Concrete Clarifier Help 1

[vincentpa]

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?

 

[JedClampett]

You need to, quick like a bunny, buy, borrow or steal "Circular Concrete Tanks without Prestressing" from the Portland Cement Association (PCA).
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.

 

[vincentpa]

I have PCA Circular Tanks. PCA doesn't give much guidance on slab design. Nothing does. I get conflicting information from every source. I have the new ACI Environmental 350 series. They talk about membrane slabs. I think membrane slabs are fine for a small clarifier but 180'diameter... I have some existing drawings for clarifiers just not any with a diamter of 180'. What about expansion joints in the slab?

Anyway, What are we doing at work on Saturday???!!!

 

[vincentpa]

Another question.

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?

 

[JedClampett]

The slab needs to be thick enough under the wall to carry the moment from the wall. As a rule of thumb, I make it 3 inches thicker than the wall. It needs to be wide enough to distribute that moment and vertical load to the soil below. Plus you'll need to put circumferential reinforcing in the slab under the wall to carry the shear (Table A-12 in PCA)from the base of the wall. This isn't explained very well in the PCA reference.
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.

 

[vincentpa]

I don't understand the circumferential steel to take the shear. I use dowels in shear friction to transfer the shear into the footing much like the "D" bars in the CRSI retaining wall. I also use a bigger heel to help resist the "overturning". I treat it as a retaining wall with the base moment. My toe can get to be 3' wide. Is this a proper approach?

 

[JedClampett]

If the toe is the portion that goes into the tank, 3'-0" is reasonable. I usually use more due to the layout of the tank.
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.

 

[vincentpa]

No,
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?

 

[JStephen]

"There is a base moment at the bottom of the wall much like a retaining wall."

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.

 

[JedClampett]

JStephen, there is a moment at the base.
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.

 

[rholder98]

I agree with all of the above, except that the PCA reference gives charts for designing the wall as fixed or hinged at the base, so there can indeed be zero moment at the base. So, JStephen is also correct.

 

[vincentpa]

Do you ever put expansion joints in the base slab? I have seen it both ways, with and without joints in the base slab. With a 180' diamter tank I would think that there needs to be an expansion joint.

 

[JStephen]

Re-reading the above, it was the retaining wall analogy that caught my eye. In a cylinder with "fixed" base, there would be a moment- but not as in a retaining wall.

 

[civilperson]

No expansion joints are required unless the tank is restrained by connection to buildings or other structures. The change in dimension or the change in internal stresses can easily be handled by the reinforced concrete. Construction joints can be accomodated with bulkheads, (allowing the continuity of the reinforcement), but any volume under 1000 cubic yards of concrete can be placed in 12 hours with a pump and a spare and batching plant capacity of eight ready mixed trucks per hour.

 

[JedClampett]

As far as the PCA giving a hinged case, it's much easier to model than design. If you can figure out a way to design a watertight slab/wall connection that takes significant shear and no moment, I'd like to see it. I recommend using both cases and taking the most conservative values from both.
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.

 

[vincentpa]

JedClampett,
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.

 

[JedClampett]

Yes, you have a large diameter tank and I would expect the shear forces to be quite high. Now multiply that by 1.3 and divide by 54 ksi. I get 64 square inches or 32 #9's T & B in the slab. If you want to keep a 6 inch minimum spacing, you'll need a 15 ft. wide thickened portion. Other options are to put in three mats of circumferential slab steel or reduce the spacing to 4 inches.
I think when you work out your wall steel, this will seem reasonable.

 

[vincentpa]

How do you transfers 2664 kips from the wall into the slab? What would the dowel requirement be? What would the arrangement look like?

 

[vincentpa]

Would I not be multiplying by 1.65 instead of the 1.3 for the sanitary factor in tension? The tension would then be 4396 kips. Divide that by 60 ksi and I get 73 sq. in. That is 73 #9. Holy Cow! Why did you divide by 54 ksi instead of 60 ksi?


i see the logic and completely understand why this needs to be done. What a detailing nightmare!

 

[JedClampett]

1) Transferring the shear is done at 9423 lb./ft. Practically any thickness wall with any reinforcing can do that.
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.