Tank ring wall foundation question 1

[suocsiv]

Which is the best method for a tank foundation(D=11,46m/H=12m) made with a concrete ring wall:

A)Design the concrete ring wall, as described in API 650 Apendix B.4.2,where the centerline diameter of the ring wall is equal to the nominal diameter of the tank??

Or

B)Design the concrete ring wall with a bigger ID so that the tank can settle on a 95%compacted fill in the ring wall??

In other words in the first case, tank shell/bottom plates joint, is resting on the concrete ring wall(bottom is resting on the compacted fill) and in the second tank shell joint and bottom is all resting on the compacted fill.

Anchor bolts are also going to be used.
No annular plate / bottom plates will exceed 50 mm out of nom diam. of tank.

Any advice or recommentation would be appreciated.
Thank you.

 

[SlideRuleEra]

Suggest following API 650. Also, in his classic paper, "Oil Storage Tank Foundations", I. E. Boberg puts the tank wall in the center of the ring wall foundation (your Case "A"). The paper addresses your concerns about settlement.
You can download the paper from my website, link below.

http://www.SlideRuleEra.net

 

[BigInch]

The EC companies I have worked for design tank foundations such that the soil bearing below the tank bottom resting on fill + depth to bottom of ring wall x soil density equals the bearing pressure of the shell + concrete ring wall soil bearing value. The ring width centerline is offset radially to the tank wall as necessary to ensure that the soil bearing below is constant and not the result of any eccentric load.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[suocsiv]

"The ring width centerline is offset radially to the tank wall as necessary to ensure that the soil bearing below is constant and not the result of any eccentric load."

Hello again BigInch,
So if I understood well do you suggest to offset the ring wall to a larger ID and rest the tank inside the ring wall??

 

[BigInch]

Since differential settlement is very important for tanks, extreme care was used to equalize the soil bearing under the ring with that of the soil bearing load under the tank bottom and to try to eliminate any eccentric twisting of the ring whenever possible. This is more important for large diameter tanks, where the lateral soil loads may not as readily resisted by additional ring tension.

As I recall, the design sketch looked like this. The tank wall is still on the ring. We would try to balance the wall load x eccentricity to equal the overturning moments from the lateral soil load contained within. I didn't show any shear load from the tank wall, which you may or may not have, nor any reinforcing.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[suocsiv]

Mr SlideRuleEra & BigInch,

Please correct me if I am wrong:
Calculating "W"(W=4310lbs/ft) shell&roof plates weight, and using "I.E.Boberg equation" for a ring wall height of 3,6ft I get the thickness for my concrete wall ring "T".(T=20in)

Also calculating Horiz. Pressure "P" on ring (again as described in I.E.Boberg article) I get "P".(P=2852lbs/ft)

So from the above equation...W*e=P==>e=P/W==>e=0.66ft=~8inch.Does this mean that I can move the shell centerline within 8in closer to ring wall edge??

Thank you in advance!

 

[SlideRuleEra]

suocsiv - No, I don't get 20 inches. You have not told us the "Weight of the Stored Product". But even using water (62.4 lb/ cu ft) for this value the computed ring wall thickness for a 12 meter (39.4 ft) high tank is not reasonable. Recheck your all numbers and metric conversion factors, especially the unit weight of the shell/roof.

http://www.SlideRuleEra.net

 

[suocsiv]

My mistake,
I used the total Weight (Wshell+Wroof resting on shell+Wliquid projected vertically on the crw+ Wringwall) which I calculated to compare it with allowable foundation pressure under the crw.
And:
Wshell+Wroof=768Kg/m+137.3Kg/m=905.3Kg/m=609lbs/ft
H=39,37ft
h=3,93ft
q=61,5lbs/cuft(fuel oil d=985kg/cum)
T=7in==>8in min.

still 8 inches look to me as a very narrow crw especially if I want to anchor the tank (for uplift purposes only).In API 12inch is minimum...

Is there any other problem rising if I build the crw 20 in thick(exept from the cost)?

Thanks again for your time.

 

[SlideRuleEra]

If the calcs show 8", and API requires 12", then 20" inches should be ok, as long as you have enough rebar for 20". In other words, don't design the ring wall to be 8" or 12" thick AND then arbitrarily increase just the concrete to 20" thick without increasing the rebar quantity.

http://www.SlideRuleEra.net

 

[suocsiv]

Thanks a lot SlideRuleEra!!

 

[emmgjld]

I typically keep the thickness as narrow as possible when I recommend or design. I had a VERY bad experience with differential tank settlement. A number of problems including the owners representative getting in the middle of the fill construction.

The Foundation Engineer (no previous experience with tanks) wanted to be very conservative and the wall, 12" wide, had a 20" or 24" wide footing. Settlement occurred with the tank center settling slightly more than 8" and the footing less than 1". Unhappy owner.

