Steel silos for storing bulk powders and granular materials: cement, lime, gypsum, wheat and grain, PVC and plastic granules, and animal feed. The design of every silo starts with the material inside it; density, friction with the wall and flow behaviour set the hopper angle and plate thickness.
A liquid tank and a silo are both cylindrical, but their contents behave in fundamentally different ways. A liquid transmits pressure equally in all directions; bulk materials do not. Part of the material's weight is transferred to the wall through friction, and unlike water, the horizontal pressure on the wall does not rise linearly with depth but approaches a constant limit at great depth.
More importantly, the pressure during discharge is not the same as when the silo is full. When material leaves through the hopper, the local pressure rises where the hopper joins the body. Many silo failures, such as buckling and deformation of the body, start in this area, which is why silo design has its own standard.
These figures are the starting point of the design and are replaced by actual values for each material:
A silo is never filled to the brim; at the top the material forms a conical heap at its natural angle of repose. The real capacity is slightly less than the geometric volume times the density.
The most important decision in silo design is how the material will come out. There are two flow patterns:
All the material in the silo moves downwards at the same time, and the material that went in first comes out first. This pattern is achieved with a steep hopper and smooth wall, and suits cohesive materials such as cement and materials that should not stay long.
Only a column of material in the centre moves and the material near the wall stays still. The hopper is shallower and the silo shorter, but cohesive materials may bridge or stay at the sides and go stale. It is acceptable for free-flowing materials such as plastic granules and grain.
The hopper angle is set by the material's behaviour, not by preference. For cement, a hopper of about 60 degrees from horizontal is common. A steeper hopper flows better but makes the silo taller and more expensive.
A silo of the same dimensions is almost twice as heavy with cement as with PVC granules. The table below shows the differences between common materials:
| Material | Approx. bulk density | Main design point |
|---|---|---|
| Cement | 1.2 to 1.5 t/m³ | Steep hopper, aeration pads, dust filter, relief valve |
| Hydrated lime | 0.4 to 0.6 t/m³ | Very cohesive; aeration and vibration |
| Wheat and grain | 0.75 to 0.8 t/m³ | Ventilation, moisture and temperature control |
| PVC granules | 0.55 to 0.65 t/m³ | Cleanliness, earthing, discharge without residue |
| Dry sand | 1.5 to 1.7 t/m³ | High abrasion in the hopper |
A silo's accessories are not just extras; some of them prevent serious damage to the body:
During pneumatic filling, the air carrying the cement must leave the silo without dust. The dust filter cleans this air.
If the filter is blocked or the filling rate is high, the pressure inside the silo rises. Rapid discharge can also create a vacuum. This valve prevents the roof from bursting or the body from crumpling.
For cohesive materials, low-pressure air from pads inside the hopper fluidises the material and prevents bridging.
High and low level sensors prevent overfilling during filling and unwanted running empty.
A silo is a tall structure with a high centre of gravity, and its full weight is several times its empty weight. The legs and bracing are designed for the full weight, the wind load on the body and the earthquake force. The leg-to-body connection must also be such that the concentrated load does not buckle the shell.
The foundation drawing is delivered with the silo. The concrete work is usually the client's responsibility, and once the foundation is ready the silo is installed on it by crane.
From choosing the hopper angle to the roof relief valve — what keeps a silo trouble-free
A silo is a vertical cylindrical vessel for bulk solids that is filled from the top and emptied through a hopper at the bottom. It looks simple, but solids behave differently from liquids: cement may stick to the wall and bridge, wheat is sensitive to moisture, and plastic granules pick up static electricity. That is why silo design starts with the question "what goes inside?"
The pressure of bulk material on the wall is not like water pressure either. Part of the material's weight is transferred to the wall by friction, and during discharge the pressure rises where the hopper meets the body. Silo-specific standards such as EN 1991-4 build this behaviour into the calculation of plate thickness and the support structure. For capacity in tonnes, we have written up the silo capacity calculation with a numerical example, and the online silo calculator shows the dimensions, plate thickness and weight. The price of tank and silo fabrication is also calculated from this weight.
Choosing the silo type by material, capacity, installation space and transportability
A welded silo on four or six legs with a steep hopper. It is filled with compressed air from a cement tanker and has a dust filter and relief valve on the roof. The most common silo at concrete plants and building materials factories.
Grains flow more freely and move with a shallower hopper, but they are sensitive to moisture and temperature. These silos have ventilation, a sealed roof and an inspection hatch to control grain quality.
Granules are light and free-flowing, but they must stay clean and uncontaminated. A stainless body or a suitable internal coating, earthing the body against static electricity and discharge without residue are the main points of these silos.
Each silo is designed for a specific material and capacity; these are the main design parameters
From choosing the material to installation on the foundation — with quality control at every stage
The material pressure on the wall is calculated to EN 1991-4 and the shell is designed to EN 1993-4-1. Wind and earthquake loads on the tall silo structure are applied according to the relevant codes. No silo leaves the workshop without weld and dimensional checks.
In these industries, steel silos store and distribute bulk materials
Tell us the material, capacity in tonnes and installation site to receive the proposed dimensions, hopper angle and silo price.
The most common questions about steel silo design and fabrication
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