Opinion on octogonal ring footing for a tank.

[MonsieurR]

Hi all,

I'm currently on the problem of footings for tanks, and I would appreciate some opinion on the following. I have a 3.5 m diameter 13 m height steel tank for plastic chips (80ton capacity) which is subjected preliminary to a lateral force of 35 ton.

A ring footing appears to be the most appropriate foundation since some of the static load's path is through the walls and also mainly it helps for stability. So I came up with Option 1 of the .pdf attached. Nevertheless I have been trying to minimize materials and so came up later with Option 2 of the .pdf, which is allows to save about 15%-20% on concrete. The top needs to be flat so a third option like a sloped footing type (would be the ideal I think) is for now discarded.

The code I'm using alllows a 25% of uplift and the allowable soil stresses are around 2 kg/cm2. The k of the soil is around 150 pci.

What are your thoughts on Option 2? I was counting it would work as shown in Option B but then I thought maybe the stiffness of the sloped face is not as high as the flat face and possibly the stresses would go primarily as shown in Option A.

Any opinion please express it.
Regards.

 

[SlideRuleEra]

Forget Options 2A and 2B. The cost to construct the unusual shape will far exceed any concrete material cost savings.

Is the thickness of the Option 1 footing 100 centimeters? Just want to confirm it is NOT 100 millimeters.

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[dik]

I'd probably run with Option 1... why the lean concrete bed? only use that if rain is an issue. Do you have slab reinforcing... if so, can you run with open stirrups using the slab reinforcing to provide a 'closed' stirrup? As SRE notes, is the edge thickness correct? What sort of anchor rods are you using?

Dik

 

[MonsieurR]

@ SRE
I'm aware this wouldn't save much, but would be really that costly to build?
The thickness is 100 cm preliminary, yes.
(Tank design is still preliminary so everything is not definitive).

@ dik

I haven't really put much thought on the lean concrete base, it's something we always put before pouring the concrete.
As for the slab reinforcement, I was thinking on placing it independently of the ring(see idea in attachment). In this way the ring is independent.
I don't understand what you mean, how can you use the slab reinforcing as stirrups? I'm very open to improvements.
Anchorage consists of 16 x 2 Headed anchor rods around the perimeter, 1'' preliminary, with anchorage chairs (required by the code used).

Attached some more details.

Regardless of which is cheaper, would option 2 work ok from a geotech point of view? Just wondering.

 

[DETstru]

I would go with option 1 and make the stirrups rectangular if you can. That saves fabrication cost in making the extra bends and a bit of length in the stirrups.

For Option 2: Whatever savings you get in concrete you'll likely waste on extra engineered fill, rebar fabrication, and forming the angled grades.

You can always get an opinion from a contractor.

 

[dik]

Can you use an edge detail similar to the attached? It would be easier to place steel in the edge without the continuous closed ties.

Dik

 

[SlideRuleEra]

MonsieuR - Speaking as a former bridge / heavy construction Contractor, yes, either of the Option 2 choices will be more expensive to build than Option 1.

With Option 2, the only cost reduction is the avoided material cost for the reduced concrete volume. There are no labor savings for the slightly lower concrete volume. In fact, the detailed field work to construct Option 2 will drive labor costs higher.

My statement is not hypothetical. I recall more than one occasion where we (Contractor) offered to provide additional concrete (free) in order to simplify forming and concrete placement. The offers for free concrete were always accepted and it was well worth the (material only) cost to us.

For what it is worth, can even tell you how I would bid Option 2:

1. Price the work (material and labor) to construct Option 2 as designed.
2. Add the cost of the "saved" concrete material to the bid.
3. If successful bidder, after contract award, offer the "free" concrete to allow construction of an Option 1 design.
4. Provide the "free" concrete (which the Owner paid for, but does not know it), and pocket the labor savings.

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[MonsieurR]

Thank you for all the feedback!. Indeed the builder seems to think the same.

Regards.