A 300 tonne load out bin was being constructed 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 – based on survey measurements.

The load out bin construction was completed 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 completely 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.

No – one was hurt.

The technical root cause of the incident was that 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 .

Lesson

You must consider the finest detail to ensure that the structure can be erected safely and in the proper sequence, taking into account the change in stresses as the build progresses.

This was a failure at the Engineering Constructability level.

Further & What Else

The cracks in the plinth were filled and glued together with some form of high strength araldite and the load out bin was commissioned.

If I was the owner – I would conduct a detailed inspection of these plinths to ensure there was no progressive failure evident after 15 years of operation!!

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