By: Bob Ehler
The grading classification of steel grain bins can be difficult to determine. Many variables should be considered when calculating the replacement cost of a steel grain bin. The load bearing capacity of the soil the bin is placed on can have a significant impact on the foundation and site preparation cost. The speed and frequency the bin will get loaded and unloaded is considered when the gauge of materials, spacing and number of bolts, etc. are determined. The proximity to railroad can also have an impact on how a bin is engineered as the vibration from railroad activity causes the grain to vibrate which creates stress to the exterior bin walls.
For the preceding reasons it is very difficult to give absolute guidelines on how to determine the grade of a steel grain bin. However, the following guidelines are given to promote uniformity in how we grade steel grain bins.
The replacement cost calculation in the cost manual assumes “typical” site preparation. It is very important for the weight bearing capacity underneath a bin to be uniform. If not, the weight of a filled bin can cause the foundation to shift which would likely result in a blow out. This is especially true of large bins in excess of 90’ diameter. When a bin is placed on soil with poor weight bearing capacity it is not uncommon for geopiers, pilings, etc. to be introduced. This can cause a significant increase in cost. Since this cost is incurred to rectify a substandard soil condition this additional cost is considered an “atypical” cost. It is questionable whether this atypical cost should be reflected in our determination of replacement cost.
Adding stiffeners to a bin transfers roof loads directly to the foundation, resulting in less stress on the sidewall sheets. This allows the use of lighter gauge sidewall sheets without sacrificing structural strength, making a stiffened bin more economical. Stiffened bins also provide excellent structural stability against grain, wind and seismic forces. Any bin with stiffeners will be a minimum of a 4 grade.
Many old bins that are constructed of lighter gauge materials. Typically will have dirt, or metal floor with little in the way of a foundation. These bins will likely not have built in conveying or aeration equipment. Many old government bins fall into this category.
Similar to a 6 grade bin, however these bins will have a minimal concrete foundation and floor. They will typically be filled by portable augers and may require portable unloading equipment or will have a simple unloading auger. These bins may have small aeration fans. Typically, bins at a license grain elevator will not fall into the category. They would likely be found on farms.
The vast majority of all bins will fall in this category. Will have a foundation and concrete floor. Will likely have built in aeration and unloading system. Could be filled by portable or permanent conveying equipment.
These bins will have heavy duty raised foundations. They will be more heavily engineered to withstand the pressure of fast conveying equipment, typically in excess of 15,000 bushel per hour. Being located near railroad can also be cause for a more heavily engineered bin putting it into this category. Bins located at a high speed licensed grain elevator many times will fall into this category due to the bin requiring to be engineered to withstand the pressures of being filled and unloaded numerous times a year.
Greater than 3 grade:
Grading a bin in excess of a 3 grade will be very uncommon. However, bins located in certain Industrial type applications may require a 3+10 to 2 grade to achieve cost. These bins will demonstrate all the attributes of a 3 grade bin. Will certainly be near railroad and be accompanied by very fast conveying equipment.
Licensed Bushel Capacity
To calculate the bushel capacity of a round storage unit it is first necessary to calculate the volume. This is accomplished by the following formula “radius squared x 3.1416 x height”. This will give the cubic foot capacity. The cubic foot is then multiplied by .8036 (or ÷ 1.2444) to convert cubic foot into bushels.
As a grain bin is filled with grain the weight of the grain will compact the grain below. This is known as “compaction”. This compaction will actually increase the amount of grain the unit is able to hold. The formula noted above results in a “bushel capacity” for a storage unit. Adjusting this bushel capacity to allow for compaction results in what is known as the “licensed bushel capacity”. The following factors are applied to the bushel capacity to convert to licensed bushel capacity.
Diameter Compaction Factor
36’ or larger 1.10
The compaction factor rates have been extrapolated from the Federal Warehouse Examiners Handbook. This handbook is used by the USDA and State Grain Licensing agencies when calculating the storage capacity for commercially licensed grain facilities. The licensed capacity should be the “unit of comparison” when comparing the per bushel value of one grain facility to another. On farm storage is many times expressed by using “bushel capacity” rather than “licensed bushel capacity”. This is why it is not uncommon to get differing answers when asking “what is the bushel capacity of your grain bin”. The answer lies in “did they calculate in the compaction factor”?