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Tank ring wall foundation question

Tank ring wall foundation question

Tank ring wall foundation question

(OP)
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.

RE: Tank ring wall foundation question

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.

www.SlideRuleEra.net idea

RE: Tank ring wall foundation question

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.  

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: Tank ring wall foundation question

(OP)
"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??

RE: Tank ring wall foundation question

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.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: Tank ring wall foundation question

(OP)
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!

RE: Tank ring wall foundation question

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.

www.SlideRuleEra.net idea

RE: Tank ring wall foundation question

(OP)
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.

RE: Tank ring wall foundation question

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.

www.SlideRuleEra.net idea

RE: Tank ring wall foundation question

(OP)
Thanks a lot SlideRuleEra!!

RE: Tank ring wall foundation question

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.

RE: Tank ring wall foundation question

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.

RE: Tank ring wall foundation question

(OP)
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.

RE: Tank ring wall foundation question

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

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: Tank ring wall foundation question

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.

RE: Tank ring wall foundation question

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.

RE: Tank ring wall foundation question

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.

www.SlideRuleEra.net idea

RE: Tank ring wall foundation question

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.

RE: Tank ring wall foundation question

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.

RE: Tank ring wall foundation question

I agree.  Since the low oil prices in the mid 80's, and early 90's there has been a general exit of experience from the petroleum industry.

Although I would be interested to know how you avoid edge settlements greater than that at interior points, as what you say pretty much contradicts what every geotechnical engineer I've ever talked to has said.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: Tank ring wall foundation question

    Almost all the tanks we have done were in refineries or storage areas. Our clients always were good but were mainly big players.
    Good point though, SRE, about using the ring wall due to other considerations - most definitely not, though, for tank support. It is more of a safeguard then than really for support. As we knew settlements would be more than rigid connections could take, our recommendations were always for flexible connections between the tanks and in/out piping.  For large tanks the bulk of the lateral spread would likely be below any shallow ringwall in any event.  Look at the horizontal pressures at the edge, at the mipoint to the centre and at the centre.
    We have supported tanks on a 5 ft. granular pad takeng some 10 ft beyond the edge of the tank over some quite soft soils.  I remember once where Dames and Moore got sued in that their tank settled some 40 inches - doubt it had a ring wall but although the tank was still serviceable, the owner removed it and installed a lot of piles and then sued. I agree that the ring walls seem to be one of those "we have always done it this way" or some are not used to founding without a footing.  A granular ring wall would probably work just as good as a concrete one in many cases.
  Interesting that some think that the edge of the tank will settle more than the centre.  A tank base is not rigid - the base of the tank (large ones) will be such that it will act as a flexible foundation (whether with or without ringwall).  Flexible "footings" settle more in the interior than at the exterior.  See any text of elastic theory (Polous and Davis (1975)).  The ring wall can help minimize differential settlements of the tank edge from one point to another along the edge and may allow it to be not as rigid as if not on a ring wall, but it won't help much for the total edge settlements, I would surmise as these are based on the loading of the stored liquid/silage.
  A lot of good points in the thread - thanks.

RE: Tank ring wall foundation question

Easy... theoretically, there is more load at the edge.  I think how much depends on if the tank has a fixed or floating roof.  Fixed roofs must transfer the roof weight to the shell, then down to the soil, whereas floating roofs do have an inherently more uniform load, with only the additional shell weight acting at the edge, which is probably not going to be all that much pct-wise greater in most cases, but non-the-less is heavier at the edge.

As for regidity, the ring addresses that point quite well, taking into consideration that it is there only to equalize the soil load as much as possible, it recognizes that the system is definately NOT rigid.

Total edge settlements are not of much concern and can be allowed for by pipe flexibility at the connections and by preloading the tank farm soils before construction, remembering that it is always the differential settlements that cause the most concern.

A granular ring wall would probably work just as well as a concrete ring in many cases, as long as the granular material equalizes the underlying soil loading from the shell with the interior tank load going to the underlying soils, given there is no need to elevate the tank from the surroundings and interior lateral load can be considererd nil, or the padding can extend past the edge to the point where there is no effective lateral spreading and a resultant greater settlement of the shell.

Theoretically, I don't think anyone can argue that a uniform load is not be the most desireable condition for rigid or flexible foundations of any kind.    

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: Tank ring wall foundation question

Most of my tank experience is with water tanks and small to medium sized petroleum tanks, fixed covers. I tend to use structural fill beneath and beyond the tanks. I also do a lot of overexcavation and replacement, many times with native soils, even silts and clays. The use of 1 or 2 layers of geotextile or geogrid is commen. With silts and clays the geo-products don't develope very much of their potential strength but, the improvement to the native soils is impressive. Many of my sites are 20 to 50 miles from concrete batch plants, over poor roads. The use of native soils, if possible, is desired.

The problems of settlement vary across different climates and the reasons for settlements.  If normal soil consolidation is the issue, with very moist to wet soils, The center portion of the structure will 'crater', after everything has stabilized.

If the subsoils are damp to dry, the settlement will begin with whatever portion is getting wetted. Most tank sites have horrible surface drainage. The edges get wet first, so the exterior settles first. If the situation is allowed to stabilize, the center will still 'crater', probably.  If the distress is caught early, edges are down and the center is up or 'hogged'. Usually repairs begin at this point and the ultimate soil settlement may not be seen.

In my case, I deal at lot with collapsible soils, thankfully less than 20 feet thick, except in the Rangely Area, where 70 to 100' is commmen. Rangely is the area where I have had the worse experience with poor initital construction and have earned a lot of fees doing repair.  Collapsible soils can have erratic stratigraphy and are often wetted unevenly, so settlements are unpredictable.

RE: Tank ring wall foundation question

Most of my experience is the opposite, with expansive clays where settlement occurs first with drying, not with wetting and dishing occurs in the center of the tank as those soils tend to dry as they squeeze moisture down and out.  For expansive clays, it seems like a ring wall might inhibit water entry into undertank edge areas, especially if treated and backed up with a vapor barrier.

You don't mention stripping topsoils and preloading to eliminate most of the initial consolidations before construction of the tanks ever begin.  Is that something you do?

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: Tank ring wall foundation question

I see very little Preloading as the projects are usually moving along very quickly. Overexcavation and replacement, often with native soils for a portion of or all of the fill, is more commen. Everyone is in a hurry. It is notable that the vast majority of the foundation plans are done at the time the Geotechnical exploration is being accomplished. Then changes are rapidly produced. No wonder problems of design and construction occur.

Your idea of the ringwall acting as a partial barrier is correct, in my experience. My usual problem with using the foundation as a barrier is the foundation/barrier is usually not deep enough and the moisture loss/gain by the environment is still exerting as large effect on the foundation soils.
When I construct moisture barriers, I try to get them 4 to 10 feet away from the foundation and at least 5 feet deep. For my climate, that seems to work. I am assuming you are doing what seems to work in your area.

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