structural design of a clarigester
structural design of a clarigester
(OP)
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.
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.






RE: structural design of a clarigester
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.
RE: structural design of a clarigester
RE: structural design of a clarigester
RE: structural design of a clarigester
RE: structural design of a clarigester
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.
RE: structural design of a clarigester
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!)
RE: structural design of a clarigester
RE: structural design of a clarigester