Eccentric load bearing capacity VS overturning check

[LearnerN]

I'm considering the design of a footing for a 25' tall, 24" OD process tower that only weighs 7500 pounds. It'll be on a large footing mainly due to the overturning effect of wind loading. However, I'm trying to figure out how to get the eccentric loading bearing pressure calculation to work out for the foundation. I have P=8k and M=30k-ft...overturning checks fine, but bearing pressure calcs are making this have a huge footing to get enough bearing contact. I know it has to have a huge footing since it's lightly loaded anyway, so just a little applied moment will create uplift under some of the footing. Is there any alternative to having such a massive footing for such a small tower?

I recently considered a similar footing but for a heavier similar tower. However, I think this smaller tower will have to have a larger footing due to the eccentric loading issue.

 

[EngineeringEric]

Deeper footing for counter weight? Can you account for fill on top of the footing?

I've never done something like your problem, besides poles which get smaller size but deep footings. This way lateral earth forces help handle the moment and don't add to the overall bearing capacity (4'dia x 12'deep)

 

[LearnerN]

Am I understanding correctly that with an eccentric loading bearing calculation, that you don't consider the weight of soil or concrete or anything...just the applied vertical load and moment? So if my understanding is correct, a deeper footing wouldn't help in this case.

 

[JStephen]

When checking the foundation for overturning, include the gross weight of concrete and soil above the concrete.
You can also allow uplift. Doing so complicates the problem considerably, but will allow smaller foundation sizes. On a small footing like that, you probably can't save enough concrete to pay for the extra engineering.

 

[EngineeringEric]

Let me clarify. If you make your footing thicker, say 24" as opposed to 12" then you get more self weight.

As for the soil, i typically do not account for it, i assume the top of my footing is exposed... However, under certain situations and checks if i can guarantee some conditions i have used some soil. If i have a footing that TOF is 10' below grade then soil is going to help out. will i rely on it for everything, no, but will it make feel better about being on the edge of the Kern or a FS of 1.45, yes. I am curious what over people think or do.

PS. i am not a geo/civil so my foundation design is limited. Please take what i say with a large grain of salt.

 

[LearnerN]

JStephen, right, I included the foundation weight and soil weight when doing the overturning calculation. However, I don't think I can include those items when doing an eccentrically loaded bearing capacity calculation. I understand the footing surface doesn't have to be in 100% bearing contact, but the footing still has to be large even to get a small amount of bearing contact at the full wind loading.

 

[LearnerN]

Just to clarify on everything I've said so far: if the footing has the weight of the tower (8000 pounds) spread over a 7'x7' footing, then that yields a uniform bearing pressure of 140 psf. But once you consider the effect of the wind load generating a moment on the footing, you can see it wouldn't take much for the footing to be mostly in uplift.

 

[LearnerN]

Here is the KEY question I am considering and trying to conceptualize: how could I have this foundation with plenty of safety factor against overturning AT THE SAME TIME as it being completely in uplift?

 

[BadgerPE]

If you have a 7'x7' footing that is 1' thick, you are adding an additional 7.4k of resisting dead load for global stability checks. When I run the numbers, it is close to being acceptable in my book.

However, in my part of the world, we usually try to provide a minimum 4' frost coverage on all exterior footings, including process equipment. By thickening the footing, you will increase resisting dead load as well, and work yourself back into good stable bearing pressures.

Spreadsheets are your friend for this one. You can generate a very simple one in a few minutes that allows you to rapidly adjusts your dimensions to determine what works best for your situation.

 

[LearnerN]

BadgerPE, you said: "If you have a 7'x7' footing that is 1' thick, you are adding an additional 7.4k of resisting dead load for global stability checks. When I run the numbers, it is close to being acceptable in my book."

But you're speaking of the global stability check for overturning, not the eccentric loading bearing pressure calculation where you can't consider the weight of the foundation. OR can you include the weight of the foundation in the eccentric loading bearing pressure calculation?

