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Circular Concrete Slab Cover (Roof) for Circular Tank

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sengineering

sengineering

Structural
Apr 27, 2017
17
I'm designing a concrete cover for a 22' diameter existing concrete circular tank. The Tank is embedded in the ground and there will be people walking on top of it. Here are the conditions:

- The slab on the bottom has a slope.
- The depth of Tank varies from 9'-6" at the edge of the wall to 15'-6" at center.
- I can add columns in between to support the roof/cover, since I don't know the thickness of the bottom slab, I'm thinking to add a concrete slab on top of the existing one to support the columns.

I have a hard time finding a good design guide for this and I'm not sure if there are going to be any special considerations in comparison to a two way slab here. Do you have any recommendations for references or things I need to consider in my design?
 
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You could just take the maximum span (22 ft.) analyze it for dead and live load and be done with it. So you're going to end up with a 14 inch slab for deflection reasons and reinforce it with #5's at 6". Or you could find someone with Roark, dust it off, go to Table 24, case 10 and get 10.4 foot kips and put in minimum reinforcing of #6 at 12".
 
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Continuous support around the periphery plus a single column in the middle would be my choice if the existing perimeter footing is adequate. That would require a single new footing which could be poured on top of the existing slab. You could use a circular column on a circular footing. A circular drop panel could be used on the upper slab, but my preference is to just use a flat slab for simplicity in forming.

BA
 
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Thank you for the replies. Actually the diameter is 44 ft not 22. I think I would probably end up with a flat slab with 3 columns. The center of the tank has openings and equipments and I won't be able to add a column there...
 
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If the slab is to be reinforced as a two way slab, perhaps four columns in a square pattern would be better than three columns in a triangular pattern.

BA
 
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Thanks...

I came up with 4 columns and a 12" slab on top. The problem is the bottom slab of existing tank is only 6 inches thick. I'm worried there is not enough room for rebars to be embedded in the old concrete for a new slab to be poured on top of the old one to act as a foundation. Does using HILTIs help?
 
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Hilti anchors don't really improve the situation. Moreover, you may well have a failure of punching shear etc at the slab on grade. I believe this is what BA was getting at with this statement:

BA said:
That would require a single new footing which could be poured on top of the existing slab.
Click to expand...


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
 
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Not sure why you would need a 12" slab on top. The diameter of the tank is 44'. The live load is 100 psf so if you place the columns at 16' o/c in each direction, you would have a maximum interior span of 16' and a maximum end span of 14'. I would estimate a 6" slab would be adequate, but let's say 8". The dead load is 100 psf plus weight of waterproofing.

A continuous slab over the existing slab seems a bit excessive. Why not cut four holes through the existing slab to accommodate four new footings, maintaining as much of the existing reinforcement as possible, then excavate down as required for the new footings. The top of footing should match the top of existing slab except that the bearing area of each column should be depressed and level to provide good bearing for the columns.

The joint between new and existing concrete must be waterproofed to prevent leakage.

BA
 
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BAretired, I'd lean toward not cutting into the slab at all. Pour a new footing on top of the slab large enough to carry the column loads. Size it based on the bearing capacity of the subgrade. If you want to anchor it, use small bars (#3's?) epoxied shallowly into the thin slab. It's going to be a major pain to seal the footing to the slab, so why fool with it?
 
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sengineering said:
Actually the diameter is 44 ft not 22.
The depth of Tank varies from 9'-6" at the edge of the wall to 15'-6" at center.
The center of the tank has openings and equipment...
Click to expand...

Does that tank happen to have an internal rotating rake, also?

Tank-With-Rake-1_lx5vlp.png


[idea]
[r2d2]
 
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Yes JedClampett, you may be right; waterproofing between new and existing concrete could be problematic.

If there is a rake as suggested by SlideRuleEra, columns cannot be used at all. That would be a whole new ballgame.

BA
 
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There isn't a rake. This is a clearwell built in 1950s for a waterplant and it's just holding water. Drilling through the existing concrete would make it difficult to seal it after, but if that's the only way then we have no choice. With pouring the new slab on top of existing one, I'm more worried about the new concrete sliding on top of the old one as the vertical pressure will be high. I"m thinking to specify HILTI rebars embedded in concrete, but that's such a low penetration I'm not sure if it works. Do you have any recommendations?
 
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If the slab is called out as 6 inches, it's probably a little more. Contractors typically over pour slabs on grade. It's easier than grading super accurately. But even if it's 6 inches, you can pretty much develop a #3 bar in a 4 inch epoxied hole. And if you have a 4'-0" square footing and space them at 6", that's 64 #3 bars. That's a lot of reinforcing. And there's nothing prohibiting (actually there's nothing allowing it, but I'm a glass half full kind of engineer) the use of epoxied bars for shear friction. Make sure to hook the tops and you're good.
 
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Forming the the bottom of a cast-in-place roof, supported by only 4 interior columns is going to be a real challenge. This forming work will take place inside the sloped-bottom tank which, with the forms in place, becomes a confined space. Shoring and maybe even the forms will have to be removed after the top is in place.

Any chance of not using interior columns at all, but span the the entire tank with structural steel? Probably about W27, maybe W24 sections for the longest members. A (relatively thin) cast-in-place roof uses stay-in-place forms installed from the top. Since top of the existing tank is near ground level, the steel beams can be supported by independent footings (outside the existing tank walls). Walking surface of the roof would be, say, 30" to 36"+ above grade.

[idea]
[r2d2]
 
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Why not use precast hollow core concrete panels? No forming required.
12" hollow core can span the 44 ft. per my literature, 8" would require interior support.

These have been used in many water tanks in my experience.
 
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Clear spanning is an attractive idea. A concrete slab on steel deck on open web steel joists is a possibility but the steel would be in a continuously humid atmosphere, so would need to be protected from corrosion.

BA
 
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