Lateral Earth Pressure in Ringwall foundation - Active/Passive or At-Rest

[kapoA]

I am designing the ringwall foundation for a 150,000 BBL API 650 Tank. While calculating the hoop stresses, what value of earth pressure should be used (At rest/Active/Passive)?

One view is that the ringwall will not move much and at-rest pressures should be used.

A design spec of a company (won't name them) specified to use active earth pressure.

Third theory is that the ringwall is rotating a bit, so there should be passive pressures in the top half while active earth pressures in the bottom.

If anyone has any insight or references on this, it would be really helpful.

Thanks

KapoA

 

[oldestguy]

I suppose you know what the future life of the structure will be and what others in the future will be doing to the site, such as adding another one nearby, changing the piping, etc.

When you can't answer that clearly, you assume the worst. No outside fill, at least in some places. Why stick you neck out?.

 

[kapoA]

I am talking about the inside fill.
Outside fill is neglected.

 

[a7x1984]

Sounds like at-rest to me. The ring's geometry would likely prevent any sort of significant movement to allow active pressure.

Before you doing anything creative, make sure the code likes the artwork you are trying to wow the world with.

 

[kapoA]

@a7x1984

any references (API 650 section no's) to back your thought?

 

[a7x1984]

I don't have any code reference sections to back my thought in addition to my own deduction. Well, okay, a general reference: Every foundation engineering book contains the percentage of straight cantilevered wall height movement required to develop active and passive pressures e.g.(>=.001H) is for active pressures: That is 1" of movement for every 12ft of length. "the ring is rotating a bit"... The ring will certainly not behave like a cantilever wall when its wall thickness is large when compared to its diameter and height; that makes it difficult to determine. Since the ring will behave more rigidly than the cantilevered wall, it will need to move more than 1" per wall length. Can you guarantee that?

Unless I am not understanding your specific loading, there are only two options. The wall isn't moving into the soil mass, right? My question is: is it worth the time to investigate whether the wall will move enough to develop active pressures? Also, if the soil being contained is very granular and self-consolidating, it will have a very hard time developing active pressures.

Maybe if there is an additional moment at the top of the wall (inducing a large and predicable movement), a soil with good arching action, and the right aspect ratio and wall thickness it will happen. Again, is it worth the research? If the research leads you to "I am not sure", was it still worth the effort?

Before you doing anything creative, make sure the code likes the artwork you are trying to wow the world with.

 

[kapoA]

For a twist moment, the ringwall will behave such as the portion of the ringwall above the neutral axis will rotate inwards (compression) and the portion below will rotate outwards (in tension).

neutral axis being the central axis of the cross section in the plane of the ring. ROARKS FORMULA FOR STRESS AND STRAIN SECTION 10.9
Also Refer PIP STE 03020 Page 18 section 4.6.5.6 for reference

Now the displacement for cantilevered wall you have mentioned is 0.001H. So the angle of rotation will be 0.001 rad or 0.057 degrees.


Rotation angle for a ringwall (behaving as a narrow ring ) is MR^2/EI where M is twist moment, R is tank radius.

Say my ringwall is 2 feet by 5 feet. Assume Tank radius of 70 feet. Twist moment Mt approx 700 kips. E say 29000 ksi.

Angle of rotation comes out to be 0.039 radians (2.25 deg) > than 0.001 to mobilize active and passive.

So based on this, the upper half of the ringwall is moving into the soil while the bottom half is moving out. The lateral earth pressure coefficient for passive will be much higher and will really bump up my hoop stress values. for clay Ko is about 0.7 but Kp is about 1.82. while Ka is about 0.55.

I should take an average of Ka and Kp to calculate hoop stress acroos the wall height but even that average value is much higher than Ko. Average value is 1.18 and Ko is 0.7. My area of steel calculated is about 70% more!!

 

[Ron]

At rest. Any lateral pressure from the soil transposes to hoop stress in the ringwall and the resulting lateral movement of the wall will be negligible.

 

[a7x1984]

kapoA: It looks like you have made your decision. Good luck!

Before you doing anything creative, make sure the code likes the artwork you are trying to wow the world with.

 

[kapoA]

a7x1984 - Do you see anything wrong with the math here?

Thanks.

 

[Ron]

kapoA...if you want to play with the numbers, then compute the tensile strain due to the hoop stress. The added length of the circumference can then be used to compute the change in diameter....I'm reasonably sure that you'll find it to be negligible and that no mobilization of the earth pressure will occur.

 

[a7x1984]

I agree with Ron. Your arithmetic wasn't wrong, kapoA; I just wasn't convinced the mechanism you were applying the math to was true to reality. I would be careful not to let the extra time spent equal the cost of "saved" rebar.

Before you doing anything creative, make sure the code likes the artwork you are trying to wow the world with.

 

[kapoA]

@Ron - The phenomenon considered here causing soil displacement is not expansion of the ringwall; instead it is the ringwall being turned inwards due to tank loading (more loading on the inner half of the ring).
I agree that tensile strain is most likely negligible and that should not cause mobilization of earth pressure.

@ a7x1984 - this approach is actually giving more rebar than the at-rest case; so i was concerned that the at-rest case may give less rebar than needed. Since there is a FOS of 1.6 it might absorb that difference.

All - thanks for your inputs and valueable feedback so far.

 

[a7x1984]

I do agree the extra precision you are looking for may be "absorbed" into the margin of error that load and resistance factors provide.

In Russia building design you!