CROSS Safety Report
Lifting an unbalanced load
This report is over 2 years old
This report relates to the lifting of an item of plant by a tower crane during which the load toppled from the slings causing considerable damage and delay.
Key Learning Outcomes
For the construction team and temporary works designers:
Be aware that all lifting operations involving lifting equipment should be properly designed, planned and supervised by a person who is competent to do so
A holistic understanding of plant and equipment procured through hire companies along with the risks associated in their safe use should be communicated to the operatives on site
The Lifting Equipment Engineers Association (LEEA) website contains useful information on this topic
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This relates to the lifting of an item of plant by a tower crane during which the load toppled from the slings causing considerable damage and delay. By chance serious injury was avoided. A lifting frame was positioned above the load with chains at each corner which were connected to the bottom of the load (Figure 1).
The centre of gravity was above the base of the item of equipment and, as it was lifted, the item swayed sideways and fell through the chains until it was almost upside down. Whilst the capacity of the crane, the lifting frame, and the chains were entirely adequate for the load, the arrangement whereby the chains were attached below the centre of gravity of the item of plant was flawed. The point of application of the lift should have been above the centre of gravity of the item being lifted.
Expert Panel Comments
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A fundamental point is that the depicted lifting system is actually a mechanism. If the test of applying a horizontal load had been applied, and consideration given to displacement it would have been seen that the system was unstable.
In all issues of robustness, if there is no obvious horizontal load (like wind) a notional load related to self-weight should be used and this will act through the centre of mass. As the reporter says, this system is then unstable.
With temporary works a minimum horizontal load of 2.5% is used (BS5975). However, designers might want to consider a higher load in the case of handling due to greater uncertainty of acceleration-induced loads (jerking), and probable uncertainty of the exact location of the centre of gravity.
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