While steel bin failures rarely occur, those that do fail could have been prevented in most, if not all, cases. While some will say there are design flaws with these bins, the fact that there are hundreds of thousands in use today dispels their claims.
The vast majority of failures fall into three main categories: improper construction; improper loading and unloading; and loss of structural integrity due to rust and wear and tear.
IMPROPER LOADING AND UNLOADING
What can you do to make sure you do not have a failure with your bins? Steel bins are thin-shelled structures. They rely on uniform pressures around the bin to maintain their structural integrity. Off-center loading or unloading creates non-uniform pressures on the bin walls that can result in bin damage or even catastrophic failure.
During loading it is imperative that the grain drop straight down from the center at the top of the bin. This allows the grain to flow to the outside uniformly, creating even pressures. It is fairly easy to tell if a bin is being loaded improperly. The uneven pressures caused by grain flowing in at an angle causes the bin to egg shape and force two sides out while pulling two sides in. The most obvious location to see this is at the roof. The roof will overhang the sidewall further on the sides where the bin wall is pulled in while barely covering the sidewall that has been pushed out. You may also see some flattening of the sidewalls.
The same situation can occur when side-draw baffles are used. If the grain is not center-cored after unloading from a side draw unit before refilling the bin, the same egg shaping can occur. Each time the side draw is used without the proper refilling, the egg shaping can continue to get worse.
You may also hear the term "side draw" used when someone unloads from an intermediate well before the grain funnel has reached the floor from unloading through the center well (sump). When this occurs, the pressure on the side of the bin being unloaded by the intermediate well no longer exists. The bin will start to buckle in on this side, and if allowed to go long enough, a catastrophic failure can occur. On some bins this can occur when only a few hundred bushels are removed.
The biggest contributor to failures in improper construction is related to the foundation and bin anchoring. While there have been a few cases where the body sheets or stiffeners were placed in the wrong location, it is very rare. When this occurs, the entire ring will simply recorrugate. The first signs of this are bulging of the stiffeners and subsequently recorrugation of the body sheets. If the stiffeners are on the inside of the bin, the only thing that will show up is the recorrugation of the body sheets. Before unloading the bin, be sure to contact the manufacturer or an expert in the business to ensure that the bin is unloaded in a way that will not cause additional failure.
The foundation is probably the most important part of any grain bin. The load from the grain is transferred to the foundation by the stiffeners and sheets. The greatest part of the load is through the stiffeners.
Research at the University of Kentucky many years ago showed that as much as 80% of the grain load could be in the sidewalls. This, of course, depends on the diameter of the bin and the height, but suffice it to say that the greatest part of the load is in the stiffeners. When the concrete is not level, there is the possibility that one of the stiffeners may not be carrying its design load if not properly shimmed. This forces the stiffeners on each side to carry more than their designed load. While there is always a factor of safety in any design, such an increased load can cause stiffener failure. A stiffener starting to bulge, or signs of bulging or recorrugation in the body sheets with inside stiffeners, is a warning that a failure has started. High spots at stiffener locations can also cause significant stresses higher up in the bin which may show up as buckling stiffeners or body sheet recorrugation.
The other issue with foundations is whether they are level. It is not uncommon to see settlement in the foundation when the bin is filled with grain. If the settlement is uniform, there is no problem. However, if the settlement is not uniform, the bin can lean toward the low side. The top of a bin leaning out 6 inches to 8 inches over the base is probably not going to be an issue, but a check with the bin manufacturer would be very prudent. With some of the very large diameter and tall bins, even 12 inches has been allowed but not preferred. While this may not cause a bin failure, it can have a detrimental effect on the filling and unloading equipment.
The steel used to construct these bins is covered with a thin coating of zinc. The zinc will sacrifice itself to protect the steel. During the first few years in the life of a bin, it is not uncommon to see various colors on the sidewall sheets. This can be caused by steel from different manufactures, different thicknesses of sheets and even different amounts of a protective coating that is put on the steel prior to shipment to prevent wet storage stain. While it may cause some concern, there is nothing wrong with this action. Over time, all of the steel should have the same basic color appearance as the zinc coating goes through its normal oxidation process.
While water alone may not cause the steel to show signs of rust for a long time, wet grain lodged against the steel can cause this to happen very quickly. Once the bin is emptied, the inside should be inspected for any grain that may be sticking to the sheets. If this is found, it indicates water getting into the bin or grain spoilage from other causes. If the grain sticking to the sheets is not removed immediately, severe corrosion of the steel can begin to occur. The area that is most affected usually appears at the floor. Breakdown of the weather-tight seal at the base angle allows water to get under the bin and leads to spoiled grain and damage to the zinc coating on the steel.
When these situations are found, it is imperative that the wet grain be removed and the bin properly sealed to prevent further occurrence. If the zinc coating has been destroyed at this location or any other, the steel should be wire brushed or pressure washed to remove any loose scale and then painted. While a zinc-based paint would be preferable, any type of paint recommended by a paint manufacturer is acceptable. The most important thing is to replace the destroyed protective zinc coating with something that will stop the corrosion of the steel.
The other area where rust may appear early is on the roof. While the roof itself does not start to rust, equipment that extends over the roof such as catwalks, downspouts and loading equipment may start to rust. When this occurs, material is washed onto the bin roof and the rust process is then transferred to the roof. Again, it is essential that the loose scale be removed and a protective coating be placed on the roof panels.
Temperature cables have often caused roof damage. As the size of the bins has increased, both by diameter and height, the grain load on the cables has also increased. If the attachment point at the roof is not properly designed or the cables are improperly hung, the loads on the system may cause roof failure. Weights should never be left on the bottom of the cables to hold them in place, but the bottom of the cable must be restrained. If the cable is allowed to move sideways as the grain is loaded and unloaded, the loads on the anchorage point at the roof are not acting straight down. This can also cause roof failure.
Proper use of steel bins that are properly constructed and set on properly designed concrete will give you many years of use. Even though bins themselves are not a mechanical piece of equipment, they do require maintenance and upkeep just like any other piece of equipment. If problems occur, it is essential that they be properly addressed to prevent further damage or even catastrophic failure.Harmon L. Towne is retired from Brock Manufacturing where he was vice-president of engineering and product development. He can be reached at firstname.lastname@example.org.