Building a strong foundation
Feb. 23, 2012
How important is a well designed foundation to the success of your grain bin and facility? The Reader’s Digest Great Encyclopedic Dictionary defines a foundation as “a base on which something rests” or “that part of a building or wall wholly or partly below the surface of the ground that constitutes a base.” In the case of grain storage facilities, the foundation is the footing and wall on which the bin rests. Without a solid base for the bin to rest on, there is the possibility of damage or failure with the system.
It was not uncommon back in the 1960s and early 1970s for many bins to be constructed on foundations designed without the benefit of soil borings to determine the strength of the soil. In many cases there was just a slab and shallow wall poured together. In most cases, this did not present a problem because the bins were relatively small compared to the sizes today. But by the late 1970s, it became imperative that soil borings be taken and foundations designed based on those findings due to the taller and larger diameter bins that were becoming prevalent in the industry. These larger bins caused increased sidewall and floor loadings.
At that time, it was not uncommon for companies to refrain from spending the small amount of money to get soil borings or build the foundations based on the test results. Quite often when this occurred, the cost of repairs was considerably higher than the cost to do it right the first time.
Several years ago, a company wanted to build a series of bins in a row. Soil tests were taken and a foundation design was submitted. However, the buyer decided that the recommended foundation was too costly and put in something similar to the foundation that had been used on smaller bins installed several years earlier. When the bins were filled, two of the foundations started settling between two of the bins, causing the bins to tilt toward each other. The conveyors that were attached to the roof caused roof damage to the bins. This problem continued for a few years with roof repairs being made multiple times. Finally, a contractor was brought in to raise the bins and pour new floors on top of the existing ones to level the bins. The costs to correct this situation were much higher than doing it right at the beginning.
To make sure you don’t encounter a similar situation, it is important to have soil tests performed by a reputable geo-technical engineering firm. Depending on its report, the bin company may furnish you with a foundation or they may recommend that you have the geo firm do the design. One of the questions that must be answered is what settlement is expected. Uniform settlement will not pose a problem, but differential settlement can be a major problem as was evidenced in the previously mentioned case. Even if settlement is not predicted, there is sure to be a small amount. Because of this, downspouts or conveyors attached to the roof must be connected with some type of slip connection to prevent damage to the bin roof or the items connected to the roof.
If towers are built beside the bin or between two bins, the tower foundation will not settle like the bin foundation. The same rule applies again to make sure rigid connections are not made to the roof or sidewall of the bin. Even if the tower is on the same foundation as the bin, rigid connections should still not be made.
It is very important that the reinforcing steel that is put into the wall and footing is properly placed. There have been many confirmed cases where failures have occurred because of not enough steel or improper placement of the steel. It is wise to have an observer on the scene as the wall forms are installed and the steel placed. It is also necessary to make sure the correct strength of concrete is used. Ask the concrete supplier for the mix being used if the contractor doesn’t furnish it to you.
When putting tunnels under the bin for the use of conveyors, make sure the tunnels are wide enough for maintenance access on both sides of the reclaim conveyor. If the tunnel base is below grade level, make sure a good drainage system with sump in installed. Also make sure that the tunnel walls are properly designed and installed to prevent wall failure from the pressure of the ground with a grain load surcharge when the tunnel is located beneath the ground surface. While it is more costly initially to have the tunnel above the ground, there are many advantages when this is done. Lighting becomes less of an issue as well as ground water. It is also much easier to inspect the reclaim conveyor and perform regular maintenance on the equipment.
Getting the floor level
One of the most difficult tasks when pouring the floor is getting it level. It is essential that this be accomplished so that the sweep augers operate correctly. It is also imperative that proper supports be placed at all access tunnels and aeration tunnels. The bin manufactures will give you the necessary information on methods to do this correctly when asked. In many cases they will provide you with foundation plans that call out methods to support the bin wall and any stiffeners that may be over the tunnels. In some cases, it may be easier to use steel I-beams as opposed to using all poured concrete and rebar. The concrete contractor will probably be the best person to decide which way would be preferable.
It is also important to make sure that any aeration tunnels are level. The perforated flooring used to cover these tunnels must be adequately supported and the flooring must be level with the concrete bin floor. If this doesn’t occur, the sweep auger can catch the flooring during the final unloading process, causing damage to the aeration floor and/or the sweep.
Another reason to have a level floor is to make sure the bin sidewall loads are properly transferred to the foundation. This also makes it much easier to get a good watertight seal at the base of the bin. If possible, the edge of the floor or wall that extends on the outside of the bin wall should have a slight taper away from the bin wall to help drain water away from the bin. This must be very slight as the outside stiffeners will be resting on the floor or wall, and they need to be able to properly transfer their load straight down to the spot on which they are resting. It is not uncommon to need shims at some locations to make sure that the bin is level. Doing a good job of making sure the concrete is level will make it less likely that shims are needed.
When the wall is poured, a decision must be made on how to set the anchor bolts. The best method is to set them when the concrete is being poured. It is difficult to get them properly placed so that they line up with the stiffeners. To do this properly, have the bin manufacturer give you the chord measurements between two stiffeners, five stiffeners and 10 stiffeners. If you chose to use drill-in anchors, be sure you use those that are rated for the loads provided by the bin companies. One of the problems with using drill-in anchor bolts is getting the drilled hole deep enough without hitting rebar. Most companies will tell you that the adhesive anchor systems are preferable to the expander types.
Make sure that inspection of the concrete is part of your maintenance plan. Small hairline cracks are not unusual in grain bin foundations and floors. However, if the cracks are more than just hairline, the cause must be determined. Letting these conditions continue without determining the cause and then making repairs can lead to major bin failure. If the problem is at a stiffener, there is concern for major damage to the bin. If one stiffener is not capable of carrying its design load, that load is transferred to adjacent stiffeners. Under these conditions, there may be stiffener and sidewall buckling of the bin. If this becomes severe enough, there could be catastrophic failure. Concrete cracking away from the top or side of the foundation wall also is a warning that something isn’t right. If this situation occurs, you need to get some of the grain out of the bin to lessen the load on the foundation. A structural engineer should then be consulted as to the best way to repair the wall and correct the problem.
Remember, without a solid base, many things can go wrong. You are making a large investment in your facility in the interest of making a reasonable income. Don’t jeopardize that investment by skimping on one of the most important parts of the system — the foundation.