Load Out Bin – Base Plate and Plinth Design
- A 300 tonne load out bin was being erected, starting with the pouring of 6 concrete plinths to support the bin steel columns.
- Upon completion of the plinths, the bin steel work was erected starting with the support columns loosely bolted to the concrete plinths.
- The bin support columns need to be aligned both in the horizontal and vertical planes to ensure all structural elements linking into the columns will align and can be erected and held in place.
- To get the final vertical height of the column base correct – small flat steel packers were inserted between the plinth and the column base plate.
- To ensure the column was approximately vertical, steel wedges were inserted between the column base plate and the plinth and adjusted (hammered in) to get the correct column alignment.
- The load out bin was lifted by crane and fixed into position, leaving a minor task to insert concrete grout between column base plates and each plinth.
- The grouter needed to remove the steel wedges to allow the grout to be inserted between the base plates and the plinth.
- Whilst removing the steel wedges – some were found to be difficult to remove. Several wedges were removed when the entire steel load out bin structure including the support columns fell approximately 50 mm and the concrete plinths suffered a structural failure evidenced by major cracking around the remaining steel wedges.
- At the time an active EWP was located inside the bin structural support beams. No one was injured.
- The technical root cause of the incident was the steel packer bearing area was too small and was placed too close to the open core of the plinth, which caused the plinth concrete to fail once the wedges were removed.
- The steel packers were a standard dimension – primarily used to support conveyor idler frame base plates.
- This was a result of flawed design for the support of the steel base plates onto the concrete plinths.
- The incident could have resulted in much greater damage and injury to nearby employees, again good luck!!
- Design and construction engineers need take into account operational requirements and the construction build sequence, to ensure that the structure can be erected safely in the proper sequence, taking into account the change in stresses as the build progresses.
- This was a failure of Engineering Design and Constructability.
Further & What Else
The cracks in the plinths were filled and glued together with some form of high strength araldite and the load out bin was commissioned.
Hopefully, routine structural inspections have been conducted on this structure, to ensure there are no signs progressive failure after 20 years of bin operation!!