Designing & Pricing A Silo
Q. Besides the amount of material that needs to be stored, what other information do you need to come up with a design and eventually a price for a silo?
A. The silo design is a complex problem involving a number of factors — including the type of material to be stored, the functionality or operational requirements of the silo, and the geographical location of the structure.
Here is a list of some of the items that need to be determined for the engineer to design the silo and foundation… and to provide pricing for the silo.
Material To Be Stored
Silos are designed for a specific material with specific characteristics which impact the general arrangement and structural details of the silo. Some of these characteristics are:
- Density for volumetric calculations
- Density for structural load calculations
- Angle of repose range
- Coefficient of friction between the material and the silo wall, hopper, etc.
- Moisture content
- Particle size (maximum and minimum)
- Degree of explosibility or Deflagration Index (KST Value)
If there is not adequate experience in storing and handling a particular material, it would be a very wise investment to have material lab testing done to determine and quantify as many of the characteristics listed above as possible.
Functionality or Operational Requirements
Understanding the operational requirements is paramount to the silo design. Considerations include:
- Required storage capacity
- Long-term storage or frequent cycling
- Concern for product degradation
- Concern for product segregation
- Concentric or eccentric discharge and number of required discharges
- Concern for complete clean-out
- Fill and discharge flow-rate capacities and requirements
- Aeration or fluidization requirements
- General required life of the silo
Wall and roof penetrations need to be defined and accounted for in the silo design. These include:
- Level probes
- Temperature probes
- Truck or rail car drive-through doors
- Personnel doors or openings for access
- Fill and discharge points and clearances
- Explosion relief
- Negative air
- Aeration or fluidization
Peripheral and Imposed Loads
Internal and external loads due to anticipated equipment and structures must be accounted for in the design. Examples include:
- Ladder and stair access
- Access or support floors
- Conveyance bridge and tower support
- Temperature and level cable support
The specific location may affect such design parameters as:
- Building code requirements
- Zoning requirements or limitations that may require a variance
- Wind load requirements
- Snow load requirements
- Seismic requirements
- Frost requirements
- Available space (i.e. footprint and height) for the silo
A soil analysis — inclusive of borings and a geotechnical report – is imperative to finalize the design. The report will typically provide:
- Allowable soil-bearing capacity
- Elevation of groundwater table
- Elevation of boring refusal
- Expected or anticipated total and differential settlement given the anticipated dead and live loads of the silo structure
- Recommendation for ground improvement (i.e. overexcavate and backfill, etc.) if required
- Recommendation of foundation type (i.e. shallow, piling, etc.)
This information will help the engineer determine the best solution for the foundation design.
In addition to defining the material to be stored and the operational requirements of the silo, the engineer can work with the owner to define and determine the remainder of the required data. As with any design, the level of success is determined by accurately defining and understanding the design assumptions.
Answered by Tim Lease, P.E., President of WL Port-Land Systems Inc.