Silo Failures Caused by Unrealistic Design Considerations
August 15, 2018
August 15, 2018
Many silo failures have occurred as a result of unrealistic loading conditions assumed during the design phase. Other failures have been attributed to changes in the use of silo structures without proper consideration of the implications. In some cases, more stringent loading conditions were introduced with changes to the stored material properties or the implementation of different discharge mechanisms. The limited existing design guidelines have contributed to oversimplified designs, as they fail to consider complex loading conditions, such as those caused by the asymmetric flow of stored materials.
Silo failures can cause loss of life and significant damage to adjacent structures and equipment when they occur at industrial plants or storage facilities (Figure 1). Silo structures tend to lack redundancy, and their failures can be catastrophic. Contributing factors may include poor design considerations, construction deficiencies, change of use, deterioration and lack of maintenance, and others (Figure 2). The cost of remediation after a catastrophic failure can be significant as it may include forensic investigations, temporary emergency response measures, shoring, demolition of the damaged structures, repair designs, and reconstruction.
During remediation of a catastrophic silo collapse, the permanent repairs may require the reconstruction of a silo as well as the repair or reconstruction of adjacent damaged structures. The repair or replacement of damaged mechanical equipment, electrical instrumentations and control systems, and other components may significantly increase the remediation cost. An insurance claim may also need to cover significant expenses or losses related to the business interruption of an industrial plant or storage facility that may need to remain closed for a significant period after a catastrophic failure of a silo structure. In some cases, litigation may occur after a silo failure, causing a possible surge in the cost of remediation as well as significant delays. In some cases, the litigation occurs after remediation of the structures to curtailed rising business interruption cost.
Figure 1. Partial view of a report by “fin24” of a coal silo and conveyors that collapsed at Eskom Majuba Power Station in Cape Town, South Africa.
Figure 2. Chart of possible consequences resulting from a silo failure
The design, construction, operation, maintenance, and repair of silos requires the involvement of many stakeholders. The obvious stakeholders include the owner, the design engineers, the construction and maintenance contractors, and the operational managers and personnel. Other stakeholders include the insurance companies that provide coverage in the event of damage or failure, the banks or financial institutions that finance the construction or maintenance of projects related to the use of silos at industrial or storage facilities, the owners of neighbouring properties that could be affected by silo failures, and others.
To prevent silo failures, regardless of the age of the structure, the property owner and stakeholders should assess the condition of silo structures and monitor any changes on a scheduled basis (e.g. semi-annually or annually). Qualified personnel should verify if the original design considerations are consistent with current loading conditions, and ensure that the silo structures are capable of supporting the current loading conditions including the possible asymmetric flow of materials. The loading conditions related to the specific industry can be complex, and failures can occur due to failure to perform the proper analysis and testing that goes beyond the limited recommendations provided in the available industry standards and guidelines. A preventive engineering assessment can significantly reduce the risk of failure and avoid the corresponding remediation cost (Figure 3).
Figure 3. Possible Stakeholders involved in preventing a silo failure on a regular basis
Silo structures are used in many industries as an essential component of process plants and storage facilities. For instance, silos are commonly used to store bulk raw and processed materials. The following list includes a few examples among many other applications:
-Grains and other food-related products in the agricultural and food industries;
-Cement, clinker and other materials related to the cement manufacturing industry;
-Materials used for energy generation, such as coal and woodchips; and carbon black used in the production of rubber and other processed materials.
Although storage structures have been used for many centuries, the first modern upright silo was built in 1873 by Fred Hatch in Illinois, USA, for the storage of grain for the agricultural industry. Since that time, use of the upright silo for storage has spread across many industries.
Initially, most silo structures were relatively small and simple regarding the design, materials, and loading conditions. Over time, the need to increase production and store greater amounts of bulk materials has pushed the innovation and design limits to build bigger silos with more complex configurations, loading conditions, and material handling technologies. However, the research and development of codes and standards governing the design of silo structures have not advanced at the same pace, leaving a gap between the challenges imposed by more advanced technologies and the limited existing design guidelines and regulations. Thus, silo failures have occurred because of unrealistic design considerations.
Through most of the 20th century, the traditional design of silo structures assumed a concentric flow of the stored material through discharge mechanisms which, in many cases, included concentric hoppers. Therefore, the structural design was performed by estimating concentric material pressures at specific heights. The assumed lateral pressures were to be resisted by the cylindrical silo walls, mainly through uniform hoop tension. The vertical traction loads were to be resisted through uniform compression of the silo wall in the vertical direction. Since the flow of material was considered concentric, the material pressures were assumed to be uniform along the perimeter of the silo wall cross‑section at any given height. The assumption was that the silo would be subjected mainly to axial stresses. While this is true of small silos with concentric flow of material, increasingly larger and more slender silos were constructed, which necessitated an asymmetric flow of material.
With the increase of storage capacity and industrial applications, more modern and sophisticated discharge technologies utilized in large silos required eccentric discharge outlets, further generating asymmetric flow and eccentric pressures (Figure 4). Problematically, in some jurisdictions, the code provisions allowed for overly simplified designs which failed to take into account the critical loading conditions caused by asymmetric flow. The lack of detailed provisions to account for the effects of eccentric material flow was evident in North America and other parts of the world where the applicable standards, such as the editions of ACI 313 before 2016 (e.g., ACI 313-97, and previous editions) did not provide proper guidance. For instance, ACI 313-97, “Standard Practice for Design and Construction of Concrete Silos and Stacking Tubes for Storing Granular Materials,” provided little guidance to the design engineer to properly estimate eccentric loads.
