Reprinted from GRAIN JOURNAL March/April 2014. Written by Wayne Bauer, safety and security director, Star of the West Milling Co., Frankenmuth, MI; (989) 652-7026.
Over the years, we have seen a number of deaths from grain engulfments, where people have been totally engulfed by grain, when the steel bin they were standing near split open, and the grain rushing out buried them.
If a bin that has an eave height of 80 feet or more ruptures, the grain can spill out and reach points 250 to 350 feet from the original sidewall.
I posed this question to Dr. Carol Jones at Oklahoma State University and asked what the typical angle of repose is for various commodities at different moisture levels. She shared that “wheat is about 25-27 degrees, and corn is about 22 degrees, according to some sources. Other sources state that 13% moisture corn is at 28 degrees, while 23% corn is at 38 degrees. It is pretty intuitive that the wetter the grain, the steeper the pile will be. However, not all industry data sources report that difference.”
It would be alarming to learn how many people have died in this manner over the past 10 to 15 years.
During that time, the grain industry has been experiencing an increasing number of steel bins collapsing and concrete silos blowing out.
Alarming statistics of grain bin failures
I asked Nationwide Agribusiness Insurance Company how often the commercial (off-farm) side of the business experiences bin failures due to a number of factors including aging of the bins, improper loading and unloading, structural defects, weather, and seismic activity. The insurer estimates that the number of catastrophic failures may be in the range of six to eight per year.
Sidewall failures that do not end in a complete collapse of the bin may be one or two times the number of catastrophic failures, six to 12 per year.
Nationwide was extremely helpful in assisting with this article. The company’s limited sample size, which is not necessarily representative of the entire industry, indicates the following types of bins involved with these bin failures, where the data was known.
Ages of bins involved with collapses, according to Nationwide’s unofficial records:
- 37% were 1 to 15 years old
- 23% were 16 to 25 years old
- 40% were 26 or more years old
Capacity of bins involved:
- 17% had a capacity of 10,000 bushels or less
- 55% had a capacity of 10,001 to 100,000 bushels
- 28% had a capacity of 100,000 bushels or more
Bin diameters in feet:
- 16.7% nine to 22 feet
- 26.2% 24 to 30 feet
- 35.7% 36 to 60 feet
- 21.4% 60 to 118 feet
In the case of steel bins, the opinion prevails that the industry is seeing 40-50-year-old bins that were badly in need of repair collapsing, and the bins were experiencing pressures they were not designed to handle. Randy Tinker, P.E. at Nationwide has pointed out that the industry actually is experiencing a higher number of new steel bins falling than old bins.
New grain bin construction
Tinker states, “Soil testing and the engineering design of bin foundations are critical as bins get taller. If there is no soil-bearing information available for a site, the default conservative value used by engineers is a 1,500-psf capacity for small farm bins. If a bin eave height exceeds 30 feet, this assumption no longer is safe.
“Rodney Carpenter with Clear Creek & Associates Inc. points out that the large 750,000-bushel and larger bins require foundations that need allowable soil pressures of 4,500 pasf and higher, which many soils are not able to accommodate.
“For new bins, the main issue is inadequate foundations. This can be avoided by having a geotechnical investigation conducted of the proposed bin site and then providing information to a professional engineer to design the foundation.
“The owner also should hire a materials testing company to verify the installation of the foundation. The testing company should inspect the forms and rebar placement prior to the concrete arriving on-site to verify form dimensions and rebar (size, grade, and placement). The testing company also will collect concrete samples for test cylinders to verify that the concrete meets the requirements of the design.
“The larger the bin diameter, the more chances for differential settlement because of the potential for verifying soil conditions at the site. It is important that the foundation excavations be inspected by an experienced soils engineer to determine if soil conditions could result in differential settlement at the site. The loads for grain bins have become so large that it is important to involve a professional engineer in the design of the foundation, and it is critical that the connection of the bin to the foundation be designed properly.
“Rodney Carpenter also points out that normal foundation settlements of two to three inches will cause a differential settlement again half of the total settlement. Many manufacturers limit differential settlement to one to two inches, or the warranty will not be valid.”
After the first grain bin fill and unload
- After the proper preparation of the ground, construction of the foundation and pad and the actual bin erection, the next biggest concern, according to Sid Fey with Nationwide, is the first fill and unloading of the bin.
- When full, the integrity of the anchor system needs to be inspected. Anchor bolts should be inspected for movement, straightness, and connectivity to the ring wall.
- The stiffeners need to be inspected for uniformity and straightness. The reason I prefer outside-stiffened bins is the fact that you can see the action of the forces upon the stiffeners, when the bin is full.
