structural design of a clarigester

huangyhg

structural design of a clarigester
has anyone ever heard of a clarigester, and if so, are you familiar with the stuctural design of one?  a clarigester is a combined clarifier and sludge digester, and they are usually circular.  they are divided into 2 chambers, with one on top of the other.  a reinforced concrete slab with a hole in the middle sepparates the two, and has a 1.5" layer of grout as a topping.  
i am reviewing the structural design of one from 1964 that shows radial and circular reinforcing in the elevated slab in one layer only, with 1.5 inch clear cover from the top.  the diameter of the slab is 40 feet, with a 4 foot diameter hole in the center, and the slab thickness is only 6".  my analysis shows that there are positive and negative bending moments, (both radial and tangential) but they only have rebar at the top of the slab.  i think the only way this works is if they considered the grout topping to act composite with the concrete slab.
it is my opinion that 6" is too thin for a slab with this span, and that the slab should have reinforcing for both positive and negative radial and tangential bending moments in 2 layers of steel.  i also think that the grout topping should not be considered to act composite with the slab.
any comments are gratly appreciated.
does this always have fluid on both sides?  in which case, the only load would be dead weight of the concrete, with even that reduced by buoyancy?
it's possible they assumed the outer edge was free, not fixed, and essentially eliminated much of the moment in that way.
you could develop both positive and negative moments using the rebar near the center of the slab.  whether the amount of reinforcing actually present is adequate for this would be another matter.
i saw a post in this forum a while back about the same type of design and have been curious ever since, i would like to learn more of this design.  from what little i re  
roark's formulas for stress and strain has formulas for circular plates with holes in the middle.  i make an excel spreadsheet and crunch the numbers.  the radial moment goes to zero at the edge of the hole, but the tangential moment is at its maximum, at least if you make the assumption of a fixed outer edge for the plate.
as you say, roarke's formula's for the fixed end (table 24, case 2e) show positive and negative bending for b/a=0.1.  seeing you have no positive bending strength, see if the plate can resist the weight of the concrete and grout  through negative bending alone.  model the plate as a cantilever beam, 18 feet long, with decreasing uniform load.  i get a negative moment around 6.2k'/ft at the support.  #6@6" could work, but the concrete may be overstressed.
are you saying that there is a decreasing load on the cantilever because the design strip would really be a segment that is decreasing in thickness?  i will try your suggestion, thanks.  i think an analysis using this approach may be the load path that is functioning to keep the slab from collapsing, but i think that the slab may have some significant cracking because i see compatability problems with deflections, especially near the inner edge of the slab.  i have not seen the slab myself yet.
i have the seventh eddition of roark's, and i analyzed it with table 11.2, case 2e outer edge fixed, inner edge free, and i get significant radial and tangential moments.
when i design similar structures, i provide reinforcing steel for both positive and negative radial and tangential moments, and i design the slab using roark's formulas for stress and strain, and i design the rest of it using the pca guide for circular concrete tanks.  maybe in 1964 versions of these references were not available and the designer tried to use a rational approach that was easy to use.
bjb,
i am still not sure exactly how this thing works, but is it possible that the original design was not completed using elastic theory.  it may be that it was completed using plastic theory and the original designer proportioned all of the bending forces into the negative moment resistance, as yield line theory allows you to do this, although i have never designed anything this way.  i assume that this is one of those situations where you are stuck trying to prove something works that has been their for 40 years, and are struggling to see how. (been there and done that!)
you hit the nail on the head aggman regarding trying to justify something that apparently works in the real world but not on paper.  as for your suggestion, i hadn't thought of that, and it is not how i design either.  
there is a decreasing load on the cantilever because the unit width at the support carries a pie shaped section.  yes, the slab may crack in the areas that are in tension.  (i have the 5th edition of roarke, and it shows significant radial and tangential moments.)  i'm not saying i would design the slab this way.  i'm simply trying to explain why the slab isn't sitting on the base mat.