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CROSS Safety Report

Tower crane - failure of loadbearing part

Report ID: 399 Published: 1 October 2014 Region: CROSS-UK

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Overview

A tower crane suffered a failure of one of its four main legs approximately 12m below the slew ring.

Key Learning Outcomes

For the construction team:

Full Report

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A tower crane suffered a failure of one of its four main legs approximately 12m below the slew ring. The fault was discovered, says the reporter, as a result of the tower crane operator noting some unusual banging and vibrations from what he believed to be the slewing ring (Figure 1). The crane was immediately taken out of service and an inspection carried out.

Image
Figure 1: location of the damaged mast section

This revealed that there was no fault with the slew ring but identified a failure in the leg (Figure 2). The crane was dismantled, the part replaced, and a new Certificate of Thorough Examination was issued by an independent company. At the time of reporting, it was not known what caused this failure and a full investigation is underway.

Image
Figure 2: failure in leg

This incident has been reported to the Health and Safety Executive (HSE) as a dangerous occurrence. The contractor issued an instruction to all sites to immediately carry out appropriate inspections of such cranes in consultation with the tower crane provider/operating company.

In an update the reporter has pointed out the recommendations from the Construction Plant-hire Association (CPA)Tower Crane Interest Group TIN042 Selection of Tower Cranes - Anticipated Utilization.  

These include the following ‘high intensity operations’ that are likely to increase fatigue in tower cranes:

  • Usage in excess of 10 hours per day

  • Planned lifts in excess of 6 lifts per hour

  • Risk of shock loading (emptying muck skips for instance)

  • Use largely within a restricted arc of operation

  • Frequent use above 70% SWL

  • Short mast (20m or less)

  • Short jib (30m or less)

  • Permanently sited tower cranes in ship repair facilities, factories and plant yards

  • Use of radio controls with consequent loss of operator feedback

Expert Panel Comments

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The investigation is not yet complete but an obvious candidate for a failure of this kind is fatigue cracking. There are two phases of fatigue damage: crack initiation and crack propagation. Both phases are linked to repeated application of stress. Fatigue cracking is a potential mode of failure on any moving structure, but its initiation and rate of propagation are rather uncertain.

Hence a fundamental plank of safety is the ability to inspect with the objective of detecting a crack before it progresses far enough to be dangerous. This is a really good example of a near miss from which we can all learn.  Any piece of steelwork which is subjected to repeated and/or cyclic loading should be inspected for fatigue loading. 

The incident supports the need and obligation for regular inspections but also to consider whether the usage of a crane in some circumstances might warrant more frequent inspection? For example, lighter loads but very frequent and repetitive use (creating reversals of stress) may be more harmful than occasional heavy loads.

The recommendations in CPA TIN042 are to be welcomed. There are other structures which are composed of a kit of parts each of which may have had a different history of use; for example, temporary stages or even scaffolding. The history/residual fatigue life of the individual components will probably not be known. Is there a need for more rigorous systems for detecting fatigue in construction equipment components? It is taken very seriously in the aircraft industry.

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