Conversion of Existing Concrete Tower Silos to Dry Grain Storage: Part A
Table of Contents
Existing open-top, concrete tower silos, originally built for silages, can be converted to dry grain storage provided that a number of important items are looked after. These include evaluation of present silo condition, increasing structural strength, ensuring complete moisture control, providing an aeration system and installing proper loading and unloading facilities.
Figure 1. Conventional concrete tower silo converted to dry grain storage.
Figure 2. Basics involved in converting a silage silo to dry grain storage.
In order to use a silage silo for dry grain storage, it must be structurally sound, with no evidence of major deterioration or over stressing. Silos with foundations and/or walls that show any signs of having been over-stressed during previous use, or that have been badly deteriorated by silage acids, should not be considered for dry grain storage.
The inside surface of a silo wall that shows any pitting should first be thoroughly cleaned, then have a suitable cementitious coating applied to bring the surface back to its original smooth condition. Deteriorated doors and door frames should be repaired to prevent possible air leakage during aeration.
A concrete silo (cast-in-place or stave) built for silage usually requires some modification to make it strong enough to withstand the pressures created by dry grain. There are two areas of pressure to be concerned about - a) the horizontal pressure on the side wall, and b) the vertical load carried by the side wall due to the friction effect; both require that the wall be structurally sound.
The horizontal pressures created by dry grain are somewhat different than those created by silages. In general terms the pressure of dry grain at rest is greater than silage pressure in the upper portion of the silo; this difference decreases toward the bottom of the silo, and in larger diameter silos grain pressure is actually less than silage pressure (Figure 3). This means that in converting a silage silo to dry grain storage, extra strength must be added. Ignorance of this fact has lead to several failures with converted silos in the past.
A converted farm silo, because of the ration of depth of grain to width (diameter), is usually considered to be a "deep bin". Horizontal grain pressures in deep bins vary according to the circumstances. For flat-bottom bins (recommended construction for converted silos) the horizontal pressures created during emptying are usually somewhat greater than those encountered during filling and while the grain is at rest (Figure 3). The magnitude of this increase will depend on the height-to diameter ratio, the wall roughness, and the location of the discharge opening.
Figure 3. Comparison of lateral pressures on silo wall (16' diameter) from silage and dry grain both at rest and during unloading.
The strength of concrete silos in a horizontal sense is developed through the use of circumferential steel - either as reinforcing bars within the wall of a cast-in-place silo, or as external hoops on a precast stave silo. The only practical way to add extra strength to either type of silo is to add external steel hoops as required.
Recommendations on the number of extra hoops required, their spacing and location should come from someone thoroughly knowledgeable about silo design. Assistance may be available from the original manufacturer or builder of the silo being considered, consulting engineers or agricultural engineers from the Ontario Ministry of Agriculture and Food.
One way to make use of an existing silo, without having to add any extra hooping, is to only fill the silo to a depth that the existing steel will safely carry. Such maximum depths are shown in Table 1. These figures are based on the assumption that the strength of the silo has not been affected by any sort of deterioration.
**Depths shown are based on a well maintained silo with steel reinforcing or hooping schedule equal to that recommended for a new silage silo.
One of the problems encountered when trying to decide how much extra hooping to add to a silo is determining how much strength presently exists. This is relatively easy to estimate with a stave silo where the hoops are all external and thus easily checked. In this case all that is required is to compare what exists with what is required for grain pressures, and then make up the short-fall. Additional hooping may be advisable in those situations where the existing hoops show signs of rusting.
In the case of a cast-in-place silo the existing steel is buried in the wall and unless an accurate record regarding the reinforcing schedule is available from the original builder, it is difficult to determine existing strength. If: (a) the silo has been used for regular silages up until the present time; (b) it has been filled to the top regularly; and there is no evidence of over stressing of the existing steel (no major vertical cracks in the wall), then it would be reasonable to assume the reinforcing schedule is fairly close to the minimum recommended for silages. In this case the extra hooping required is the difference between the strength provided through the reinforcing schedule and that required for grain.
Some cast-in-place silos have a continuous door opening from top to bottom. In these cases the wall reinforcing on either side of the doorway is connected by a single rod spaced at regular intervals that correspond with the joints between the doors. In these situations the actual wall strength may be limited by the size and strength of the door rods. This must be considered when trying to determine the extra steel required for dry grain storage.
If the silo has not been used for some time, one should be conservative and give less value to the existing reinforcement. The same is true if the inside surface of the silo wall shows serious deterioration due to silage acids. The concern here is the possibility of decreased strength due to acid attack on the reinforcing steel, and/or a reduction in the bond strength of the concrete surrounding the steel (lapped joints in the rings of steel depend entirely on bond strength to hold the sections together).
