Section 13 example of Circular Concrete Tanks without Prestressing

[Guastavino]

JEDClampett and other Concrete clarifier folks,

Can anyone enlighten me on where the "38,000 lb-ft" of moment comes from in section 13 of the thread title book (on page 16, second paragraph)? Seems like it just was pulled out of thin air to use in the example.

I've got a 20' diameter, 14' tall clarifier with the slab/wall tied together. I'm trying to use this guide since it's my first clarifier rodeo, but am stuck on where that number comes from.

Thanks!

Nick

 

[JedClampett]

It's an assumed value for this example.
So yes, pulled out of thin air.

 

[Guastavino]

Jed,

Thanks much, I thought they might have just made it up to work the example. So in a situation where you have no upward hydrostatic pressure and uniform soil characteristics, I'm thinking the applied moment is next to zero at the base. I do have a continuous slab (that is pretty thick at the edges) at the base, so I'm thinking it's closer to Fixed based than free, but I'm thinking I design for the envelope of "fixed-base free-top" and "hinged base free-top" and neglect the "moment applied at base section".

Does that seem right? I'm not 100% confident because I can't think of a situation where a base moment would be applied other than maybe a hydrostatic upward pressure.

Thanks,

Nick

 

[Guastavino]

Also, I'm beginning to figure this out more and more as I study this PCA document. It's great, but you have to do some reading between the lines. With that said, just omit my last post and I'll try to restate it:

1. It's my understanding that the only time you would have an applied moment at the base is when you have something like hydrostatic pressure etc. acting on the slab? Are there other times when you could have an applied moment at the base. I'm having a hard time envisioning when this would apple.

2. So section 11 talks about Wall with shear applied at the base. In a previous thread (good one at that) between Jed and Vincentpa, Jed mentioned that the shear at the base needs to be resisted by the foundation. I'm assuming I have two options:

A) I can resist the tension at the base with the foundation by taking the shear load from the hinged base and multiplying it by Pi*radius and designing hoop steel in the footing for that, thereby eliminating the requirements of section 11. (This is what Jed mentioned)
B) I can resist the tension in the base by using section 11 and increasing hoop stresses in the wall itself. If I go this route, the foundation doesn't need to resist any shear/thrust.

3. If I go with the hinged base, I'm assuming I can neglect all moment interaction between the foundation and the wall. I could envelope it too by using the "fixed-end" moment, but I don't see that as necessary, albeit, I may do it as good practice.

Thanks

 

[JedClampett]

You are correct. I've done quite a few of these and never had an applied moment at the base.
As far as 2., you are correct. But if you reinforce according to option B, you're basically just designing a hoop structure. I've never checked, but if you do it that way, you don't even need the reference. Just reinforce for the pressure times the diameter and make sure the slab/shell joint doesn't crack.

 

[Guastavino]

Jed and other clarifier designers,

Thanks. How are these clarifiers designed for seismic and wind? I'm in a 2009 IBC area with 90 mph wind and low seismic (sds=0.280 or so). I can't see wind having any impact whatsoever. I don't see seismic as a big deal either, other than maybe for the foundations. But I'm not sure where to start on that. The loads/structure is so complex I'm not even sure how I would apply those other than maybe a base shear/overturning on the foundation

Also, are there any important things to consider when specifying the concrete? Like type of cement, etc.

Thanks for all your help.

Nick

 

[JedClampett]

Wind is not an issue. Seismic is. All that water sloshing around adds loads. We have internal proprietary software that one of our guys developed. I've never bothered to get into it up to my elbows, so I can't give a step by step procedure.
ACI 350.3 has a lot of guidance. Whether it can get you over the finish line is another question.
One more thing. The mechanisms also see the sloshing and pressure. And they're more likely to fail. So we have another program to analyze them and give the forces to the vendor so that can consider them for the anchorage.
You are pretty lucky. At that seismic level, you probably could just ignore it and be fine. But you (and I) don't know that for sure.
As far as cement, as long as chemical damage from the soil or ASR is considered, regular cement is fine. We like fly ash, as it enhances some of the chemical resistant properties of the mix. ACI 350 will tell you to keep the w/c down to .45 for water and .4 for wastewater treatment.

 

[Guastavino]

Jed,

Thanks much. I really appreciate it. I'm watching the ASCE short course now to see if there are any other things I need to consider that I'm overlooking. I just now see all the 350.3 stuff. Luckily also, it's only a 18' wide inner diameter clarifier that is only 14' tall with virtually no soil retainage. So, it's about as simple as you get. But I also want to check my blindspots.

Thanks again.

 

[Guastavino]

JED/others,

1. One thing I get stuck on is section 11 with shear at the applied base. It seems as though it's not enough to analyze for Section 7/8 and use the envelope design and call it a day, because they both assume the base is free to move, which obviously isn't the case for a typical clarifier with a base slab and footing designed to take the hoop tension. It seems like the foundation is "applying a shear at the base" even though it's not an external force (it's a resisting force, but the tank walls don't know that). Am I right about that?

If so, It looks like the two options I have are on page 13:

A) It lets me design the hoop bars for the maximum tension from the max point down to the base. (And when it's a 14' tall 18' diameter tank, #5@12" o/c seems to take it all anyway).

or

B) I can use the "average" shear (say V/2) apply it as a load and superimpose it on the hoop stress/bending moment tables I already calculated for the hinged/fixed cases.

2. If I go with "option B" above, then I would guess I would really only need to design the hoop steel in the footing for V/2, but may just go with V to envelope it?

3. It seems like none of the PCA tables are to be used on their own. It seems as though you always have to layer them in one way or another based on how it's all detailed. Am I missing something? The thing that bugs me is that they do a design example for each, but without the layers I'm thinking their design examples don't always mean anything final (such examples as the one in sections 7 and 8 where they give you a base shear and then say later, "oh by the way, you have to resist that in section 11"). Not saying I know a better way to present the information, but I'm just confused how to apply all the layers correctly (and when).

4. Does anyone know of commercial software for this? I'm in the process of developing a spreadsheet, but with hundreds of behind the scenes calcs, it would be nice to check myself.

5. Does anyone know any good references/examples of clarifier tanks other than the PCA and ACI 350? I looked online but didn't find anything that looked valuable.

 

[Guastavino]

Or maybe I'm looking at this wrong? As I study section 11, it says "if the base were free to slide, the reaction at this location would be zero. Therefore, the shear at the base, not including the sanitary coefficient, will be somewhere between 0 and 6238#."

Maybe the design in 9 assumes that the shear is already present? And you can use it stand alone as long as you develop the shear through hoop stresses in the footing? Then I can eliminate using section 11?

Sorry, I'm just a bit confused as when and when not to apply provisions.

 

[Guastavino]

BTTT (Back to the top). Any help greatly appreciated!

 

[JedClampett]

In Table A12, you can calculate the shear at the base of the wall. You integrate (basically multiply it by the diameter) that force per foot to get the force that the base is required to resist. That force is resisted by hoop bars in the foundation. I usually add top and bottom circular bars in the thickened portion of the footing. These can be minimal (5 #5's top and bottom) or substantial (10 #7's top and bottom) depending on the tank diameter and fluid depth.
It is poorly explained in the PCA Manual.
I've also seen many cases where these were forgotten and nothing happened.