Extending existing circular concrete water tank 1

[txeng91]

I have a client who wants to add an additional 8' in height to their existing water tank. The existing tank has an inside diameter of 22' with 12" thick walls and has a depth of 10'-6". I have not seen the tank empty so I do not know what the conditions are at the bottom or what kind of footing it has, but it felt like a concrete slab when I hit it with the tape measure. The existing reinforcement is unknown so we are having it scanned so we can determine if the reinforcement is adequate for the additional water pressure. My question is how to extend the tank wall up. My initial thought was to dowel into the top of the existing wall and build up on top of it and have a waterproof joint in between the two (Option 1). The builder also mentioned possibly coming around the outside of the wall with the extension (Option 2), which I am thinking might be a better way to go, although I'm not sure exactly how to waterproof that joint. I attached a sketch showing both options. Also, any tips on waterproofing the joint would be greatly appreciated.

 

[JedClampett]

First of all, you have to make sure the remaining portion of the tank can take the hoop and other stresses. If it was designed for a 10 ft. deep tank, it's not likely to be OK for a 18 feet deep tank. But if you can convince yourself that the reinforcing and wall thickness are acceptable, option 1 is the way to go. Greenstreak makes retrofit waterstop. Some engineers will suggest strip hydrophilic waterstop, but I'm strictly a PVC guy. Make sure dowels have adequate edge distance and spacing to actually do something. Staggering them will help spacing. Place your hoops on the outside of the dowels to help edge distance as moments won's be too large.
Get a copy of the PCA pamphlet "Circular Concrete Tanks without Pretressing" to help your analysis. It's the best 45 dollars you'll ever spend.

 

[SlideRuleEra]

An earlier edition of PCA's "Circular Concrete Tanks Without Prestressing" that JedClampett recommended is in the public domain. It can be viewed free and even (legally) downloaded... one page at a time:

"Circular Concrete Tanks Without Prestressing", c. 1943, PCA, 32 Pages

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[JedClampett]

SlideRuleEra, I noticed that the "current" $45 PCA version references the ACI 350-89, so the public domain one can't be that bad. Still, it's pretty good.

 

[SlideRuleEra]

Jed - Yes, the current edition is from 1993. I've downloaded the 1943 version and attached it.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[JoelTXCive]

What sort of allowable bearing pressure do you have under the tank. Eight feet will add 500psf (8'x62.4pcf) to the structure.

 

[BAretired]

The additional 10' of wall height will add 1500 plf to the existing wall. Hopefully the existing wall has an adequate footing to sustain this additional load. No footing is shown on the sketch.

If the existing wall reinforcement is not adequate to resist the additional water pressure, perhaps it could be reinforced with prestressed strands wrapped around the exterior and protected with a pneumatically placed concrete cover.

BA

 

[txeng91]

I've been through all the calculations in the PCA handbook and determined the wall will need 2 layers of #5 @ 12" oc to support the additional pressure, so I'm hopeful the scan comes back with good news so we don't have to mess with strengthening the existing.

Jed, thanks for the Sika link, I usually spec the swelllstop but did not realize they had a post install product.

As far as the additional load on the soil, the total water pressure at the bottom is about 1200 PDF which isn't too bad, but we don't have a soils report. I'm more worried about the additional wall weight as BA mentioned, which will total to about 2800 plf at the base of the existing wall. I guess we might need to have them excavate at the exterior of the wall to see what's going on with the footing down there. We will probably need to approximate a conservative bearing capacity and see if it can handle the additional load.

One mor question. For option 1 would you think the tank needs to be analyzed as a monolithic structure and the dowels sized to transfer moment and shear? I know on paper it seems like a stable structure assuming a pinned base between the two, but I'm a little afraid of separation between the two developing and compromising the water tightness. On the other hand, it would obviously be preferable from a construction stand point to keep the dowel spacing and embedment to a minimum, either staggered on each face as Jed mentioned or down the center

 

[JedClampett]

I would envelope the monolithic case and the top as an unsupported pipe or tube. I think for water tightness reasons you're going to want to dowel the top portion in, and once you do that, it's going to transfer moments and shears.
In addition, I'm a believer is a belt and suspenders approach, especially when you have significant unknown conditions.