Catastrophic failure of silo – bolted conical bottom section

Health and Safety Executive - Safety Alert

Introduction

A serious risk of catastrophic collapse of cold formed bolted silos has been identified, involving separation of the conical bottom section (or conical hopper section or cone section) from the main cylindrical structure. The failure is directly attributable to the design and fabrication method used in forming the cone section.

Background

Design

The silo involved in the incident was approximately 28 m tall and 8 m in diameter and in service for around twenty years before it failed. This type of silo is shown in figure 1. The conical bottom section was constructed from three rows of overlapping 5.3 mm carbon steel (galvanised) plates bolted together. The upper edge of the top plate had tabs folded at 45° and was bolted to the horizontal ring beam at the base of the cylindrical section of the silo, known as the 'hip ring'. A schematic of the cold formed edge against the hip ring beam is shown in figure 2.

Failure

Forming the tabs to fasten the cone to the structure involved folding the plates through 45°, and in this case, the fold had been made against a sharp edge as opposed to a radius. This sharp fold had left an indent in the plate and micro-cracking on the outer edge of the fold (figure 3). These micro cracks propagated over time leading to the formation of significant fatigue cracks and causing an initial leak of grain

Once this stage was reached, the cracks grew quickly leading to the failure of the folded tabs at the top of the plate (figure 4). This led to the failure of the adjacent bolts, due to overloading, and, as a result the cone section detached, releasing the entire contents of the silo. The sudden release of the grain induced a suction effect on the silo's roof, resulting in an implosion of the roof structure and causing further damage.

Action required

The actions required are in line with duties required by health and safety law and in particular regulation 4, regulation 5 and regulation 6 of the Provision and Use of Work Equipment Regulations 1998.

• Users of cold formed bolted silos (of the specific design discussed) should make sure that the cold formed edge (fold) of the conical bottom section is subjected to a thorough inspection.
  • In particular, check for cracks at the fold, including the tabs at the top of the vertical joints.
  • It is important that both surfaces (inside and outside of the fold line) where micro-cracks were detected are inspected.
  • The inspection should be capable of detecting small cracks and, non-destructive techniques such as penetrant testing would be considered necessary.
• Given the location of the fold line beneath the hip ring (see figure 1), it might be required to dismantle the conical section to facilitate a thorough inspection.
• It is also necessary to repeat such inspection at suitable intervals in order to detect future crack growth.
• Leaks around the hip ring and conical hopper joints should be taken as a sign of impending failure, and missing bolts or irregularities noticed on the sidewall are to be considered as early warning signs of possible issues. If such signs of distress are noticed measures to reduce the loading on silo such as emptying the silo should be taken, if it is safe to do so. Attempts to carry out any repairs can be dangerous as the Silo may fail during such work.  Furthermore, the creation of an exclusion zone is also recommended.
• In the event of a catastrophic collapse of a silo and uncontrolled release of its contents appropriate control measures should be put in place to mitigate the remaining risks.  If close approach is required to recover parts and clean up, risks associated with falling structures, risk of asphyxiation, health hazards associated with breathing in dust and the potential for dust explosion need to be addressed.

Relevant legal documents

Provision and Use of Work Equipment Regulations 1998. Approved Code of Practice and guidance. L22 (4th Edition) HSE November 2014. http://www.hse.gov.uk/pubns/books/l22.htm

Further information

Any queries relating to this safety alert should be addressed to:

Health and Safety Executive
Operational Strategy Team
Local Authority and Safety Unit
Redgrave Court
Merton Road
Bootle
Merseyside
L20 7HS

General note

• Please pass this information to a colleague who may have this product/equipment or operate this type of system/process.
• The designers, manufacturers, and suppliers of silo equipment are reminded of their duties set out by Section 6 of the Health and Safety at Work etc., Act 1974.