Circular Concrete Slab Cover (Roof) for Circular Tank

[sengineering]

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?

 

[sengineering]

Thanks for the great comments.

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...

 

[SlideRuleEra]

Make one end of the W27 / W24 members 3'+ higher than the other end, keep each W10 level (along it's length)... there is the required 16:1 mono slope. Include framing for pumps in the structural steel layout.

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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[sengineering]

Thanks for the sketch and comments SlideRuleEra. Yes, I have used 100 psf for live load and added 0.3 kips point loads for the pumps.I will do a cost estimate to see if using steel beams is going to be cheaper than columns.

 

[SlideRuleEra]

Good, glad it helps. Keep in mind that all of the weight from the new sloped cast-in-place roof + live load (a few hundred kips total) will bear on either the existing tank walls (which most likely were not designed for that load) or the proposed (corrosion resistant) interior columns which put high load on the 6" thick, 60+ year old slab that has been in a corrosive environment since built. Depending on soil conditions, expect some differential settlement (cracking the existing tank bottom) when the columns are loaded.... dead load alone will probably be enough to do that. Suggest rereading my comments on construction concerns (i.e. dollars) for building a cast-in-place roof on columns.

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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