CROSS Safety Report
Lifting an unbalanced load
This report is over 2 years old
Overview
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:
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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
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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
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The Lifting Equipment Engineers Association (LEEA) website contains useful information on this topic
Full Report
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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.
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Feedback
This report suggests that all lifts where the suspension point is below the centre of gravity are inherently unstable and should not be used. This is not correct. The diagram does not even show an unstable lift unless the combination of lateral load and lifted weight are way outside what could normally be expected. My main concern is that by stating that the point of application of the lift should have been above the centre of gravity the report infers that no lifting should be carried out where the suspension point is below the centre of gravity of the load. This is not something that I would like to appear in contractors' site procedures manuals as a rule of thumb because it is likely to lead to more problems than it solves. There are clearly a very small number of circumstances where the geometry of the lifting points can lead to instability but, as with fork lift truck lifting, I think that it is probably more important to consider the base to height ratio and the weight to horizontal load ratio and resulting moments than focussing on the height of the centre of gravity relative to the suspension point. These issues have clearly been considered on these lifts which appear to have been successful: My opinion is that the majority of lifts are self stabilising rather than just some and that centre of gravity vs lifting point level should not be given precedence when looking at a lift.
Expert Panel Comments
Expert Panels comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-UK Expert Panels page.
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.