Load Combinations

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ThoroughPM

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I wanted to reach out for advice-I am doing preliminary design on a Silo foundation. Using the load combinations in ASCE 7-10, the governing load was generated by equation #7 (0.9D+1.0E). The result of that load is minor downward force and a very large overturn moment.

However, equation 2 (1.2D + 1.6L + .5S) resulted in a very large downward force. This combination also seems much more likely to come into play based upon the area that I live in (Dayton, Ohio). I wanted to combine the results of the heavy downward force resulting from equation #2 with the large overturning moment resulting from equation #7. My main concern is that combining the two load cases will result in a very conservative design. However, I don't see how I can justify not considering the live load for the silo (1124kips) at all and using equation #7.Curious to hear any one else's opinions here.

 
Silo's can be tricky, just like large oil tanks. Since they are not always full, you have to ensure there are no problems in the empty condition. The large downward force in Equation 2 could counteract the overturning moment, so it won't be as conservative as you may think. I would not recommend adding the load combination results together.

Therefore, I would design the foundation for the empty condition where the overturning moment controls. Then check the foundation design with the silo fully loaded to check for adequacy. Remember, grain is only a live load when it is being moved around (per some reference manuals).

One reference manual, the Handbook of Farm, Dairy and Food Machinery Engineering, states that "The loads caused by the grain stored within a grain bin or storage building can be characterized as either a dead or live load depending on how they act within the structure. The variation in load type (dead or live) is caused by the emptying and filling of the structure which creates uncertainty in the magnitudes of these loads. Grain loads caused only by the weight of the grain are normally considered to be a dead load. An example of this would be the total vertical load transmitted by the weight of the grain to the foundation. However, design codes and standards note that because of uncertainties in the lateral pressures caused by granular materials that lateral pressures are considered live loads and ultimately have a higher load factor applied to them."

I found the above quoted reference with a quick google search. I have not performed a silo design, but have designed several oil tank farms in remote areas. We always design for empty conditions first, then back check for fully loaded, then 50-75% loaded with lateral / sloshing. The sloshing usually ends up controlling the overall foundation design, but it is an iterative process and can take a while to get correct. I believe it to be similar to your situation and hopefully the above helps with your design.

Good luck.

 
Well, I don't know what your foundation looks like, but I agree that combining the results of LCs 2 and 7 may not be conservative -- not if the design is controlled by uplift.

You also have load combination 5, which I would interpret as a half-full silo subject to an earthquake. If you wanted to check it 75% full as Bly suggests (and based on knowledge of how full the silo likely is most of the time), I'd say that's reasonable.

Regarding your statement about certain combinations seeming to be more likely due to geography... that's why environmental loads are variable based on your location, correct?

 
You don't pick one load combination over the other; the foundation has to meet the demand for both (not concurrently). In other words, you need to check the foundation twice if that makes sense...

 
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