Eccentric load bearing capacity VS overturning check
Eccentric load bearing capacity VS overturning check
(OP)
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.
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.






RE: Eccentric load bearing capacity VS overturning check
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)
RE: Eccentric load bearing capacity VS overturning check
RE: Eccentric load bearing capacity VS overturning check
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.
RE: Eccentric load bearing capacity VS overturning check
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.
RE: Eccentric load bearing capacity VS overturning check
RE: Eccentric load bearing capacity VS overturning check
RE: Eccentric load bearing capacity VS overturning check
RE: Eccentric load bearing capacity VS overturning check
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.
RE: Eccentric load bearing capacity VS overturning check
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)
RE: Eccentric load bearing capacity VS overturning check
RE: Eccentric load bearing capacity VS overturning check
RE: Eccentric load bearing capacity VS overturning check
RE: Eccentric load bearing capacity VS overturning check
"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.
RE: Eccentric load bearing capacity VS overturning check
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.
RE: Eccentric load bearing capacity VS overturning check
RE: Eccentric load bearing capacity VS overturning check
How big is huge?
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
RE: Eccentric load bearing capacity VS overturning check
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!
RE: Eccentric load bearing capacity VS overturning check
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.
RE: Eccentric load bearing capacity VS overturning check
RE: Eccentric load bearing capacity VS overturning check
You are misreading the intent of what Das said. You must include the weight of concrete in your calculation. Why wouldn't you? That is the load that the soil/footing interface experiences. If the maximum resulting pressure exceeds permissible soil pressure plus overburden, you have to try again.
In the case at hand, I assumed that the top of footing was at grade. The maximum resulting pressure was found to be 1428 psf based on a moment of 30'k. If the moment was taken at grade, it should have been increased to account for the one foot footing thickness and I neglected that.
I'm pleased that you got your numbers to work out but you still need to understand the concept which Das discussed in his book. Read it again.
BA