When Should You Consider a Silo Structural Assessment?

Silos are commonly used structures for bulk solids storage (see Photo 1) in many different industries. While silos are an important part of industrial operations, they are also the reason for many accidents in industrial facilities. Among various hazards in these facilities, silo failures are one of the most common. Hundreds of industrial and farm silos, bins and hoppers experience some degree of failure each year.

Photo 1: Common silo geometry, skirt and column supported

The root cause of silo failures may be due to one of the following or a combination of them:

Like other important pieces of equipment, proactive silo inspection and proper maintenance is required on a regular basis to limit the detrimental effects of deterioration. Regular observations of the silo structures are the best way to detect problems at early stages by the plant personnel. Inspections by engineers specialized in silo structures are also required as a trained eye can spot signs of distress that the simple observer may not be able to capture. While proactive silo inspection can help to detect distress and track deterioration, it cannot determine the level of deterioration and/or strength capacity of the structure. Therefore, a silo structural assessment is required at some point during the life of the silo whenever its structural behavior/condition or loading effects vary from the original design.

Photo 2: Silo before unexpected failure

Photo 3: Silo after unexpected failure

A structural assessment is a procedure utilized to check the adequacy, structural integrity and soundness of structures and their components under the current loading conditions or expected ones. The following paragraphs describe circumstances when a structural assessment is required to ensure a safe operation and usage of a silo.

When Signs of Distress in a Silo and/or Component Are Observed

Generally, signs of distress in a metal silo include deformations (see Photo 4), distortions, cracks, rust, holes among others. Whenever an obvious deformation in metal silo shells or a large crack and/or hole is observed during routine observation or inspection, it may be the time to consider a structural assessment to determine if the structural integrity of a silo has been compromised. Because a metal silo structure is sensitive to surface imperfections, the margin of safety will be reduced once deformations appear on the shell. The overall or local strength of the silo structure in this condition may be questionable to take the operating load combinations of stored material and environmental loads.

If your silo is constructed out of reinforced concrete, large cracks wider than common hairline cracks are normally the first sign of distress in a concrete silo. Large cracks allow moisture and ambient chemicals to penetrate at the interior of the concrete shell and reach the rebar reinforcement. If this occurs, the rebar and concrete can deteriorate causing their structural capacity and bonding being reduced or, in extreme cases, lost. Therefore, the safety margin of the silo structure is reduced and failures with larger consequences may occur. 

Photo 4: Deformation in silo

When Stored Material Needs to Be Changed

During the life span of existing storage silos, there are some instances when the plant operations require changing the stored material due to either processing or supply requirements. For example, the material being stored may be purchased from several sources depending on availability and economics or the original source could have stopped supplying the material. This may be the perfect time to consider a structural assessment to determine the consequences of storing a different material in a silo structure designed to store a different material.

     
In the original silo design, the product being stored was predefined. The properties of that product, such as the bulk density, range of moisture content, particle size distribution, etc. have been fixed or their variability already taken into consideration as part of the design basis. Although many bulk solids are described by a generic name, such as coal, this can cover an extremely wide range of chemical and physical characteristics. Changes in those characteristics can significantly alter the bulk density and the friction between the stored material and the silo wall. In addition, variations in moisture content (particularly surface moisture) and particle size distribution strongly affect wall friction as well as having an influence in bulk density variations. As a consequence, the solids-induced loads on the silo wall may vary significantly from the original design. If the loads exerted on the silo structure are larger than those calculated for the original design, the strength capacity of the silo structure may not be able to resist the new imposed loads.

     
In a similar way if the new material is combustible, dust combustion or explosion may be another potential risk which may have not been considered in the original design.

When Changes in the Silo Configuration Are Required

Due to process changes during the silo life span, the user may need to implement changes which will impact the silo configuration. Therefore, the silo or some of its structural components need to be modified to accommodate the new configuration and a structural assessment is required to evaluate the consequences of such modifications.

     
How bulk solids behave during storage and discharge in a silo, defines the kind of loading will be imparted on the walls of the structure. Bulk solids have unique flow behavior that transfers different loads to the silo structure compared to liquids and gases. Since bulk solids exert shear stresses on the silo walls in their static and dynamic conditions, this changes the pressure profile along the height of the cylindrical section to a nonlinear pattern. Concentric discharge from a silo usually results in symmetric pressures along the circumference of the silo, while eccentric discharge results in asymmetric pressures developing on the silo walls. For example, changes of inlet or outlet location (or addition) or discharge equipment in a silo may change the material flow pattern from a concentric one to an eccentric one; therefore, having an impact on the bulk-solids induced loads. It is unlikely the silo structure has been designed to take into considerations these changes and their consequences.

When a Different Usage of the Silo Is Considered

Some silo owners may consider the silo structure for different or additional usage compared to its original one. When a silo is considered for a different usage, it is important to understand that different loading scenarios that this new use or additional use will be imposed on the silo. A structural assessment of the silo structure can be a great tool to simulate those loading scenarios to understand the weaknesses of the structure and how it can be modified and/or reinforced.

     
One example of additional usage is as a tornado shelter for the entire silo and support system. A tornado shelter should be designed to withstand high winds and flying debris in order to protect individuals from extreme weather conditions. Unless specified in the original purchase order and design documentation, silos and their support structure are not normally designed to resist such high wind speeds and flying debris.

     
nother example is when silos are utilized as living spaces or public places such as museums. While the loading imposed by the stored bulk materials will be eliminated, there are several other aspects that need to be taken into consideration. For instance, adding equipment for utilities that are not part of regular silo operations but crucial for human spaces. Addition of floors, access openings, windows, ducts for utilities, etc., all play a role on the modifications to the silo structural system that needs to be assessed to come up with a safe design for human utilization.

When the Silo Life Span Is Reached

Like any structure, a silo deteriorates over its life span. A silo is usually designed for 20-30 years unless a different requirement is specified in the documentation of the original purchase order. Silos can usually have a long and useful life if they are maintained, used adequately and inspected on a regular basis as well as problems (if any) are addressed. Many silos around the world have been used for over 40 years and proper inspection and/or maintenance of many of them has been overlooked, postponed, or performed shoddily. A structural assessment can provide crucial information to determine if an aging silo structure can continue in operation, needs maintenance or reinforcement or needs to be taken out of service.

     
Over the useful life of a silo structure storing wet and abrasive material, the silo wall may experience corrosion and erosion (i.e., abrasive wear). The combination of these two phenomena drastically reduces the wall thickness and, therefore, its strength at a fast pace. This is particularly true of a metal silo shell, since a silo shell’s critical buckling strength is a function of its radius-to-thickness ratio. Therefore, the thinner silo wall will have a lower compressive strength capacity compared to its original designed thickness. If extended usage of the silo structure is considered, a proper structural assessment should be conducted to ensure its continued safe operation.

Components of Silo Assessment

In general, a proper silo structural assessment includes the following parts:

Photo 5: Silo inspection

Conclusion

Due to its unique structural behavior and solids-induced load effects, the structural assessment of silo structures requires specialized knowledge. This includes not only understanding the structural analysis and design of these types of structures, but also bulk solids flow behavior, their loading patterns as well as their operational constraints.

     
Safety must always be the priority in all industrial facilities. Besides routine inspection and regular maintenance of silo structures and their mechanical equipment, it is the responsibility of the user/owner to conduct timely structural assessments when the circumstances above are present to ensure the structural integrity of the silos and avoid regrettable structural failures.

     
Jack Wu is a senior project engineer in structural group at Jenike and Johanson Ltd (Tyngsboro, MA). For more information, call 978-649-3300 or visit jenike.com.