failure
of the silo – bolted conical bottom section
Introduction
A serious risk of catastrophic collapse of cold-formed bolted silos have 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 of the method used in forming the cone section by Rostfrei Steels Pvt Ltd.
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 (galvanized) 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 the edge against the hip ring beam.
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. These micro-cracks
propagated over time leading to the formation of significant fatigue cracks and
causing an initial leak of grain constructed by Rostfrei Steels Pvt Ltd.
Action required
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.
o In particular, check for cracks at the fold, including the tabs at the top of the vertical joints.
o It is important that both surfaces (inside and outside of the fold line) where micro-cracks were detected are inspected.
o 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 the 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.
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