silo foundation overturning

huangyhg

silo foundation overturning
if seismic overturning is based on the live load (bulk storage) in the tank, should the live load be used to assist in resisting the seismic overturning?  
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for a storage stucture, i would think you would check it both ways, empty and full.  there's no telling when the earthquake will hit, so you can't count on the extra mass all the time.
in this case, when the silo is empty, wind controls. i plan on checking it for seismic stability which would be for the tank full, and wind stability which would be when it is empty.  
i would say the answer to your question is that maybe only part of the weight can be available for stabilization, for earthquake shakeout has a vertical component that can have full lateral force at the same time than the weigth being lifted (or pushed upwards by some amount). however for bulk dry solids i would say that maybe average one should encounter over 70% of the weight stabilizing. hence the safe approach will be to use less than average stabilizing, say a characteristic value expected to be counteracting lateral forces at about 50% of the weight. for liquids maybe about the same values or a bit less should be standing.
that seems reasonable.  it's conservative, but the 50% or so allows some use of the weight of the bulk solids.  
ctw,
the amount of product that may be used to resist seismic overturning is entirely depended upon the details of construction.  
if the silo is tower or skirt supported, then all of the product that is present will resist overturning.  
if it is a ground supported silo, its more complicated.  for an unanchored silo, the amount of uplift resistance from the product will depend on the thickness, material strength and details of construction of the silo bottom at the shell-to-bottom joint.  if the silo is anchored, the uplift resistance of the silo wall will not include any product component and will be entirely dependent on the anchorage details.
in addition, the foundation design will also govern.  if its a solid slab, you may be able to count all of the product.  if its just a ringwall foundation, you will not be able to assume all of the product is effective.
unfortunately, it's one of those "depends" answers.  more info would be need to provide proper guidance.
hope this gets you started.
steve braune
tank industry consultants
stevebraune-
that's helpful.  the tank is skirt supported with a slab foundation.  the thickness of which will be dependant upon how much material i can include in resisting overturning.  which from your post, i can use the entire weight of the material.  this is what i was assuming in the preliminary design, but wanted to get some other opinions.
can you elaborate on why only some of the material weight can be accounted for in a ringwall type foundation?
ctw,
imagine this situation.  a flat bottom silo for granular produst storage is anchored to a ringwall.  the unit uplift seismic force (lbf/foot) in the silo wall exceeds the unit weight of the silo roof, wall and the foundation.  upon uplift, the ringwall will pick up part of the granular product that is situated directly over the inner portion of the ringwall.  in addition, a "wedge" of product will also be picked up... how much will despend upon the internal friction factor, angle of repose, etc.  yes, the seismic activity may cause some product shifting, so therefore, you may wish to count only the product directly over the wall.  hope this helps.
steve braune
tank industry consultants
makes sense.  thanks.
ctw,