 

[IFRs]

If you are using anchors, you want the tank on the ringwall. You don't want the tank to settle indside the ringwall. The tank is on the ringwall at least partially because keeping the tank shell planar and true is agruably the most important thing to keep the tank from being a maintenance issue in the future. As others have said, differential settlement of a tank shell is to be avoided at all costs. Uniform, consistent, predictable support is the key and a ringwall (sometimes with a footer) is a good method to achieve this. If your soil conditions are guaranteed stable over the long term you may be able to do without a ringwall and just put the tank on a gravel pad aka figure B-2.

 

[suocsiv]

In wich cases is a footer needed below ring wall??

My ground is stable rock at a depth of about 1.5m and what the civil contractor is suggesting to do, is to level the foundation up to 1.2m with concrete mixed with pieces of rock(without reinforcement) and build a foundation on top of it :1,2m height by 0.7m width properly reinforced(0,7 below ground level-0,5m above ground level, with anchors).

My civil engineered agreed that there is no problem with this method ,but it is his first time doing this ,so I am wondering about "emmgjld"'s case...

From the other hand, calculations ,from many relative literature, point that dimensions of ring wall are within limits (with the appropriate reinforcement always).

Plus we have built in the past (10 years ago) tanks with max Diameter 9,5m /12 meters height with the same method at the same location and since today no settlement has occured.I strongly believe that the great advantage we luckily have is the soil conditions (pure rock), for either building the tank with a ringwall or without it.

...But sharing thoughts in this forum is something I couldn't do 10 years ago...

Any opinion is appreciated,
Thank you.

 

[BigInch]

You need a footing below the ring when you can't make the moments, shown in the diagram above, balance without using one.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[emmgjld]

You need to directly address BigInch's latest comment.
To wit, what are your conditions???
... Being on rock, what are to horizontal forces being applied due to the tank weight and any soil/fill?
... What other methods are possible to mitigate some of the horizontal forces?
... What is the previous experience in this area, if any?
... What purpose is the footing fulfilling? Bearing capacity problems? Moment problems?
... Do you even need a Ring and why or why not?
If you are trying to 'fine tune' a generalized design and a Geotech is not involved, why not?
This is engineering and I think the Geotech should provide most of the information required for your answers.

 

[BigH]

I'll say it again - I've been involved with some 60 tanks - I know others and have talked to them about concrete ring-walls and they have been involved in up to a thousand tanks and they can't remember ever using a concrete ringwall. This includes large (150ft) storage tanks on till, on firm clay, on bedrock - a full range. They do use a granular pad of a metre of so thickness to sit the tank on. I'd really like to get to the bottom of why everyone seems to think that concrete ring walls are a necessity.

 

[SlideRuleEra]

BigH - I can tell you why we use ring walls for tanks on industrial sites in the typical "swampy" soils of coastal South Carolina - it contains the soil within the ring. After the tank is installed, it is common to dig (deep) trenches right next to the ring wall to install lots of piping, ductbank, electrical grounding, etc. Many of these items have nothing to do with use of the tank. Also as the years go by, we do as much as possible to allow digging anywhere on site (maintenance, plant modification, etc.) without having to use temporary (or permanent) shoring.

In fact, sometimes even a ring wall is not good enough, a pile supported concrete mat foundation for the tank is needed. I have designed a few of these, at existing plants, where the location of the tank was so important, that we had to exclude piling from certain corridors in the pile plan for existing pipes & ductbanks (underneath the new tank) - the foundation mat had to be designed to to work using this "crazy" pile layout.

http://www.SlideRuleEra.net

 

[emmgjld]

BigH - I agree with SRE regarding stabilization and the ability to excavate at a later date. This very point has influenced some of my recommendations.

I also think a lot of ringwalls have been constructed because some of the Structurals want them. Some structurals have a distrust for things which cannot be calculated in a 'rigid' manner. Soil properties are not 'rigid' enough. Reinforced concrete can have some very nice numbers applied. I have had this argument and it gets tiresome.

 

[BigInch]

I'd guess you probably don't do alotta' work for oil companies.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[emmgjld]

No, I don't do alotta work for oil companies. I don't even pursue it.

Much of the work I have done on new oil & gas construction in Colorado has not been enjoyable. Too much foolishness. The construction managers and companies tend to be rather pushy and, when encountering something a little out of their experience, tend to be a little arrogant and oftentimes have not paid much attention to those who have answers which are not on the 'Standard Plans'.

I do a lot (and I mean A LOT) more work in repair of facilities. I have found the companies (usually 2nd or 3rd owner) tend to be interested in getting things to work. I have similar experiences with Coal Companies.