load on walls of grain containment building

huangyhg

load on walls of grain containment building
i have a friend that is constructing a grain containment building.  the overall size is 37.5ft wide x 72ft long with load-bearing walls constructed of cca/acq lumber as detailed below.  a tarp frame kit will be added to the top of the walls to complete the enclosure
the stub walls are 4' high and are constructed of 6x6 #2syp cca posts placed on 4' centers (36-40" of post set in concrete below garde, 4' above grade).  the posts are lined on the inside with 2x6 t&g #2syp acq planking.   modeling the grain as a fluid, i calculated the force on the 72' walls to be 57,000 lbs distriubted across 18 posts.  this would be an average load of 3000 lbs acting perpindicular to the post 1.5' above grade.  the load on the 37.5' wall also works out to about 3000lbs per post(30,000lbs across 9 posts).  is this design sufficient, or do we need to add some support cables across the width of the building?      
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what are the reactions supplied by the 'tarp enclosure' manufacturer for your locale. hope they include wind and snow loads. then do a beam with one fixed end as a cantilever. put in the loads and see if the post will snap or deflect at the top too much.
calculating the pressures as though the grain were liquid can overestimate the pressure considerably- depending on how it's done.
if the grain is leveled flat with the top of the posts, the "liquid" approach will overestimate the pressure.  if the grain is piled up in the center, and you calculate the pressure using the total height, you'll overestimate the pressure.  if the grain is piled up, and you figure it as a liquid just using the depth at the edge, you could be over or under.
the grain-pressure problem is essentially a soils problem, and post strength would be limited by soil strength among other things- suggest you find a local civil engineer and/or hunt up references on bin design (structural engineering handbook has a section on the topic; aisi has a book available, among others.)
how do you empty the grain?  it occurs to me that bumping the posts with a front-end loader could be your major post-design condition.
note that if you put cables across, you are assuming that loading is always symmetrical- which may or may not be a valid assumption, depending on your method of loading/unloading.
i believe the liquids approach will work only in certain circumstances due to the extra issue of angle of repose.  check with a structural engineer.
griffy