Overturning of footing with uplift

[irishengdave]

Hi -- I'm looking at a footing and checking overturning. Criteria as follows:-

1) Uplift = 200 kN
2) Shear = 100 kN
3) Weight Soil Backfill = 200 kN
4) Depth = 2 m
5) Width = 2m

Refer to sketch attached.

Question: Is the footing stable in overturning or not? Assume that loads are un factored and required FOS overturning = 1

I see two two scenario's

a) The 200 kN uplift cancels out the fill and concrete weight and it has no overturning resistance.
b) The 200 kN does not affect the overturning stability. The foundation is just "floating" with zero pressure underneath and the mass of the fill is still there and resist's the overturning.

This seems like a simple problem but I can't get my head around this as to what will happen here.

Comments welcome!

 

[BAretired]

Sketch attached?


BA

 

[mike20793]

The sketch isn't attached. I don't see how there is overturning on this without any moment unless it retaining on one side. You need to sum moments about a point on the footing and include all forces acting on it. That will tell you whether it is overturning or not. You can use the appropriate load combinations or use service loads and proportion it so you have a factor of safety (usually 1.5 for overturning).

edited because sketch was provided

 

[irishengdave]

Cant't get the upload function to work, tries several times.

 

[irishengdave]

Trying to upload sketch

 

[BAretired]

The moment on the base is 200 kn-m. The footing has no resistance to overturning and insufficient resistance to pullout.

BA

 

[irishengdave]

Thanks BA

What if the uplift is eccentric, say 1/2 meter from the right end of footing.

How do you calculate the overturning moment and what is it?

 

[BAretired]

The overturning moment is the moment of all applied forces about the edge of the footing. If the uplift is 1/2 m eccentric, it provides a stabilizing moment of 100 kN-m which reduces the overturning moment to 100 kN-m.

Edit: The above is not correct. The overturning moment is the moment of all forces which would cause overturning about the edge of the footing. The stabilizing moment is the moment of all forces which would prevent overturning about the edge of the footing. If the uplift is 1/2 m eccentric, it produces an overturning moment of 100 kN-m about one edge and 300 kN-m about the other edge of footing.

We have not taken into account the passive soil pressure on the side of the footing and pedestal which would provide additional stabilizing moment.



BA

 

[BAretired]

Also, if there is any possibility of a high water table, the full weight of the footing and overburden cannot be used to provide stability. Only the buoyant weight could be used in that case.

BA

 

[mike20793]

How do you calculate the overturning moment and what is it?

You sum moments about any point on the footing and compare resisting to overturning moments. You must include all forces acting on the foundation, including passive soil pressure.

 

[irishengdave]

Mike, In my sketch, I think the uplift cancels out the downward load, so the O/T=200 kNm.

I don't think you should take moments of the uplift times the distance to edge of footing as an overturning moment, it just reduces or cancels out the downwards load on the footing, correct?

 

[mike20793]

Well, the result ends up being the same, but it is incorrect to say you don't include it. You call the backfill out as 200 kN but you show it as a distributed load. Also, is there backfill at the pedestal? Since the footing is symmetric and the loading is also symmetric these issues are just semantics, but for more complicated layouts, they must be considered.

 

[irishengdave]

Well, the result ends up being the same, but it is incorrect to say you don't include it."

Say the uplift is moved so its 1/2 m from the edge to the right. Now the uplift still cancels out the downwards load.

By your argument, If you take it as a moment @ the edge that gives you 100 kNm overturning moment, which means the overturning has reduced. Doesn't make sense.

Or else I'm missing something

 

[BAretired]

OTM = V*h + U*b/2 = 100*2 + 200*1 = 400 kN-m

Mstab = W*b/2 = 200*1 = 200 kN-m plus passive pressure effect

V = shear, U = uplift,
W = weight of ftg. plus overburden
h = height from V to bottom of ftg.
b = width of ftg.

BA

 

[Lomarandil]

Even for coincident locations, if you just let the uplift and weight cancel out, you'll have a different result when computing the factor of safety against overturning.

While it's a matter of semantics rather than behavior, the convention is that the FS is the ratio of all moments causing overturning versus all resisting moments.

 

[irishengdave]

OK I think Im getting it now,

The eccentricity will cause the footing to rotate in the opposite direction of the overturning, thus reducing the overturning.

If you check OT including the uplift in the overturning calc. the end result is the same as reducing the direct downwards pressur. You still have to consider the reverse moment due to the eccentricity of the uplift and take it as a restoring moment.

What got me is the uplift is not really causing the footing to overturn, but just reduces the OT moment by relieving the downwards pressure.

 

[msquared48]

A while back, the OP asked...

What if the uplift is eccentric, say 1/2 meter from the right end of footing.

How do you calculate the overturning moment and what is it?

In this case, there will be two overturning moments, one in each direction, and unequal. You will need to design the footing for both cases.

And I do not know why the FS for OT would only be 1.0. It's usually 1.5.

In addition to this, with the footing eccentric, you will need to check the maximum soil stress as it is more likely to be overstressed with an eccentric footing.

Mike McCann, PE, SE (WA)

 

[irishengdave]

Mike, The one in the direction of the shear will govern, no need to check in the opposite direction.

Its a hypothetical example, The FOS in a real world problem would be normally 1.5 or greater (depending on the level of uncertainty of the applied load, surcharge effects etc...)