Concrete Clarifier Help
Concrete Clarifier Help
(OP)
I am designing a 180' diameter concrete clarifier and I have some problems to work out. Does anyone design concrete clarifiers out there?
1) I read ACI 3502-04 and it suggests using a membrane slab with no construction joints. I think that is impossible for a 180 diameter clarifier. That is 472 yars of concrete. I would prefer to use radial construction joints at 30' on center with an expansion joint at radius = 60'.
2) My other problem is vertical construction joints in the wall. Do I weaken the plane at the construction joint by decreasing the section (sawing or forming a joint), seal it, and provide a water stop or do I just provide a waterstop without weakening the plane at the construction joint? Is it better to use a vertical key or dowels at the construction joint to trasfer the out of plane shear?
3) How do I calculate the base shear due to thermal expansion outwards so the wall does not slide off of the slab?
1) I read ACI 3502-04 and it suggests using a membrane slab with no construction joints. I think that is impossible for a 180 diameter clarifier. That is 472 yars of concrete. I would prefer to use radial construction joints at 30' on center with an expansion joint at radius = 60'.
2) My other problem is vertical construction joints in the wall. Do I weaken the plane at the construction joint by decreasing the section (sawing or forming a joint), seal it, and provide a water stop or do I just provide a waterstop without weakening the plane at the construction joint? Is it better to use a vertical key or dowels at the construction joint to trasfer the out of plane shear?
3) How do I calculate the base shear due to thermal expansion outwards so the wall does not slide off of the slab?






RE: Concrete Clarifier Help
1) If that's what they say, they're wrong.
2) You need to provide construction joints in the wall with waterstop that align with the construction joints in the slab. No weakening is necessary, unless you want to chamfer the joint.
3) The wall should be supported by the slab via dowels. You'll need to transfer the out of plane shear and bending moment through these dowels. Once this is done, the thermal shouldn't be a problem.
It would be a good idea to find an existing clarifier and look at the details. Everyone's look pretty much the same. If you don't have typical construction joints with waterstops, take a look at those also. I'd send you ours, but I don't know if my bosses would appreciate it. But usually the municipal designs are pretty much an open book and you can review those for ideas.
RE: Concrete Clarifier Help
Anyway, What are we doing at work on Saturday???!!!
RE: Concrete Clarifier Help
I've seen toes and heals (like retaining walls) at the bases of walls of clarifiers and tanks and I have not seen toes. I don't know how the tanks and clarifiers without toes are tranfering the base moment into the soil. There is a base moment at the bottom of the wall much like a retaining wall. I just design the base slab at the wall like a retaining wall. Can anyone explain how the walls without toes deal with that base moment?
RE: Concrete Clarifier Help
Use the pinned case to design the wall horizontal steel and the fixed case to design the wall verticals.
The heel (beyond the wall) is handy to put in the circumferential reinforcing, add hooks to the radial steel and for overturning. As a minimum, I'd put in a heel length as thick as the wall.
I like to do an overturning type calculation with the calculated moment from PCA on just the thickened slab portion. Note that the liquid in the tank can resist this overturning. Beyond the thickened portion, you can reduce the slab thickness to about 8 inches and use the "membrane" slab as mentioned in ACI 350.
As far as being in on a Saturday, I got to keep ahead of you guys who are learning to design clarifiers.
RE: Concrete Clarifier Help
RE: Concrete Clarifier Help
As far as the circumferential steel in the slab, you need to multiply the shear from Table A-12 by pi times the radius to get a total force. It's similar to a pressure load. Design the slab steel for this force. The easiest way to picture this is that it's the difference between the pure pipe stress and the stress obtained from Tables A-1 through A-11. As you'll notice, the stresses drop down to nearly nothing at the bottom of walls. If you look at Figure 10, you'll see a illustration of this effect. These stresses need to go somewhere.
RE: Concrete Clarifier Help
I'm saying that the toe is on the exterior of the tank and the heel is on the water side. When I am checking overturning, I look for a factor of safety greater than 1.5. But I also check the soil bearing pressures for overturning using a retaining wall model much like the retaining wall examples in CRSI. I use the base moment at the base of the wall as the overturning moment. I then calculate resistance and bearing pressures using the toe (outside of the tank) and a portion of the tank slab (the heel) to resist the overturning moment, weight of the wall and water over the heel. I find it difficult as times to keep the bearing pressures within allowables if the tank wall is high. Is my methodology correct?
RE: Concrete Clarifier Help
I'm not familiar with the specific construction here. But a cylindrical shell filled with liquid resists the outward force of that liquid by hoop tension in the shell, not by being cantilevered up from the ground. Also, a cylindrical shell can resist some degree of uniform rolling moment applied to the end- see "Formulas for Stress and Strain", for example.
RE: Concrete Clarifier Help
vincentpa, as long as you're using unfactored loads, it sounds like you're doing it right. But I usually don't have an overturning issue. If you extend the heel (by thickening) so that more water can be used to resist your overturning it will shorten your toe. Plus you have more weight in your heel.
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RE: Concrete Clarifier Help
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As far as expansion joints, I never seen them in a round clarifier. Even for a 180 ft diameter tank, I would stay away from them. I'm not sure where you could put them except a round expansion joint at the thickened slab/membrane slab interface. Anything continuing through the wall would void the hoop analysis. Besides, expansion joints are nightmares to design and construct.
RE: Concrete Clarifier Help
If I take the shear from table A-12, it gives me 9423# (including LF=1.7). If I take that shear x pi x radius, I get a shear force of 2664 kips. That sounds really high.
RE: Concrete Clarifier Help
I think when you work out your wall steel, this will seem reasonable.
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i see the logic and completely understand why this needs to be done. What a detailing nightmare!
RE: Concrete Clarifier Help
2) You're right. I usually use working stress and use a low allowable of 16 or 14 ksi. I used 54 ksi to account for the .9 reduction factor.
You can't trifle with 180 foot diameter clarifier designs. They're large structures with significant loads.
If you think they're bad wait until you do a digester that's 80 foot in diameter and 35 ft. tall. 36 inch thick walls, three layers of reinforcing etc.
RE: Concrete Clarifier Help