2550.1 : Cranes, hoists and winches - Safe use - General requirements (non-Queensland Government link). After these tests have been completed, a competent person must complete a full assessment to ensure the vessel and crane combination has withstood the test loadings without structural damage, and the mechanisms function free of any defect that will affect the safety of the crane.
15.2 Mobile cranes on suspended slabs
Mobile cranes are sometimes lifted onto a suspended floor or other elevated parts of a building for either construction or demolition activities. It must be noted that suspended floors are not originally intended to support mobile plant and there may be a risk of the floor or structure collapsing. This is particularly the case for heavier mobile cranes. Prior to lifting the crane onto the elevated area, written documentation is to be provided that demonstrates the design floor loading of the building is not being exceeded. Where the floor requires strengthening (e.g. by propping), an engineer is to provide written instructions that detail the dimensions, locations and other specifications of the propping to be used.
If the crane can only be used in particular areas of the building, due to inadequate strength in other areas, access to such areas is to be prevented by the use of barricades or other types of barriers.
15.3 Use of mobile cranes for demolition
Not all mobile cranes are sufficiently robust to withstand the stresses of demolition ball work. The use of mobile cranes for demolition ball work should be restricted to cranes designed for arduous duty, such as convertible dragline excavators. Hydraulic boom cranes should not be used for this type of work, as overstressing of the sliding points can occur.
All cranes used for demolition ball work must be fitted with a suitable falling object protective structure (FOPS), to protect the operator from falling objects.
Precautions are to be taken to ensure the hoist rope is prevented from leaving the boom point sheave. This may include fitting heavy duty rope guards to the sheave to control the slack rope condition that may occur as the ball falls. Damage is likely where the demolition ball is attached to the hoist rope. Hoist ropes should not be fixed directly to the demolition ball. A length of chain should be used to join the hoist rope to the demolition ball. The chain should be at least 16 mm, and at least two metres in length.
A crane that has been used for demolition ball work must be thoroughly inspected and verified by a competent person to be in a satisfactory condition before it is used for general lifting. The results of the inspection must be noted in the crane's service logbook.
15.4 Use of mobile cranes for tree-lopping
It is recommended that mobile cranes not be used for tree-lopping. Tree-lopping is the activity of removing or pruning a tree by the systematic removal of the tree limbs, foliage and trunk. During tree-lopping, the tree is connected to the crane hook prior to cutting and the theory is that the crane will be able to assist to lower the tree parts in a controlled manner in designated areas. Cranes have been used for tree-lopping, particularly where parts of the tree are above buildings and other structures. There have been a relatively large number of serious mobile crane incidents associated with this activity.
Tree-lopping with cranes is considered to be of very high risk due to the likelihood of the crane overturning or structural failure of the boom. During the activity, it is difficult to determine both the mass of the part of the tree being cut and the direction in which the timber will fall. In addition, it is extremely difficult to eliminate shock loading that will be applied to the crane. These issues will either cause the load radius to increase or apply a side loading to the boom. Safe crane operation requires that only vertical loads be applied to the hoist rope and the loads be applied gradually. This requirement is generally stated in crane manufacturer's instructions.
Mobile cranes must not be used for tree-lopping unless the following can be ensured:
- the crane operator does not perform the role of a dogger;
- a dogger is present during all lifting activities;
- the arborist, crane operator and dogger consult with each other;
- side loading will not be applied to the crane boom;
- any loading to the crane is well within the crane's safe working load;
- the crane will not be shock loaded;
- wind will not adversely affect the safe use of the crane;
- only vertical loads will be applied to the hoist rope and crane hook; and
- at completion of the saw cut, the radius of the load will not increase.
If the above can be ensured, and there is no other practicable alternative, a mobile crane may be used for this activity in very limited situations. However, the work procedure must be documented and hazard control measures listed in the procedure. The tree arborist and crane operator are to participate in, and be satisfied with, the documented procedure.
Last updated June 9, 2006