grain bin evaluation

huangyhg

grain bin evaluation
i have been asked to give my opinion about the structural integrity of an existing 400,000-bushel circular butler metal storage bin.
the structure is about 18 years old (no records) 96 feet in diameter about 60 feet high and set on a circular concrete foundation (unknown design) 100 feet in diameter.
i looked @ the bin and foundation. there are no cracks in the foundation and none of the metal rib siding show any signs of displacement and all bolts are intact and in place.
however when i took elevations on the top of the footing there is about a 5 陆鈥?inch difference in elevation in a100 feet.
questions how concern should i be about the elevation difference and how far do you investigate?  this is not a public work project and it has been in use since construction.
how about the top of the wall at both points?  are they 5.5" difference also?  what type of soil is it?
did not take elevation on top of bin wall, would required
personnel lift.
soils are most likely gw or gp
generally, if you tilt the base of a tank or silo while holding it in a plane, nothing much happens.  you develop a bit of a lateral loading that could be treated similar to seismic loading
if the base deflects out of a plane, it can cause problems in the structure itself, which would generally show up as buckles of one sort or another.  api-653 includes an evaluation criteria for welded steel (smooth) tanks, but it would not be applicable to a corrugated-type structure.
given that it's an agricultural facility, there's not much telling how the foundation was built, whether a soils report was done, etc.  if it doesn't seem to be having problems otherwise, i'd note the settlement, but otherwise not take any action.
it is possible it was built that way, too.
  the soil loading when full is near 3.1 ksf. if this is less than the allowable, then a bit of differential settlement,(5" in 100'), should not render the structure unusable for the use that it is experiencing unless there is a head house with augers requiring level placement.  even then, shims may be installed for the machinery requirements.
so is it fair to say that the wall is tilted about 3.14 degree (arc tan of 5.5/100) ?  since you dont see any cracks on the foundation then i assume the whole structure is tilted 3.14 degree?  or is it possible they made and error with the foundation and the wall on one side is taller than the other when they build it?  since the foundation is good and the shape is circular, i wouldnt worry about it.  just my oppinion.  
oopss correction.. .262 degree.. forgot to change it to foot.
perhaps it's sloped on purpose for water drainage?  i know you would have bigger problems if you have water in it while full of grain but perhaps there is a need for washdown or some other reason.  just a thought.
stroker has a point there.  sometimes tanks are built where the shell is vertical but bottom slightly sloped- called a "shovel bottom".  with corrugated stuff, you probably wouldn't want to do that, but it might have been tilted intentionally.
it would help to know generally what type of soils are in the area.  around here, you don't expect to see that much movement, but i've seen sites that anticipated two or three times as much settlement.  if you're in an area where movement is pretty minimal, chances that it was built that way are a lot higher.  if it's beside the river or in the swamplands, it could have moved that much.
one quick, easy and unsophisticated way to get an idea if the tank is leaning is to take a 4' carpenter's spirit level, walk around the bin and check to see if the walls are plum at numerous points. plot your results.
not the most accurate check, but if there is a problem it may tell you where to investigate further.