Structural collapse of bin hopper

Mine type

All mine types

Incident

A product bin hopper fell into the tray of a dump truck while discharging. The truck operator had pushed the lever to open the pneumatic gate and hit the button to start the bin vibrator and the flow of crusher dust when the hopper section separated at the flanged joint and collapsed into the tray.

The bin was one of a bank of six bins below an elevated screen house, each with a rectangular top section measuring approximately 5.3m x 5m x 2.2m attached to structural columns at each corner and tapered bottom section of 3m in height suspended in tension and connected to the top section by 88 x M20 bolts.

Hazard

Suspended structures

Cause

The failure is attributed to inadequate design:

• Flanged connection was of a cantilever design with no stiffening, and combined with incorrectly spaced bolts contributed to high prying tension loads and bending loads causing substantial overloading at the flanges.
• Bolt design was inadequate—the connection bolts used were of grade 4.6 (minimum tensile strength 400MPa – commercial grade). The structural connection was found to require bolts of grade 8.8 (minimum tensile strength 800MPa – high strength grade).
• Bolts were spaced closer towards the middle of the bin than at the edges despite the fact that load concentrations tend to be higher at the edges.

Comments

• Investigation revealed that the flange connection bolts failed. Some bending and cracking of the bolts as well as prying of the flanges and distortion of the straight bin faces were evident. There were no drawings, lay of the structure/building or records of modifications available.
• Hopper plates were not stiffened, consequently inducing flexing and bulging of the plates. Although not a significant contributor to this incident, it could have caused fatigue cracks of the welds at the hopper edge seams.
• There was no formal structural examination / monitoring regime implemented which contributed to the failure to detect an impending catastrophic event.

Recommendations

• Ensure the design or modification of structures is carried out by competent persons with appropriate qualifications.
• Apply quality assurance process during installation and construction (for example correct tensioning of bolted joints in bin structures).
• Implement appropriate condition monitoring / structural examination regimes to detect problems and reduce the risks associated with critical structures.
• Maintain records of design, construction and modifications (including process changes) and other appropriate critical information required for managing risks.
• Use records (including results of previous examinations) to establish maintenance strategy for managing risks.

Authorised by Roger Billingham - Chief Inspector of Mines

Contact: minesafetyandhealth@dnrm.qld.gov.au

Issued by Queensland Department of Natural Resources and Mines