base slab design for a circular concrete clarifier

[bjb]

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.

 

[rdengsc]

Usually when I design a baseslab for a clarifier, The Uplift due to groundwater governs the design of the slab. I assume that the tank is empty and apply the groundwater pressure to the base slab. Depending on the Diameter of the tank, the Slab can get pretty thick. Even providing pressure releif valves, the uplift still usually governs.

Hope this helps

 

[bjb]

rdengsc, do you design the base slab assuming continuity with the walls, or do you assume a hinged connection? From a detailing standpoint I think that the detail for a hinged base is more difficult than doweling the rebar at each face of the wall into the base. The problem with the continuous base is that moments from hydrostatic uplift in the base have a big effect on the ring tension in the wall. When I design a circular concrete slab I use Roark's Formulas for Stress and Strain.

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

 

[JedClampett]

I use a combination of the hinged and fixed case in my designs. That is, for the moment at the bottom I use the fixed case and for the hoop stresses I use the hinged case. It's a belt and suspenders design. I always detail the wall to slab connection as fixed. As far as thicknesses, I don't see any reason to make the slab any thicker than 2 or 3 inches thicker than the wall. I continue the thickened portion for about 8'-0" or until the water pressure down (in the thickened portion)is about equal to the moment in the wall. I then taper the slab to 8 inches thick. Our company has literally done hundreds of clarifiers this way.
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.

 

[tincan]

We use a combined stem wall / ring footing for the wall support. Hoop steel in the clairifier wall is checked for both fixed and pinned conditions. The floor is provided with relief valves. The floor is cast on a crushed stone /granular base. Floor thickness normally 4 to 6 inches. Floor is normally independent of the wall footing with the floor resting on the wall footing w/waterstop. Have had no problems with the floor or wall. Also, high water tables are not a serious problem in our area.
best tincan

 

[bjb]

JedClampett, sorry to have been away from my post for so long. Are you saying that you extend the thickened portion of the base slab until the moment from the water pressure (wl^2)/2 equalls the moment at the base of the wall? That is how I have designed rectangular tanks without a roof in the past. I also agree with providing circular bars in the wall for hoop stress based on a hinged wall to slab connection, and for providing dowels to the foundation assuming a fixed connection. What was giving me a hard time was the thick base slabs with the wall rebar detailed as for a hinged connection to the base. Of course, I was just looking at someone else's drawings and don't know all of the conditions that directed their design.

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.

 

[SlideRuleEra]

The clarifier foundation designs that I have participated in had thick slabs to keep the structure rigid, not for strength. This was done since even minor distortion of the foundation can cause operational problems.

http://www.SlideRuleEra.net

 

[JedClampett]

It's hard for me to justify someone else's design without seeing it. Heck, it's hard for me to explain designs I did six months ago.
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.

 

[bjb]

JedClampett,

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.

 

[JedClampett]

We always space the bars at 12 inches maximum. We would do the same for any slab on grade.
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.

 

[blake989]

I hate to jump in at the end, but now I have a question. If you provide orthogonal reinforcement in the base slab of a circular tank, how does one design it? I am familiar with radial and circumferential reinforcement steel design in circular tanks (PCA manual), but not orthogonal. Is there a text somewhere for this.

 

[JedClampett]

The orthogonal reinforcing is in the region where the base is just a slab on grade. It's there for temperature and shrinkage. The thickened edge is the load bearing portion.