SafeWork NSW recently issued a safety alert following an incident in which the top section of a tower crane (the jib and machine/counterweight deck) detached from the slew mount and fell to the worksite below.
There was no-one on-site when the incident occurred.
The design of the crane has the top section bolted onto the slew mount by eight vertical bolts, two in each corner of the slew mount.
The bolts are pre-loaded during installation, meaning they’re in tension when both in-service and out-of-service.
Investigations are continuing, but initial indications are that one or more of the bolts at the front/jib side were weakened by fatigue cracks, which began within the threads of the bolts.
Once these failed, the load transferred to the remaining bolts, which failed due to overload.
While the reason for this incident is yet to be determined, things that can cause cracks and propagation in bolts loaded in tension include:
As such, the safety alert recommended a number of actions to reduce the risk of such incidents:
The crane involved was a Terex Comedil CTT561, and the safety alert said owners/operators of CTT561/561A cranes should contact Terex for the correct inspection, service and maintenance frequency for their particular model.