I'm talking about the calculation P(vertical force)/A(area) +/- 6*eccentricity/L(length)

 

[BadgerPE]

Yes. I am including the weight of the footing in my eccentric bearing pressure check. So P = 7.5+7.4 = 14.9k

 

[LearnerN]

BadgerPE, I think I understand now and have resolved my confusion. With the standard bearing pressure calculation of P/A, when the engineer is using the soil's "allowable bearing capacity," this is the reduced value per a safety factor from the "net bearing capacity" (which is the ultimate bearing capacity reduced by ONLY the soil overburden, not anything about an assumed concrete weight - my misunderstanding!). So none of this is accounting for the weight of the concrete, so YES, that should be included in the eccentric load bearing pressure calculation. And so even in the standard bearing pressure calculation of P/A, the weight of the concrete should be included in the vertical load "P".

 

[LearnerN]

Regarding what I said above, P/A does includes the weight of the concrete in the value of "P" (the vertical load), not just the applied load, right?

 

[LearnerN]

Per the 6th edition of Braja Das' book "Principles of Foundation Engineering" page 129:

"The net ultimate bearing capacity is defined as the ultimate pressure per unit area of the foundation that can be supported by the soil in excess of the pressure caused by the surrounding soil at the foundation level. If the difference between the unit weight of concrete used in the foundation and the unit weight of soil surrounding is assumed to be negligible, then q_net = q_u - q"

I think this resolves my confusion. "Net" bearing capacity, like I said above, considers the max pressure the soil can support IN EXCESS of soil overburden at the foundation level. The above "net" bearing capacity calculation ASSUMES that the difference between the unit weight of concrete in the foundation AND the overburden soil is NEGLIGIBLE, since the "q" value is simply the depth x the unit weight of soil (and typically these values are fairly close). So, NO, the weight of the concrete cannot be added as part of the vertical load in the bearing pressure calculation, because this would be considering it twice in the calculation.

However, I still don't think there's any alternative for this foundation in my opening post than a huge footer. Plus, I'm still uncertain about the "key question" I mentioned above in the 8th post on this thread.

 

[KootK]

I find that adherence to kern rules etc makes these kinds of foundations painfully un-economic. My recommendation is shown below. You can work with either net or gross soil bearing pressures as long as you're consistent. I always convert to gross as I find that simpler conceptualize.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[LearnerN]

KootK, that's a great way to analyze the situation - thanks. Related to what you said, I figured I'd add the difference between soil and concrete unit weights as an additional vertical load, and that's helping to make my numbers more reasonable for a more reasonably-sized footing.

 

[BAretired]

However, I still don't think there's any alternative for this foundation in my opening post than a huge footer. Plus, I'm still uncertain about the "key question" I mentioned above in the 8th post on this thread.

How big is huge?

Here is the KEY question I am considering and trying to conceptualize: how could I have this foundation with plenty of safety factor against overturning AT THE SAME TIME as it being completely in uplift?

If it was completely in uplift, it would be floating away. Assuming a footing 7'x7'x1', weight of footing = 49x150 = 7350#, say 7000#.

W = 8000 + 7000 = 15,000
eccentricity e = 30/15 = 2'

Effective width of footing = (3.5-2)2 = 3'
Average pressure = 15000/7x3 = 714 psf
Maximum pressure = 1428 psf
1428 psf is a pretty low bearing pressure, so what is the problem?

BA

 

[LearnerN]

BAretired, the point where I'm disagreeing is that you can't factor in the weight of the footing for a bearing pressure calculation - it's already assumed per the definition of "net bearing pressure" per the quotations I provided above from Braja Das. However, I think you can add the difference between the concrete and soil unit weights as an additional vertical load.

For those following the discussion, I am managing to make the 7x7 footing work. The wind load was a little conservative, and my applied moment was also originally a little off. This has been a helpful discussion!

 

[BadgerPE]

As KootK mentioned, use gross bearing pressures. It is easier to follow.

As far as your "KEY" question, can you provide a pressure diagram showing how the footing is stable for OT, but is fully in uplift? We may not be on the same page with that one.

 

[LearnerN]

BadgerPE, I fixed my problem. The overturning stability ratio came out around 3.5-4, and I was able to get 70% bearing contact. I realized I miscalculated the moment from the wind when I was doing the bearing contact pressure calculation, so it was higher than it should've been. That got my numbers to work out. And the conversation in this thread has been helpful to conceptualize, and gave some useful tips for understanding the relationship between overturning moment and eccentric loading bearing capacity. Thank you.