A large iron ore screen bin had been assembled in China and shipped to a construction site in the Pilbara – Western Australia.
The bin was one of several bins that were craned into position onto a steel frame structure and bolted in on the horizontal and vertical axes.
The client did an engineering check on the installation and found that the top flange of the supporting structure which was bolted to the underside of the bin had been distorted as no packers had been used to ensure that the bin sat evenly on the beam prior to being bolted and torqued up.
It was evident that the riggers doing the installation always torqued up bolts to the allowable setting of the rattle gun and were not concerned about any structural distortion which may have resulted.
A client directive was issued to the contractor to remove the flange distortion and over stressing by inserting suitable thickness packers between the bin bottom flange (over 12 metres long) and the supporting beam flange.
Work commenced with riggers removing the over 50 bolts from the joint.
The unbolting commenced from one end of the support beam.
Now pause here and reflect on this methodology!!!
It became increasing difficult to undo the nuts – using an air powered rattle gun.
Why would that be???
The riggers were used to applying brute force to erect structures and applied the same approach to removing the bolts.
They had removed about three quarters of the bolts when there was an explosive noise and the remaining insitu bolts all suffered tensile failure at the same time – no one was hurt. Lucky!!
The riggers had no understanding of the stresses contained in a structure and how to safely unbolt a major component which had been bolted and constrained, both in the horizontal and vertical planes.
This also applied to the responsible Construction Engineer who did not provide any advice to the riggers on an unbolting methodology – He had not done any form of hazard analysis and risk assessment.
There are several lessons to be learnt from this incident – but I will leave it to the readers of this article to figure it out!!
I (Craig Power) was the incident investigator for this event – It was an enormous help to have a mechanical engineering and an extensive OHS background to be able to get to the heart of the matter quickly and ask the right questions.