On 1 December 2019, 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 this 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:
- threads being in the load path
- shock load or overload
- excessive shear or torque loading – eg from side loads on the jib
- excessive cyclic loads – eg in-service and wind loads
- an incorrect bolt – eg incorrect size, grade
- incorrect installation – eg over or under torqueing
- wear, leading to excessive clearances
- operating beyond normal service life
- inadequate connection design.
To prevent these things happening in tower cranes:
- undertake pre-erection and commission inspections to identify and repair/replace damaged components, including replacing or doing non-destructive testing (NDT) of critical bolts at each installation – some manufacturers recommend replacing certain components, such as specific bolts, after a certain number of installations (rather than NDT)
- install bolted connections to the correct torque
- do regular in-service inspections, at intervals and covering the items specified by the manufacturer
- don't add signs, lighting or equipment that may increase loads on the crane structure, unless assessed and approved by the manufacturer or a competent person
- free-slew (weathervane) the crane when out-of-service, unless specifically designed to be parked in fixed position
- operate within the cranes rated capacity and wind limits
- design connections that are less susceptible to fatigue – ie free from stress concentrations in the load path; only subjected to stresses below the endurance limit for the material.
The crane involved was a Terex Comedil CTT561. This model was produced from 1998 to 2005,then it was upgraded to a CTT561A model, which uses a horizontal pin connection instead of the vertical bolt arrangement, a substantially different connection. Owners/operators of CTT561/561A cranes should contact Terex for the correct inspection, service and maintenance frequency for their particular model.
For more information, see tower crane safety.