- All sidewall, stiffener, and wind ring bolts and connections need to be observed. Are there missing connecting bolts or any stiffener connections and connection plates that do not align, are damaged, or twisted in some way?
- Are there any obvious bulges or flat spots on the bin sidewall?
- Does the connection of the sidewall to the roof look uniform, and is the roof’s overhang uniform all the way around the bin?
- Is there any problem with the bin door frame? Many times, the first sign of bin problems show up at or near the framework not the bin door opening.
- Take lots of photos, and document the photo location and item photographed for future points of reference. If the bin is stiffened internally, it will be necessary to complete the inspection of bolts, stiffeners, and wing rings after the bin is empty.
- Sid’s preference would be that if a sidewall spout is installed on the bin that it not be used for the first fill and unloading cycle of the bin.
- Once the bin is unloaded, complete the same external inspection of the structure observing what was listed above needs to take place. Make sure everything on the outside is still tight and uniform. Take additional photos that match the photos taken when the bin was full. Compare photos.
- Go inside, and look for any floor damage or signs of settlement. Some is expected, but you should know the difference between normal settlement and excessive settlement based on the engineering completed for the foundation.
- Check the floor openings and operation of floor gates. Look for any cracking around the openings. Check the operation of the gates; they all still should work freely. And, while you’re at it, clean out the aeration runs.
- If you have temperature cables, make sure they still are hanging at the appropriate height. You do not want them tangled with unload or sweep augers in the future.
- Inspect the inside of the man door opening for any signs of internal failure.
Sid Fey also mentions that “many of the sidewall failures get turned in as wind damage, when they really are associated with improper loading or unloading or the result of being an older bin that may be worn out or was not designed for today’s heavier grains.”
Failures witnessed in the field
Dan Wambeke with Scafco Grain Systems Co. shared some interesting observations in this area with us. He commented that “we have a lot of aging corrugated galvanized steel grain storage bins here in the Northwest. I know of many that are approaching 40-50 years of age now.
“The primary failure mode I see here is via field-applied openings in the bin walls to add access doors, giant doors, or auger outlets. Millwrights can install anything, but their knowledge of how their addition to the bin might be affecting the overall structural integrity of the bin may be lacking, and portions of the bin fail around their installation.
“I have seen minor bin failures caused by non-reinforced openings installed in bin walls for inclined discharge augers or above floor aeration tubes. The giant doors often added after the fact by millwrights or contractors are usually strong enough, but their connection to the bin wall is lacking.
“Rarely do these bins completely fail by rupture. They just rip several sheets and seams open and spill a few hundred bushels. The vertical stiffeners seem to hold the bin together.
“Also, I have seen two partial bin failures due to the operator not installing all the latches or crossbars in giant doors throwing more force on the door jambs than they were designed to resist.”
Wambeke also commented that there have been two classic bin failures resulting in complete rupture and collapse of the bin in his 35-year career in the Pacific Northwest.
One happened around 1980 in central Washington, when a bin ruptured and collapsed due to under-design and failure of a field-applied factory-built access door.
A second collapse killed a man who camped in the shade of a Parkersburg bolted steel bin that had a questionable field-installed giant door. The bin ripped open during a summer night and buried the pickup camper and the man.
Wambeke shares the moral of the story. “Beware of field-applied openings. Inspect them and the bin regularly for any changes.”
Taking grain bins out of service
Nationwide’s Randy Tinker points out that the decision to take a bin out of service should be based on an annual inspection of the bin. Items to be inspected include foundation, anchor bolts, roof-to-sidewall connections, wall stiffeners, wind rings, wall sheets, roof sheets, and bin bolts.
Rodney Carpenter points out that when rust starts to creep in, and doors start to bulge, and many small cracks show up, it’s time to have it pulled out of service until evaluated. If you no longer trust the integrity of the bin, or you are not willing to pay the expense to maintain the bin in good condition, then the bin should be taken out of service.
Take into consideration the value of the man hours needed to fill and empty a bin. Smaller bins that need to have portable augers set up to fill and some type of manual unloading system to empty are not as economical as a larger bin that is filled by a permanent leg and emptied by a permanent sweep auger. Many times, the economics of the bin can be the determining factor on when it should be retired.
Issues with uneven forces from unloading
Another issue that may be the primary cause of most collapses is uneven forces due to unloading corrugated steel bins from an intermediate sump hole when the center hole plugs. Bins are designed to handle equal pressures in one direction.
Train and retrain employees always to unload corrugated steel bins according to the bin manufacturer’s recommendations. All manufacturers recommend that bins be unloaded from the center sump hole, before any intermediate sumps are opened. There should be no variation to this unloading rule.
Steel tank failures often are minor affairs involving the loss of a few hundred bushels, but when they collapse, the results may be catastrophic.