Steel bins have been utilized for grain storage since the early 1900s and are popular in the grain industry because of their durability and longevity. But even sturdy steel structures will lose their strength and eventually deteriorate if not properly maintained.
The biggest reasons for steel bin failure are misuse and lack of proper maintenance.
Steel grain bins are thin-shell structures, and to maintain their shape and structural integrity uniform loading of the sidewalls is required. Since the loads on the sidewalls and the floor can weigh thousands of pounds, a properly designed and maintained foundation is a must.
REGULAR MAINTENANCE CHECKS
When performing regular maintenance checks, the first place to look is at the foundation. It has to be strong enough to both support the grain and prevent an empty bin from overturning or being pulled off of the foundation in high winds. Thus, it is essential that the foundation doesn’t show signs of uneven settling, large cracks, crumbling concrete and/or missing or loose anchor bolts. There should be a watertight seal between the base of the bin and the foundation.
The bin examination should also include checking for buckling stiffeners, bulging sheets, missing bolts and general appearances that seem abnormal.
The bin might even appear to be leaning in some instances. If any of these signs appear, a determination must be made about the structural integrity of the bin. Keep in mind that minor bulging may be noticeable in some of the sheets. This is not uncommon, especially in the middle of a sheet. However, excessive bulging could offer a warning that the sheet is about to recorrugate (fold down on itself).
In most cases, some grain can be drawn out of the bin to relieve pressure. But you must remember that as grain is drawn out, the loads in the sidewalls will increase in weight by approximately 10%. If there are sheets or stiffeners that have already buckled, drawing out grain may create a larger problem.
If the bin has started to re-corrugate, do not try to draw grain out without contacting the manufacturer or a professional that can assist in getting the bin emptied. Trying to use the standard unloading system in such a case may bring the entire bin down in a catastrophic failure.
Closely examine any openings in the sidewalls. Most bins have some type of access door in the sidewall, but the opening must be properly supported to withstand the hoop stresses. Most doors use some type of a tie-bar system. This may be an integral part of the door, such as special door boards, or tie bars that are removable. Other openings might be for flumes, or in larger bins, doors large enough for front end loaders to enter. If any bulging is noticed in these areas, it is essential that the pressures be reduced and corrective action taken. If the bin is empty, make sure all bolts, fasteners, etc. are properly installed before filling.
When looking at the top of the bin, make sure the roof overhangs the sidewall uniformly. The strength of an empty bin against high winds is greatly reduced if it is not kept in a round shape. If it appears the roof is pushed out farther on two sides than the other, the bin is no longer circular. Fasteners holding the roof in place may no longer be there. Corrective action should be taken to make the bin round again.
The ladder should also be inspected for rusty rungs and to make sure all bolts are in place. Many times the rungs rust from the inside out. If this occurs, the rungs will be significantly weakened and must be replaced. Also make sure the safety cages are properly attached.
Check for rust on the roof, since that is often the first place it occurs on a bin. Many times this happens because dust and trash are allowed to sit on the roof. There also may be catwalks and downspouts running over the roof that start rusting, allowing water to drip onto the roof.
Rust is one of the greatest enemies to steel. If rusting has started, you should brush, scrape or sandblast it or use other means to remove loose scale before repainting. For many years only zinc based paints were recommended, but there are now many quality paints that can be used. Check with the bin manufacturer or a local paint specialist to find out what is best for your situation.
If the rusting has been going on for years, it is quite possible that the entire roof will have to be replaced. The bin manufacturer or a qualified structural engineer can assist you in making that determination.
When the bin is empty, examine the interior for problems. Make sure to follow all safety rules before entering the bin. One of the biggest areas of concern is to see if spoiled grain has been hanging on the sidewalls or at the base of the bin. In either instance, if this has occurred, rust has probably formed on the steel under this spoiled grain. If it is only surface rust, you can follow the recommendations for the roof. If the rust is much worse, you need to determine the thickness of the steel that is left. If enough thinning has occurred, the damaged sheets will have to be replaced.
While conducting the interior inspection, check the seal at the base. If there is evidence that water has entered the bin at this location, clean the area and apply new sealant. This may be done on the inside or outside, depending on the type of sealant used.
In the event that the stiffeners are on the inside, check for buckling, proper anchoring, rusting, effective splices and any other abnormalities. Be especially diligent in checking at the base of the stiffeners for proper anchoring. If there are any such problems, corrective action should be taken before filling the bin.
FILLING AND EMPTYING
Proper inspection and maintenance can go a long way toward preventing steel bin failure, but proper filling and emptying of grain bins are equally important. Because they are thin-shelled structures, steel bins rely on uniform hoop pressures to maintain their structural integrity. It is therefore essential that they be filled in the center and unloaded from the center.
When filling a storage unit, the grain should fall straight down into the center of the bin and be allowed to flow out to the sides, following the filling angle of repose. In this way, uniform pressure occurs around the entire circumference of the bin. If this method is not used, the bin will start to become obround (deviating from its circular shape), which can cause many problems.
Likewise, the center well should be used for unloading until the grain mass has reached the bottom of the bin. If the center well were to plug, an auxiliary well could be used if it is within 25% to 30% of the bin radius of the center well. For example, a 60-foot-diameter bin has a 30-foot radius, meaning the 30% figure would be 9 feet.
Never use an unloading well outside of this distance until the center has reached the floor. To do so will almost certainly cause a bin failure. Using an unloading system close to the wall first relieves the uniform hoop pressures, causing the bin to buckle one-third to one-half of the way down. Although it is hard to fathom, just a few 100 bushels (several dozen tonnes) drawn out this way can cause a failure.
If you have to unload the bin from the side for truck or railcar filling, a flume must be used. If a flume is used, some grain must be unloaded through the center well prior to refilling. If this is not done, the bin will become obround.
More grain is drawn off of one side when using a flume, resulting in uneven pressures that cause some ob-rounding or egg shaping. Filling the bin without relieving these uneven pressures adds to this situation. Emptying the bin and refilling it multiple times without relieving this uneven pressure increases the ob-rounding, which stresses the bin and the roof connections.
General observation of the bin should be done on a regular basis. Every time you walk by the bin you should look at it to see if you notice anything out of the ordinary. A more formal inspection should be done on a regular basis. It is up to the facility managers to determine if that will be once a year, every time the bin is emptied and filled or some other schedule.
A checklist should be created to make sure all necessary checks are made during each inspection. The checklist on this page is an example of how one might be structured, but it is by no means inclusive of all the items that need to be checked.
Proper use and upkeep of steel grain bins will enable grain facilities to use these storage units for many years.
Harmon Towne is retired from Brock. He can be reached at firstname.lastname@example.org.