Silo Design Best Practices 2: Exploring Structural Challenges

Based on structural inspections of more than a dozen homogenizing and blending silos at various cement production facilities across the United States and Canada, many blending and homogenizing silos are structurally deficient. This is sometimes a result of structural engineer’s not keeping pace with current technology trends or not understanding the principles of material pressures in these silos. 

Depending on several factors, the deterioration of a blending or homogenizing silo will manifest itself in a similar sequence. First, the silo wall will begin to show vertical cracks on the outside. These cracks will also be present on the interior face of the wall, but this can go unnoticed as the interior of the silo is not entered or observed as often as the outside face of the wall. In time, the vertical cracks will grow more numerous. Near the top of the elevated floor of the storage space, horizontal cracks will begin to appear as well. The edges of the cracks will begin to wear, with the edges of the cracks appearing raveled and broken. Many times, the material that is stored and blended will begin to leak on the underside of the elevated floor. Depending on construction, concrete will begin to spall at the top of the silo wall, leaving vertical reinforcement and/or jack rods exposed. As the deterioration progresses, the silo wall will delaminate. In final stages, the silo wall can catastrophically fail due to a loss of strength. 

The genesis of the cause of the issues is related to the design assumptions. Even modern codes, such as Eurocode, do not fully provide a design methodology for these silos. But, taking a step back in history, many homogenizing silos were constructed in the 1970s. At this time, it was expected that the governing design case of a silo would be a full chamber of material fully fluidized. 

This is different than typical silo design, where the weight of material is distributed, using one of several formulas (typically Janssen’s) to calculate what percentage of the load is resisted by friction load (visualized by some of the material “hanging” on the wall) and the remainder being supported by the elevated floor below. These calculations are approximations in themselves, with accuracy depending on storage space characteristics as well as material properties. Instead, a fully fluidized cylinder of material in a silo will have no friction component. Additionally, this fully fluidized material will, theoretically, have no internal cohesion or friction, and the lateral load will be much higher than that calculated. 

Certainly, a fully fluidized chamber of material will produce higher outward forces on a silo wall than a material that is not fully fluidized. This was the standard for design of a generation of homogenizing and blending silos. This would result in needing a larger amount of steel reinforcement due to tension in the silo wall than if designed using static design principles, even with code prescribed design procedures. However, this is not what typically happens inside a blending or homogenizing silo. Instead, the process of homogenizing or blending of material creates a different scenario, and one that has left many of these silos under-designed. 

While homogenizing silos vary in configuration, two arrangements are most common. In both cases, pressurized air, typically at least 25 pounds per square inch (psi), is filtered into the silos through the use of air pads in the bottom of the silo. These air pads, which are typically steel troughs covered with industrial cloth that allows air to pass through but not the material, fluidizes the material in the silo. 

In one of the two homogenizing silo arrangements, the bottom of the silo is split into four even segments. Each quarter of the silo is pressurized in sequence. When one of the four segments is pressurized and fluidized, the material in that quarter expands, and will overflow onto the other three segments. The process continues, sometimes, with additional material being added, until material is withdrawn either from an opening in the bottom or it overflows into an adjacent silo via an opening in the wall that connects to the adjacent silo. 

When this process occurs, the pressures inside this segment are much, much greater than those in the remaining three. This difference in pressures results in a flexural component in the wall, whose effects tend to be much greater than those for a fluidized cylinder of material. As a result, the deterioration discussed above begins. 

In the other homogenizing silo layout, compressed air is pumped into the silo through air pads on the elevated floor (similar to above). However, in this case, the air valves separate the storage space into twelve outer components, and at times have a central storage chamber that is separated into four segments. The aeration pressures are much lower, typically 10 to 15 psi, and one twelfth of the outer chamber is aerated for a short duration. 

At the same time, in the center area, the corresponding interior quarter is aerated, and material flows from the outer twelfth towards the center, and material is fed toward the preheater through a trough. The process repeats, with each twelfth being activated for a short (typically 30 seconds or so) period, and the inner quarter being activated for three periods before switching to the next quarter (and correspondingly the fourth, outer, twelfth segment. This can cause asymmetric flow to occur. 

Asymmetric flow is a phenomenon that occurs when material stored in the silo is withdrawn non-concentrically. When material is withdrawn from a silo, a flow channel develops. The material in this flow channel moves, while the remaining material remains static. The characteristics of this flow channel depends on many factors including the material, hopper slopes, finishes, and more. 

The pressures of the flowing material are markedly less than those in the static material. If the flow channel comes in contact with the wall, this difference in pressure will cause flexure, or bending, in the silo wall. The flexure typically will cause stresses in the silo wall that exceed those caused by uniform material pressure. These increases can overstress the silo wall. 

Similar to the above arrangement, a blending silo with an inverted cone has an elevated floor or trough around the perimeter, and material is directed to openings around the perimeter of the silo at the inverted cone. This inverted cone has the narrow end at the top, and the wider end at the bottom, bearing either on an elevated floor or a ledge on the silo wall. 

Material is deposited into the silo storage space, then low pressure air (approx. 5 psi) is injected into air pads and material flows through the openings in the cone. This process is repeated, either sequentially or randomly. In this arrangement, the material flowing against the inside face of the silo wall creates the asymmetric flow channel that results in the bending described above. The result, often, is overstress of the silo wall. 

Additional possible effects on a silo of this kind include: 

• Issues related to roof support beam bearings. A silo roof is typically a reinforced concrete slab supported by the perimeter wall as well as discrete structural steel beams that are supported by the silo wall. Due to the bending that occurs as discussed above, this can cause relative movement between the silo wall and roof slab. At the roof support beam bearing areas, a concentration of forces occurs, and the silo wall below typically is not capable of resisting these forces. As a result, the concrete wall fails in shear, and portions of the wall spall. This loss of bearing can compromise the roof support beam bearing, leading to the beam losing contact with the roof slab. Without proper support below, the concrete roof slab can fail. 

• Depending on how the elevated floor of a blending and homogenizing silo is supported, the bending that overstresses the silo wall can cause the silo wall or integral pilasters that support the elevated floor to spall where the floor bears. This spalling can compromise the support of the elevated floor. 

This blog is part of a three part series. Read part one, Silo Design Best Practices 1: Homogenizing and Blending Silos, and part three, Silo Design Best Practices 3: Repair Options.

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