Grain bin maintenance to detect and prevent potential failure
May 01, 1994
by Teresa Acklin
Part two of two: avoiding unequal pressures in the bin through regular maintenance and observation can prevent structural failure.
We are all aware of the huge vertical pressures created by grain in bins. In order for these bins to carry vertical loads, engineers design what are called stiffeners.
Many of the older bins had stiffeners on the inside, but we now are seeing a large number of bins with stiffeners on the outside. The advantages of outside stiffeners are twofold: They can be observed easily when the bin is full, and grain will not stick behind the stiffeners during unloading.
Observation of the stiffeners will easily determine whether vertical loads are being exceeded in the bin. If the stiffeners are on the outside, buckling will appear in the bin when it is full. If this condition occurs, some grain should be withdrawn from the bin to prevent the vertical type of failure.
If the stiffeners are inside the bin, the sidewall sheets will start to recorrugate, or fold down on themselves, if loads are excessive. Again, some grain should be withdrawn from the tank to relieve the pressure.
Research by several major universities has found that withdrawing only 5% to 10% of the grain can reduce some of the sidewall pressures by a much greater percentage. Removing grain alleviates the concern about complete failure until the tank is emptied.
Another cause of stiffener buckling may be voids created by grain crusting in certain areas. This condition will not be identified until the bin is empty; but if such a condition has existed, you normally can see where the grain hung to the walls above the buckled stiffeners.
This area should be thoroughly cleaned, and if the sidewall sheets have started to rust, they should be painted before refilling the bin. This condition also could indicate improper aeration, and one should check with the manufacturers on proper aeration techniques to prevent a recurrence.
Another way to check the sidewalls is to stand next to the wall and look straight up the side. The bin should be perfectly round and the stiffeners straight when the bin is loaded. If the stiffeners are bulging or sucking in, something is wrong, and the condition should be checked out with the manufacturer.
You occasionally may observe that the vertical seams of the sheets appear to be bulging out slightly. This is a common phenomenon related to the amount of slip allowed in the bolted vertical seam. But if the slight bulge appears to be excessive, the manufacturer should be contacted.
Depending on the type of stiffeners used, it is not uncommon to see some possible slippage at the stiffener connections. Where shims are used between the stiffeners, this will not occur; but when stiffeners rely on bolts to transfer the load, there occasionally can be some shifting.
When the tank is empty, the joints should be checked to make sure that no slippage is occurring. If the bolts were adequately tightened and loads were normal, slippage and bolt movement should not be observed. But if some slippage appears at these joints, the cause should be determined and steps should be taken to prevent further slippage and possible failure.
Another situation visible from simple observation is some type of split in a sidewall sheet. Usually, a split or tearing starts at one of the bolt holes or around the door and works its way up the sheet. This indicates an inferior sheet or one that was improperly placed in the bin, and immediate steps must be taken to prevent bin failure.
If the split is very short and does not go all the way to top or bottom of the sheet, drilling a small hole at either or both ends of the split may prevent it from enlarging until proper repairs can be made. It is essential that some type of temporary fix occur to keep the bin from failing catastrophically.
Often, when individuals see this type of occurrence or when they see the sidewall sheets starting to come apart at the vertical seams, the initial thought is to wrap cables around the bin. While this may be a temporary fix, one should keep in mind that once problems have begun to occur, there is always danger to the individuals wrapping cables if the bin fails. Keep in mind that cables are at best only a temporary solution until the bin can be unloaded and proper repairs made.
Cables wrapped around the bin never should be used permanently, and we do not recommend them as a temporary solution. Cables will stretch, and eventually, failure can occur even with cables. Heavy steel strapping or banding material has much less tendency to stretch, but even that solution should be considered only temporary until the bin can be emptied and proper repairs made.
Another area in the sidewall that can create problems is any opening made for access, such as frontloader doors. Because of the pressures induced around such openings, it is common practice to use some type of tiebar.
A tiebar that begins to pull apart or shear at the bolt location, or bolt shearing itself, indicates that loads are greater than the tiebar can hold. Some grain should be withdrawn from the bin immediately to reduce pressures and avoid a failure.
The bin base.
The base of the bin is another area that often causes problems. It is extremely important that the load from the stiffeners be properly transferred to the concrete footer.
Because of the nature of construction, a gap between the stiffeners and the concrete is common when the bin is built. It is essential to place shims beneath the stiffeners' feet to transfer the load properly.
