What is the efficient foundation system for elevated tanks?
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Question

What is the efficient foundation system for elevated tanks?

IJR (Structural)

(OP)

22 Mar 01 09:25

Pals

This one always makes me uncomfortable and I can use your experience.

Elevated tanks supported on 4 steel braced legs, will usually have 3units of compression on 1 pair of legs and 1 unit of tension on the other pair when wind or quake works on it.

Say +75ton on each of the compressed pair and -25ton on each of the other

spaced say 25ft apart, and taking any two legs at a time:
with medium level bearing capacity, say sandy clay

1) combined footing wont usually work

2)my favorite 2 footings with a strap beam wont usually work

and I am usually left to join all 4 legs with a mat(Piles are expensive in remote areas where these structures are built)

and if it turns out that one leg can not join the mat, for some space-based reason, I get a funny mat with 3 legs on it.

Isnt there a better simpler and more efficient system?

Lets discuss

Thanks. IJR

KAM (Geotechnical) · Answer 1 · 2001-03-22 13:52:05

If tension loading is the problem, couldn't you use individual footings with helical anchors (CHANCE or Atlas piers) to provide the uplift resistance?

IJR (Structural) · Answer 2 · 2001-03-23 00:56:35

Thanks KAM

Your single footing suggestion is most welcome.

Do you want to give me more info on helical anchors. Never heard of them before

Once more thanks

IJR

KAM (Geotechnical) · Answer 3 · 2001-03-26 09:08:17

For more info on helical anchors, go to www.atlassys.com, or to www.hubbell.com/abchance

Riz (Structural) · Answer 4 · 2001-03-28 07:26:12

IJR,
Why not a circular strip footing. I found it to be very economical for a masonry water tank (60m3). U will also realise its strapping advantage.
What do u propose against overturning when tank is empty?
Riz

IJR (Structural) · Answer 5 · 2001-03-28 09:51:23

Riz,

Nice to have you back. My problem is NOT about TANK ON GRADE. It is about ELEVATED TANK supported on 4-steel legs, braced between one another.

Thanks for commenting on ring foundations, which I usually use for tanks on grade. You mentioned strapping there which I never thought about. Thanks for that.

Regards,

IJR

Ron (Structural) · Answer 6 · 2001-03-28 11:20:12

Agree with KAM. We have had to retrofit towers for additional loading and the anchors worked well. Combinations of footing and anchors can reduce the footing mass for overturning moment.

IJR (Structural) · Answer 7 · 2001-03-28 11:42:24

Thanks Ron, I am now after helicals. Guess what, we face this problem all the time, and I usually end up relying on soil mass. There was a day I had this dream: a chunk of soil mass melting like ice-cream and a bunch of columns flying like a stick.

Thanks once more and again

IJR

Riz (Structural) · Answer 8 · 2001-03-30 08:10:49

IJR,
The circular strip footing was for an elevated tank. U can elect to taper the ends for economising concrete.
Riz

IJR (Structural) · Answer 9 · 2001-03-30 09:23:04

Riz, I havent yet understood you. If you dont mind come back and lets get you more clearly.

I have a feeling you probably have a point, insisting on circular strip footing.

I have four steel columns to carry the tank 8m (24ft above), sort of a braced frame.

Right now I cant figure out how to run a circular strip footing to contain the column locations.

Please let me see your point closely.

Respects,
IJR

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What is the efficient foundation system for elevated tanks?
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