A considerable portion of the weight of the grain contained in a silo is transferred to the footings through the walls due to the friction effect of the grain on the silo wall. Tall, narrow silos transfer a larger proportion of the total weight to the supporting soil through the walls and foundation than do short, wide structures. In general terms, once the depth of grain has reached 2 1/2 - 3 times the silo diameter, the weight of any additional grain will be taken almost entirely by the wall (with little or no corresponding increase in floor load). Thus, in a silo conversion the ability of the walls to withstand this vertical friction load must be carefully evaluated to avoid a possible wall crushing problem.
A concrete silo used previously for silage may have suffered some degree of deterioration due to the etching action of silage acids. This action reduces the effective thickness of the wall, thereby reducing its strength and thus its vertical load carrying capacity. Careful evaluation of existing wall strength is essential in any silo conversion since the area of probable deterioration is normally at the bottom of the silo where the vertical load is the greatest. Thus the strength of the silo wall (in a vertical sense) may limit the depth of the grain that can safely be stored in a converted silo.
The problem of wall strength for vertical loading is the greatest for precast stave silos where the staves are often only 2 1/2" thick. However, the capacity for vertical loading should also be part of the structural evaluation of cast-in-place walls since-a) these walls can also lose a good deal of their effective thickness through acid etching, and b) the original concrete may not have been as strong as that in precast stave walls.
In evaluating an existing silo for possible conversion to dry grain storage consideration should also be given to the adequacy of the existing footings, i.e. - whether they are large enough to take the added load that may be imposed by dry grain.
Combined Horizontal and Vertical Pressures
Stave silos should have the spacing of the hoops such that enough support is provided to the staves to prevent them buckling outward under the combined vertical and horizontal loads. The maximum safe distance between hoops will depend on not only the size of the silo and the depth of grain above the point in question, but also the effective thickness of the stave itself. For older silos whose staves have been etched with silage acids, the effective thickness may be considerable less than the original thickness. The determination of wall strength and thus maximum safe hoop spacing requires inspection and review by someone thoroughly knowledgeable in this field.
Adding Extra Hooping
Care should be taken when adding extra hooping to a silo. If the silo may be used for silage again sometime in the future, the extra hoops must be positioned so as not to interfere with the door openings. This may be accomplished by:
On stave silos, added hoops should not be tied into existing spreader bars unless this has been checked out by the silo manufacturer or some other knowledgeable person (the spreader bar may not be designed to take the extra load). If spreader bars do not have openings for extra hoops, extra holes should not be drilled as this may weaken the spreader bar. Avoid running hoops over spreader bars or door frames that will hold the hoop away from the staves for some distance (lack of support could result in cracked staves). If the silo will only be used for dry grain in the future the extra hoops can be run across the door openings as required for extra wall strength. At the same time extra hooping is added to a stave silo the existing hoops should be re-tightened to make sure the wall is tight and equally supported throughout its height.
Sometimes it is necessary to cut new openings through the wall of the silo near the bottom to accommodate the aeration fan or the unloading auger. Care should be taken to make sure any hooping or reinforcing steel that is disturbed or cut by this installations is replaced by additional hoops with proper spreader-bar arrangement around the opening.
In the conversion of a silage silo to grain storage, it is absolutely essential that the silo be made moisture-proof in order to avoid re-wetting the grain (resulting in heating and spoilage problems). Moisture-proofing involves three areas: roof, wall and floor.
Roof and Walls
All grain silos must be equipped with a rain-proof roof. This means careful inspection and possible repair or replacement. Any silo slated for conversion, should be inspected after a driving rainstorm to see if moisture has penetrated; if dampness shows, the wall must be moisture-proofed. The best way to moisture-proof a silo wall is to spray the outside surface with a moisture repellent coating that prevents the passage of liquid but allows the movement of water vapour.
A new concrete floor is usually required in a silo conversion. Where subfill is required, us a pea-gravel or well-compacted sand over-laid with a layer of 6 mil polyethylene to act as a moisture barrier. The floor should be constructed 12 inches or more above the outside grade level. A six inch thick concrete floor, with wire mesh to prevent movement if any cracking occurs, is usually adequate. The floor should be built to accommodate unloading and aeration equipment. The unloading auger should discharge at a height above grade such that it will not be necessary to excavate a hole in the ground for the receiving equipment (this can lead to water problems).
The installation of hopper-shaped floors in converted silage silos is not generally recommended. Installing a hopper that extends above ground level will add severe horizontal pressures to that section of the lower wall supporting the hopper, while below-grade hoppers often encounter moisture problems. Recommendations for strengthening an existing silo wall to accommodate a hopper-shaped floor are beyond the scope of this factsheet and require the advice of someone knowledgeable in grain storage design. Hopper-shaped floors of concrete are difficult to form with sufficient slope to be self-emptying. It is also hard to install satisfactory aeration equipment with a hooper-shaped floor.
Dividing a silo into several bins is difficult to do in an existing silo and should only be attempted after consulting with someone competent in storage design.
A separate, companion Factsheet - Conversion of Existing Concrete Tower Silos to Dry Grain Storage, Part B (Order No. 88-070) gives additional information on such topics as - aeration, silo loading and unloading and maintenance of grain quality.
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