One of the most significant areas of possible rust problems is at the base of the bin. If the base is not sealed properly, moisture gets inside the bin and can spoil the grain. Once the bin is emptied, this entire area should be thoroughly cleaned out and dried, the sheet should be repainted and the joints sealed to prevent rust from developing.
There have been many documented cases of bin failure occurring because the bottom ring had rusted and deteriorated to the point that it would not withstand the pressures created by the grain. It also is entirely possible that if this condition is allowed to continue, the anchor bolts holding the stiffeners can fail, allowing the sidewalls to push out at the failed bolts.
Other areas to observe while walking around the bin are the concrete footings and foundations themselves. If the footing is cracking beneath the stiffener, the load supported by that stiffener is being transferred to the adjacent stiffeners, causing them to be overloaded. Immediate steps should be taken to make sure that proper support is provided under the suspect stiffener.
In many commercial installations, it is common for the unloading system to include tunnels running beneath the bin. Checks should be made where the tunnels exit the bin foundation to make sure that cracking is not occurring in the tops of the tunnels.
If proper bars or I-beams were used to support the sidewall loads, no cracking will be observed. But if cracking appears in this location, a method of supporting the tunnel top should be found immediately.
One of the greatest problems seen by manufacturers is improper unloading of storage bins. Steel bins are designed to be unloaded through the center well first; unloading through intermediate wells should be avoided until no more grain runs out of the center. The only exception to this rule is if proper sidewall flumes have been installed for side loading into trucks or rail cars.
Using intermediate wells before emptying grain through the center well creates the problem of unequal pressures, causing the bin to go egg-shaped. Once this starts to occur, the law of physics takes over: for every action, there is an equal and opposite reaction.
Because the tank is egg-shaped, excessive pressures are placed on one sidewall; the steel on the wall stretching or moving out pulls steel from the wall with less pressure. At this point, the wall on the low-pressure, or unloading, side begins to buckle, the tank starts to recorrugate and stiffeners start to buckle. If this condition is allowed to progress, the potential exists for total failure.
Bin manufacturers recommend that procedures be taken to assure the center wells do not plug during the unloading process. Obviously, there is no assurance that wells will not plug, and the best remedy is to have a second well located about 1.2 to 1.5 meters from the center well.
This second well should have its own operating rod; if the center well plugs, the second well can be used to unload the bin, rather than using the intermediate wells first.
Manufacturers also recommend some type of well guard, basket or “clod buster” be placed over the wells to keep crusted grain from plugging the center well. Coring, or drawing 40 to 60 tonnes of grain out of the center well after filling the bin, is another way to prevent problems that can plug a center well.
Other items that should be checked during routine maintenance are warning and safety signs. These signs are placed on all of the bin's access points and should be kept clean and readable. If they become unreadable, contact the manufacturer for replacement signs.
Two other items should be discussed regarding safety. Although it is not common to see a fire or smoldering grain in commercial storage bins, the possibility does exist.
If fire or smoldering occurs, draw the grain out of the bin, on to the ground if necessary. Although some grain will be lost, it will be much less than the amount lost by attempting to smother or drown the fire with grain in the bin. Structural damage also could occur by trying to put out the fire with grain in the bin.
The second safety issue relates to rescue procedures in a grain-drowning situation.
Extreme forces are created around a person being pulled into a flowing grain envelope, and the person cannot be pulled out without injury until the grain pressure is reduced. If someone is totally submerged, the only way to get to the person is to remove the grain.
Aeration fans should be running to attempt to provide oxygen to the victim. V-openings, 75 cm to 1 m across the top, should be cut in the bin side to drain out the grain.
The openings must be low enough to get the grain away from the victim, but high enough that the grain can drain. By cutting the V shape, the steel can be pulled back or pushed in to regulate the flow. Several openings should be made around the bin to ensure uniform draining and to prevent tipping and structural failure.
The items discussed here are not the only items that must be checked. Likewise, there are other solutions to some of the problems mentioned. The main concern is to stimulate your thinking so that formal procedures will be established for your operation.
By Harmon L. Towne, manager of engineering for Brock Manufacturing in Milford, Indiana, U.S. Mr. Towne is a registered professional engineer and serves as technical vice-president for the American Society of Agricultural Engineers. This article was first delivered at the 1993 International Technical Conference and Exposition of the Grain Elevator and Processing Society.