CROSS Safety Report
Sudden failure of storage silo
A 5,000 ton capacity storage silo containing ground granulated blast-furnace slag (ggbs) split apart suddenly and without warning onsite at a concrete production facility.
Key Learning Outcomes
- Ensure designers and contractors are suitably experienced and competent
- Include a third party design check in the procurement process
- Arrange for regular inspections when in use
For structural engineers:
- Loads in silos can be uncertain
- Incorporate robustness into the structure to help resist unknown loads
- Thin-walled shell structures are complex and warrant an independent design check
- Regular inspections of such structures are recommended by HSE
For the construction team:
- Ensure that bolts and other safety-critical components are sourced from reliable suppliers and come with authenticated documentation
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The lower cone-shaped hopper section of a 5,000 ton capacity steel storage silo, some 30m tall, containing ground granulated blast-furnace slag (ggbs) for use as partial cement replacement in concrete, split apart suddenly and without warning onsite at a concrete production facility. The silo was almost full at the time and a large quantity of the fine powder spilt onto the surrounding area forming a dust cloud in the process.
The knock-on vacuum effect created from the rapidly falling material triggered buckling and rupture of the cylindrical shell as well as an inward collapse of the conical roof and some deformation of the steel support structure. The concrete batching plants were not in operation at the time of the incident and fortunately, nobody was injured. There were no harmful effects experienced from the dust emissions, the silo was dismantled and the fallen material safely removed.
Investigation highlights design issues with bolted joints
A detailed investigation on behalf of the owner was carried out which highlighted serious concerns over the strength of the bolted joints within the silo hopper section. With the silo in a near full condition, the bolts in the meridional joints near the top of the hopper would have likely been on the verge of an ultimate limit stress failure proceeding to a catastrophic failure of the silo.
The investigation concluded that the silo failure happened by mechanical overload, likely of a meridional bolted joint in the hopper section. The primary cause of the overload was considered to be directly related to inadequate design, mainly in relation to the bolted joints in this area.
A number of contributing factors were also recognised including:
underestimation of design loads/design error
underestimation of the impacts of prying forces on the joints
unknown impacts of cyclic straining due to thermal loads
lack of robustness to deal with potential unknown loading, and
lower than expected tensile strength in some of the bolts tested
Value of design review
Large silos of this size are special and uncommon structures. The reporter believes that an independent design review and Category 3 check should be considered for such structures. They say that silos of this magnitude should also have additional robustness built-in as loads are not well known. The meridional joints in the hopper section were aligned in this case and staggering the joints will provide increased robustness. They also suggest that independent testing of the structural bolts to verify the mechanical properties should be considered.
Expert Panel Comments
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Expert Panels comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-UK Expert Panels page.
CROSS welcomes the reporter's, and the owner's, public spirit in sharing information on this major failure so that others may learn.
This was a specialist structure containing a large volume of material that collapsed suddenly and was a near miss in terms of possible casualties. A general feature of all silo designs is that the loads are very uncertain being linked to the way materials move and settle during filling and discharge. There is a history of silo failures due to uncertainties in behaviour.
Checks by competent parties
Silos are, it is thought, most often purchased by private clients who may not be experienced in the practices of procurement and know the need for third-party checking of complex structures such as thin-walled shells. Checks not only during design and construction but during normal operations to determine if there are signs of deterioration could be desirable.
Analysis carried out after the event found several contributing factors including possible design shortcomings. The reporter does not give any information about the basis of design but several of the issues raised would be adequately covered by the Eurocodes (especially EN1993-4-1 for silos and EN1993-4-2 for tanks).
Importance of bolt sizing and strength
There is mention in the analysis of bolts in relation to prying forces, which is a design consideration, and lower than expected strength of some bolts. The requisite strength of bolts should be ensured by sourcing these from a reliable supplier. Although there is no suggestion of it here, CROSS is aware of instances where certification accompanying proprietary products has stated compliance with standards or specified requirements, but the products have been found not to be in accordance with the specification.
A degree of conservatism in bolt sizing, and design generally, may be warranted when the magnitude of applied forces is not readily determined. Robustness will improve.
It is known that some of these types of structures were often designed to working stress US codes where there is a lot of empiricism, which is fine as long as the designer does not depart from the standards in any way. Detailed assessments on tank type structures have found that some of the empirically based rules are very sensitive where stability is concerned.
In 2017 HSE published a safety alert: Catastrophic failure of silo – bolted conical bottom section which highlighted the possibility of micro-cracks in silos where the conical bottom section has been cold formed and assembled using bolted joints. This gives useful information for silo owners and designers and recommends the need for regular inspections.
There are sources of design guidance on the web and users, if not familiar with the subject, should take care to ensure that the information they might use comes from a reliable source.
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