1. Circular Bins: Silo and Bunker
Silos and bunkers are circular structures which are designed to store large amounts of granular materials. The circular geometry of these structures provides the proper shelter for the materials inside, because other options - for example box shaped structures - would suffer from large amount of load effects which would be irrational to deal with.
These two different names - silo and bunker - express a geometrical property of these bins. If the bin is deep and slender, it is called a silo; whereas if the bin is shallow and stubby, it is called a bunker. It is required to make this distinction, while the geometry of the bins changes the behavior of the stored material. The measure where a bin acts as a silo or a bunker is defined in specifications.
Silos are used in many fields, like in agriculture, in cement plants, in energy producing plants to store large amounts of materials. The building material of the silos may also vary like concrete, steel, aluminum, wood, etc. considering the necessities.
Also one silo may contain several compartments to store different types of materials in one silo, to save money and to save space.
In this article, design of a post-tensioned homogenizing silo in accordance with Eurocode will be discussed. The discussions will make it possible to apply the design in any specification.
2. A Brief Introduction for a Homogenizing Silo
The homogenizing silos, are the silos that include aeration systems inside. The aeration system installed inside the silo, makes it possible to extract a perfectly blended material from the silo. In order to do so, aeration tubes are installed on the bottom slab of the silo. These tubes have holes on them through which air is blown inside the silo to fluidize the material. The fluidized material is then drawn from the silo to conduction systems like conveyors or directly to trucks.
There are two kinds of aeration. Either a batch of material can be aerated at a time, or all the material inside can be aerated and discharged from the silo therefore flow can be continuous.
On Figure 2, homogenizing silo technology from company IBAU can be seen. In this technology, only a batch of material is homogenized at a time and drawn from the silo. The reason to homogenize only a batch of material may either be the capacity of the conduction systems, or the requirements of the plant. For example, for a raw meal silo in a cement plant, the aeration may start and stop according to kiln feed bin level while raw meal silos are feeding the rotary kiln.
On figure 3, a typical continuous flow (CF) silo, where the aeration is continuous can be seen.
A department below the storage volume is required for homogenizing silos, where the material will be discharged and placed to the conductive systems. In other words, this department is the shelter for the mechanical equipment and is the beginning of the transportation of the material. This part contains big openings for the materials to go out of the silo.
As can be seen from above, many mechanical parts take place in a homogenizing silo. Therefore, the preliminary design of these silos are carried out with the mechanical engineers while the geometrical requirements of these silos mainly depend on the requirements of the plant and the decisions given by mechanical engineers. These decisions include; “What is the volume of the material to be stored inside?”; “At what distance, should the silo be placed in the plant to optimize the efficiency of the conductive systems?”, “Which conductive system will be applied here?”, “How should the geometry be, for the below part of the storage volume?” etc.
3. Specifications Used for the Design of the Silo
Eurocode – Basis of Structural Design (EN1990 – 2002)
Eurocode 1 – Basis of design and actions on structures – Part 4: Actions in silos and tanks (EN1991-4: 2006)
Eurocode 1 – Actions on structures – Part 1- 4: General actions – Wind actions (EN1991-1-4: 2007)
Eurocode 2 – Design of Concrete Structures, Part 1 – 1 : General Rules and Rules for Buildings (EN1992-1-1: 2004)
Eurocode 8 – Design of Structures for Earthquake Resistance, Part 1 : General Rules. Seismic Actions and Rules for Buildings (EN1998-1: 2003)
4. Preliminary Design
The design adventure of the silo starts with the preliminary design of the
superstructure. Before we start the design, we may have some ideas about the silo.
For example, the height, the diameter and the amount of the stored material may give designer the clue whether this silo will be a post tensioned silo or not.
The roof of the silo is a reinforced concrete slab, on which the equipment rest. The raw meal comes to the silo through the pipes on the roof. Due to the difficulty to construct a scaffold, the roof slab will be a composite slab with steel beams and a trapezoidal steel deck. Hence, 150mm thickness will generally be sufficient for the slab thickness.
Before the design starts, wall thicknesses may also be estimated roughly. This part generally depends on the experience. But after the loads of the silo is worked roughly and convinced that the silo will be post-tensioned, the upper wall thickness may be estimated considering; the post tension cables will need some space to run through the wall, and the hot material inside will tend to expand the silo introducing extra moment forces. Generally, for a post tensioned homogenizing silo 400-500 mm thickness would be optimum to start the design. Discharge conditions may also introduce extra moments to the silo wall.
The bottom silo wall thickness, wall below the bottom slab, may be estimated roughly to be 200mm thicker than the upper silo wall, while the bottom slab will rest on the bottom silo wall. Hence, 600-700mm thickness may be a good estimation to start.
5. Loads and Combinations
Loads acting on the silo can be summarized as:
Self-weight of the silo.
Dead loads acting on the structure.
dead load of the equipment provided in the mechanical drawings
uniformly distributed dead load assumed on the platforms, provided in the mechanical drawings
Self-weight of the non-structural elements (levelling concrete, second stage concrete, etc.)
Live load on the platforms provided in the mechanical drawings. In addition, snow load which is specified by the design criteria or by the specification used.
Loads due to the stored material.
Filling and storage material loads
Filling Patch Load
Discharge loads
Discharge Patch Load
Discharge Patch Load for Large Discharge Eccentricities (if required)
Load on Bottom Slab
Earthquake Load
Wind Load (not critical)
Temperature Load
Combinations should also be studied.
a. Ultimate Limit State
For STR and GEO limit states the following combinations are to be taken.
where;
b. Serviceability Limit State
For serviceability limit states the following load combinations are to be taken.
The loads, the combinations and the analysis of the structure should deeply be investigated and the detailing should be done with great care.
To be continued...