Figure 4. Schematic plan view of silo cross-sections with concentric (left) and asymmetric (right) flow of materials
Only in the last few decades, more advanced material handling testing and modelling engineering tools, such as the relatively recent development of linear and non-linear finite element analysis software, have allowed for further research and development of more realistic models of silo structures. In the last couple of decades, some standards have already incorporated design provisions to account for more realistic loading conditions, such as those concerning the design of silos that would likely exhibit an eccentric flow of the stored material. Asymmetric flow results from the formation of flow channels inside slender silos or in silos that incorporate eccentric discharge mechanisms, such as eccentric discharge outlets and inverted discharge cones.
For instance, the European standard for the design of silo and tank structures, EN 1991-4:2006, became one of the first and most comprehensive codes, providing detailed guidelines for design engineers to estimate most loading conditions including eccentric pressures (Figure 5). In North America, the latest version of ACI 313, published in 2016, acknowledges the need to consider the effects of asymmetric flow in silos and provides the corresponding guidelines.
Figure 5. View of estimated eccentric horizontal pressures for a silo structure in accordance with EN1991-4:2006
The pressures resulting from an eccentric material flow pattern would adversely change the loading conditions imposed on a silo originally designed and constructed to operate under concentric pressures. An asymmetric flow would impose axial stresses combined with flexural and shear stresses (Figure 6), instead of only axial stresses such as the case of concentric flow. For instance, a silo wall would exhibit hoop tensile plus flexural and shear stresses in the horizontal direction, and compressive plus flexural and shear stresses in the vertical direction. The combined stresses could increase the demand versus capacity ratio of critical structural elements beyond a serviceability limit state and result in partial damage or collapse of a silo structure.
Figure 6. View of a finite element model showing the typical uneven distribution of stresses and deformation as a result of asymmetric flow of the stored material in a silo structure
When an eccentric flow channel is formed within the stored material, and its cross-section strikes the silos wall (Figure 7), the horizontal pressure distribution along the perimeter wall cross-section will vary because of an arch effect of the material around the flow channel. The uneven distribution of pressures will then impose flexural and shear demands to the silo walls, in addition to the axial stresses (e.g., hoop tensile stresses, Figure 6).
Figure 7. Schematic plan view of a flow channel resulting from asymmetric flow inside a silo cross-section.
Bulk materials and silo wall surfaces have unique material properties and behave differently under different material handling scenarios (e.g., at rest, during material discharge, etc.). The storage of advanced materials for new applications, the introduction of new material handling technologies, and the use of new design configurations and silo construction materials have also increased the loading complexities.
For instance, fluidized loading conditions (e.g., quasi-hydrostatic pressures) may develop when gases are added to change the temperature of the stored materials or to prevent or trigger chemical reactions. In some industries, such as cement processing and storage, the loading conditions would also change because of complex discharge mechanisms used to fluidize the material. For example, cement silo aeration systems, silo fluidizing pads, and inverted discharge cones are used to keep the material flowable at the base of cement silos. Fluidizing the material changes the loading conditions, and may cause impact loads as a result of arching and collapsing of the material above and around the fluidized material or flow channels (e.g., “rat hole formation”). Other industries may incorporate different equipment, such as pneumatic vibrators to improve the discharge process. The additional loading conditions including the effects of vibration, impact, and uneven distribution of pressures must be considered in the design of silo structures and their foundations.
In complex scenarios, the available code provisions do not cover all the realistic loading conditions, and further testing and analysis might be required. In addition to the safety risk, a silo failure can cause significant damage to adjacent structures and equipment in a process plant or a storage facility. Preventive engineering assessments and monitoring of silo conditions can greatly reduce the risk of failure and avoid onerous remediation expenses.
The remediation cost for a property owner or insurance company can be significant as it may include the initial remediation and emergency response, business interruption, structural repairs, equipment and material losses, as well as other costs. Therefore, it is important to prevent silo failures by verifying that the original design considers all the realistic loading conditions for the current use of the structure. It is also essential for design engineers to consider all the loading conditions related to the specific industry, stored material and silo wall properties, silo configuration, and discharge systems.
 Eskom knew about silo problem in January – Solidarity. Fin24, 3 Nov. 2017, https://www.fin24.com/Economy/Eskom-knew-about-silo-problem-in-January-Solidarity-20141103
 American Standard ACI 313-97, Standard Practice for Design and Construction of Concrete Silos and Stacking Tubes for Storing Granular Materials, American Concrete Institute, January 1997.
 European Standard EN 1991-4:2006 (E), Eurocode 1 – Actions on structures – Part 4: Silos and tanks, European Committee for Standardization, May 2006.
 American Standard ACI 313-16, Design Specification for Concrete Silos and Stacking Tubes for Storing Granular Materials and Commentary, American Concrete Institute, 2016.
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6. J. W. Carson, Limits of Silo Design Codes, ASCE Practice Periodical on Structural Design and Construction, Vol. 20, Issue 2, May 2015.
7. H. McKay and J. Durack, Implications of the new Eurocode EN1991-4 on the design of Cement and Raw Meal Storage Silos, Cemtech, September 2006.
8. J. W. Carson and R. T. Jenkyn, Load Development and Structural Considerations in Silo Design, Presented at Reliable Flow of Particulate Solids II, Oslo Norway, August 1993.
9. J. Johnson, Deterioration of Concrete Tower Silos, Ontario Ministry of Agriculture, Food and Rural Affairs, December 2008.
10. European Standard EN 1991-4:2006 (E), Eurocode 1 – Actions on structures – Part 4: Silos and tanks, European Committee for Standardization, May 2006.
11. American Standard ACI 313-97, Standard Practice for Design and Construction of Concrete Silos and Stacking Tubes for Storing Granular Materials, American Concrete Institute, January 1997.
12. American Standard ACI 313-16, Design Specification for Concrete Silos and Stacking Tubes for Storing Granular Materials and Commentary, American Concrete Institute, 2016.