Circular Concrete Slab Cover (Roof) for Circular Tank
Circular Concrete Slab Cover (Roof) for Circular Tank
(OP)
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?
- 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?






RE: Circular Concrete Slab Cover (Roof) for Circular Tank
Dik
RE: Circular Concrete Slab Cover (Roof) for Circular Tank
RE: Circular Concrete Slab Cover (Roof) for Circular Tank
BA
RE: Circular Concrete Slab Cover (Roof) for Circular Tank
RE: Circular Concrete Slab Cover (Roof) for Circular Tank
BA
RE: Circular Concrete Slab Cover (Roof) for Circular Tank
http://files.engineering.com/download.aspx?folder=...
RE: Circular Concrete Slab Cover (Roof) for Circular Tank
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?
RE: Circular Concrete Slab Cover (Roof) for Circular Tank
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.
RE: Circular Concrete Slab Cover (Roof) for Circular Tank
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
RE: Circular Concrete Slab Cover (Roof) for Circular Tank
RE: Circular Concrete Slab Cover (Roof) for Circular Tank
Does that tank happen to have an internal rotating rake, also?
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RE: Circular Concrete Slab Cover (Roof) for Circular Tank
If there is a rake as suggested by SlideRuleEra, columns cannot be used at all. That would be a whole new ballgame.
BA
RE: Circular Concrete Slab Cover (Roof) for Circular Tank
RE: Circular Concrete Slab Cover (Roof) for Circular Tank
RE: Circular Concrete Slab Cover (Roof) for Circular Tank
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.
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RE: Circular Concrete Slab Cover (Roof) for Circular Tank
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.
RE: Circular Concrete Slab Cover (Roof) for Circular Tank
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RE: Circular Concrete Slab Cover (Roof) for Circular Tank
BA
RE: Circular Concrete Slab Cover (Roof) for Circular Tank
www.SlideRuleEra.net
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RE: Circular Concrete Slab Cover (Roof) for Circular Tank
I used 100 pcf for the live load even that seems a bit much. This project is in the middle of nowhere.
Using a slab with steel beams or hollow core and skipping columns are both good ideas. However, I'm hesitant to use steel since this is going to be a very corrosive environment... I don't have much experience with hollow core, this slab is going to have a few pumps (about 2-300 lbs each) on top of it and needs to have openings, is that possible with hollow core if you want to span a slab with no middle beams?
Also, I just realized there is a requirement for the tank cover slab to be sloped by 16:1. So the roof either has to be mono slope or an A-frame...
RE: Circular Concrete Slab Cover (Roof) for Circular Tank
You need to define the live load requirement now. That number will influence the whole design. Being in the "middle of nowhere" is NOT a valid reason to reduce loading requirements. In fact, it could make higher loading on the roof more likely... say, (heavy self-powered) equipment, has to put on the roof for maintenance or repair work.
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RE: Circular Concrete Slab Cover (Roof) for Circular Tank
RE: Circular Concrete Slab Cover (Roof) for Circular Tank
A clear-span roof avoids every one of those issues. I'm not saying this will be a perfect solution, but with proper design (both structural & geotechnical) there is no dependence on the 60+ year old tank to do more than it was designed for.
Please let us know the decision.
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