Bearing pressure uplift 2

[DaveHolder]

I did design to foundation 5.5 by 5.5 meter and 2 meter deep, and I still get the uplift bearing pressure around -50 kN/m2!

Do I still need to increase the size of the foundation in order to remove the uplift? I can’t go down deeper than 2 meter as it is the mud stone level!

Any help is appreciated

 

[jayrod12]

That depends on why you're experiencing uplift. Is it due to overturning, or straight uplift from the column above. If straight uplift, you need more mass to hold it down. If overturning, perhaps a triangular pressure distribution can still work as long as at the opposite side, the bearing capacity doesn't exceed the allowable.

To answer your direct question, I'm not familiar with Eurocodes, but I can't see them in any way allowing uplift for bearing capacity.... those two things just don't go together.

 

[Ron247]

You need to outline your foundation reactions. Pure uplift is treated different than overturning uplift as jayrod indicated. It is not an uplift pressure answer, it is a how much weight do I need to hold down my foundation.

 

[JAE]

You either need enough mass to provide a comfortable safety factor against uplift, or you need to physically attach the foundation to the stone below and then verify that the stone can take the uplift force from the anchors into it.

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[Celt83]

If this is one frame column, link its foundation to it's frame pair and do a combined foundation instead of isolated.

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[Ron247]

If your uplift is 100 KN, then your foundation and any dead load you can rely on needs to weigh about 150 KN to provide a minimum 1.5 FOS against uplift. If you decide to anchor the foundation into the earth, then you still need 150 KN of resistance but you can get it with a combination of anchor strength, dead load and foundation weight.

I am lost with your terminology of "maximum compressive force" to hold down an uplift load. When dealing with direct uplift, foundations are more like an anchor than a soil pressure item. I do not see how making the steel columns bigger will help unless you make them a whole lot bigger.

100KN is about 23,000 lbs. A footing that weighs about 35,000 lbs should hold that down. 5.5x5.5x2 meters = 60.5 cubic meters. = 2136 Cubic Feet = 320,000 lbs. Are you sure about your calculations. I never work in KN, but a 150 KN foundation should work.

 

[hokie66]

Ron is correct. Without the fiddling around with conversions, your footing is about 1450 kN, so is a lot larger than required for uplift.

 

[DaveHolder]

No the foundation is not a lot larger than the uplift!

The structure dead load is around 100kN,
Foundation weight is 5.5x5.5x2x 24= 1500KN
Total = 1600kN

1.5 x 1600/(5.5x5.5)=80KN/m2 is greater than the uplift pressure 50KN/m2

Therefore, the size of the foundation is ideal to resist the uplift of 50 KN/m2!!

 

[hokie66]

You did say the uplift force is 100 kN. I get the 80 kPa bearing pressure, but have no idea why you say you have an uplift of 50 kPa. 100/5.5^2 is 3.3 kPa, but that is an irrelevant number.

 

[DaveHolder]

Sorry for misunderstanding! The vertical deal load is 100kN! The uplift pressure is - 50KN/m2

 

[hokie66]

Maybe you can explain that with some numbers, because it doesn't make sense to me. An "uplift pressure" of 50 kPa doesn't compute. If the uplift force is not 100 kN, then what is it?

 

[DaveHolder]

The foundation bearing pressure is between +110KN/m2 and -50KN/m2, negative bearing pressure means the foundation got uplift!

100KN is the dead load applied on the foundation

 

[Ron247]

Brave heart:
Give us a free body diagram of the all the loads acting on the foundation. Do not give any foundation weights in that FBD. Building DL, Uplift etc is all we need. I thought your uplift was 100KN based on your previous post. I have never expressed an uplift as a "pressure load" and that is what is confusing me. If you are getting the 50 KPa as output from some foundation computer program, it is not an applied load it is a reaction.

I assume the uplift is not a pressure, it is a vertical force that gets to the foundation via the column baseplate.

 

[DaveHolder]

I’m getting confused myself! I think there is misunderstanding about the terms have been used!

I did analysis in computer program as you mentioned! I have applied various load combinations including dead, imposed and wind loads for Equilibrium, structural and geo purpose!



The bearing pressure I got is showing tension and compression! I thought when the tension is existed in the foundation means there is uplift which it will turn the foundation over! So I always I design the foundation without any tension bearing pressure!

I will try to upload the FBD later!

 

[Ron247]

If your foundation program is showing a tension load, it assumes you foundation is actually attached to the earth via anchors. Is there an option that does not allow tension but will allow compression? That will be similar to a triangular pressure distribution with one end equal to zero.

You must have either an overturning moment on the foundation or a horizontal force that causes the overturning if you are getting both positive and negative values on the same single analysis. Overturning will make a foundation rotate on the ground and therefore give unequal soil pressures (gradient) but will not necessarily make it turn completely over. Your stability checks are used to determine if it will actually turn over or uplift, not your soil pressures. The common FOS is 1.5 for overturning and uplift. Forces stabilizing divided by Forces overturning = 1.5

 

[hokie66]

Brave heart,

This thread, along with some others you have started, suggest to me that you are relying too much on software which you don't understand. This appears, if I understand it correctly, to be a simple problem which is easily resolved with paper and pencil. Are you an engineer, a draftsman, or what?

 

[DaveHolder]

Have you seen draftsman discussing bearing pressure before?

It is not that easy to resolve by hand when you have various forces acting on 4 columns from different directions!

Dead load Fx FY FZ MX MY
Imposed load FX FY FZ Mx My
Wind x friction Fx -/+ FY Fz MX MY
Wind Y direction Fx -/+ FY Fz Mx My

I did hand calcs applying only dead load and wind.

I checked sliding and overturning, the foundation size was adequate!
I only opened this thread because when I used the software I got tension bearing pressure!

 

[hokie66]

Maybe you should discard that software, but I suspect you are not understanding the output.

Yes, I have know draftsmen to try to act like engineers.

 

[DaveHolder]

Im not sure what do you mean by I’m not understanding the output?

The software shows you bearing pressure results for different load combinations! The output I got for one of these combinations was between + 110 and - 50 KPa! and there was warning stating that there is tension in the foundation!

 

[hokie66]

If the inputs and outputs are correct, then that must be due to overturning rather than straight uplift. And you can't have tension unless you have anchorage. So as jayrod12 said in the first response above, you seem to have a variable stress distribution across the footing. A common situation.