base slab design for a circular concrete clarifier
base slab design for a circular concrete clarifier
(OP)
I have seen several designs for circular concrete clarifiers that have relatively thick base slabs, ranging from 16 to 20 inches thick, with the wall to base slab detailed as a hinge. It seems to me that this is more thickness than required, since I don't see the slab being heavily loaded. These have been clarifiers where a buoyancy force was not a factor in the design.
How do you determine what the thickness of a base slab for a circular clarifier should be? The PCA document for the design of circular concrete tanks without prestressing seems to imply that the base slab be designed as a mat foundation. Since you only need to support the weight of the concrete walls, could you just design a strip of the base slab under the walls to transfer the weight into the soils? When I look at it like this I get a low soil bearing pressures that are usually a few hundred psf.
How do you determine what the thickness of a base slab for a circular clarifier should be? The PCA document for the design of circular concrete tanks without prestressing seems to imply that the base slab be designed as a mat foundation. Since you only need to support the weight of the concrete walls, could you just design a strip of the base slab under the walls to transfer the weight into the soils? When I look at it like this I get a low soil bearing pressures that are usually a few hundred psf.






RE: base slab design for a circular concrete clarifier
Hope this helps
RE: base slab design for a circular concrete clarifier
The examples I am puzzled about are in areas where the groundwater is well below the slab base and where flooding is not an issue either
RE: base slab design for a circular concrete clarifier
Note that the PCA book on design of circular water tanks mentions that you need to design the slab for a hoop stress, but it never really tells how. You'll need to add hoop bars in the slab. See Table A-12 for the shear you'll have to design for.
By the way, we very seldom have high groundwater.
RE: base slab design for a circular concrete clarifier
best tincan
RE: base slab design for a circular concrete clarifier
Where a hydrostatic head is a problem, we put pressure relief valves in the tank. We still assume that some hydrostatic pressure will result on the bottom of the base slab, and design according to the tables in the PCA book. We allow for the groundwater getting about 2 feet above the pressure relief valves.
RE: base slab design for a circular concrete clarifier
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RE: base slab design for a circular concrete clarifier
Anyway, you're right. I thicken the slab until until the moments equal. If the clarifier has a sloped bottom for process reasons, this works wut nice.
RE: base slab design for a circular concrete clarifier
I have another question for you regarding the base slab radial bars. In your opinion, do the radial bars in the slab need to meet the 12" max. spacing requirements of ACI 350-01 section 10.5.4? I'm talking about the radial bars that are not in the thickened area of the slab.
The commentary to the ACI section states that soil supported slabs such as slabs on grade are not considered structural slabs in the context of that section unless they transmit vertical loads from other parts of the structure to the soil. It then goes on to say that reinforcement if any, should be proportioned with due consideration of design forces.
RE: base slab design for a circular concrete clarifier
By the way, I've talked to some General Contractors about the easiest way to place the slab bars. There are two choices. One is to keep an orthogonal arrangement. the other is to have circumferential bars at different radii spaced at 12 inches and radial bars of defined lengths that get closer until they splice into a smaller number of bars. The first method means a lot of straight bars, but varying bars at the construction joints. the second gives you well defined construction joints with only circumferential laps. He preferred the second way. He said he gets a price from the reinforcing in dollars per ton and after that, he doesn't care how easy the bars are to fabricate. The second way makes his slab pours easier and that was money in his pocket.
RE: base slab design for a circular concrete clarifier
RE: base slab design for a circular concrete clarifier