Overturning failure of spread footing below grade and connected to frames 4

[LuisUgarte]

Hello!

I am currently working on a PEER review for a steel moment frame building. In the past, I used to check isolated footings against overturning moment, no matter if they are below grade. At this time, there is a dicussion with the EOR, since he thinks that overturning only affects footings that are free to rotate and in our casem every footing is connected to the steel columns of the structure. I´ve been thinking on that and these are my thougths:

- Isolated footings for frames are not subjected to local overturning moment, because they are connected with the rest of the structure and soil below grade makes this difficult, even if grade beams are not present. So, if global building overturning check is satisfied there is no need to check every footing.

-Overturning check is only required for footings of bridge piers, tower tanks or retaining walls where global stability depends on a single footing.

Do you agree with that? What is your opinion? I will appreciate it.

Thank you.

Regards

 

[Celt83]

What is causing the destabilizing force on the foundation?

If its a moment an unstable foundation would negate a fixed base assumption. If its shear on top of a concrete pier localized overturning would mean a hinge at the column pier interface. If uplift then that would indicate a global stability problem.

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[LuisUgarte]

Celt83, I appreciate the reply! It is a moment. But, does the factor of safety against overturning, ensure fixed base? I think it does no, of course the FS indirectly give us more spread area and then more rotational rigidity, but I am not sure if that could be enough justification to end the discussion of the project.

Than you!

Regards

 

[phamENG]

LuisUgarte - you need to have a robust load path all the way through the structure. In other words, you have to resist the forces/moments/etc. where they occur, not just "globally".

As Celt83 mentioned, the fixed base assumption of a column will be dependent upon the fixity of the foundation. Just embedding it doesn't make it fixed. There are well established methods of verifying that it's fixed, but "it's buried" isn't one of them. So the structure may be fine for overturning globally, but failing to size the footing for a fixed base column to actually provide fixity could result in an inadequate frame and lateral instability under wind or seismic loading (or, in really extreme cases, notional live loads). If there's a moment at the footing, it needs to be checked locally.

The factor of safety doesn't give us fixity. It gives us a margin of error between theoretical fixity and approximated reality.

 

[human909]

Two good replies already. But I'll add mine...

LuisUgarte there seems to be a confusion between overturning resistance of the foundation and moment resistance of the foundation. Even if your structure and foundation is suitable to resist global overturning it does mean it is suitable to resist local moments applied at the footings.

It is a moment.

Well then you damn sure need to provide a suitably strong and stiff load path to resist that moment otherwise all your assumptions go out the window.

Providing a suitably rigid foundation for a moment base is not easy particularly for isolated footings. There is a very good reason why we often assume pinned footings.

 

[phamENG]

It just occurred to me that there may be a misunderstanding. When you say local overturning are you referring to the uplift that occurs on one column of the moment frame and the compression in the other? If so, you still need to resist it locally. The reason is simple - this is where we are assuming the loads are being resisted. The surrounding frame is likely a series of idealized pin connections. You wont have the necessary continuity to "move" that overturning out to the extremes of the building without some severe distortions. So while your building may not go rolling down the street, the inadequacy of local overturning will compromise the moment frame and, again, cause a lateral instability in the structure.

If you want to test it, run a 3D analysis model but make the reactions under the moment frame columns compression only springs. See what happens.

 

[LuisUgarte]

Thank you all!

Yes, I am a bit confused, and I am not justifiying to pass overturning check, I would like to make it clear, the thing is:

I can't see a footing below grade be overturned, before that, soil mass mut be movilized and create a condition free to rotate, which involves that soil pressure reach it's capacity. But if soil pressure check is ok and below it's capacity, how could overturning happen?

What I am understanting is that the factor of safety of local overturning check helps to improve the rotational rigidity indirectly, of course, is not the magic solution for fixed based.

Regards

 

[phamENG]

Which one are we talking about here?

 

[1503-44]

Which one? Both.
There are shear loads at both A and B, but they do not appear to be shown on A.
If bearing capacity is OK, that means instead of failing the soil, as it is apparently strong enough to resist downward rotation, there may not be enough moment resistance and the opposite side will raise upwards and overturning will begin about the point of maximum soil pressure. If you stop it from rotation and translation in the downwards direction, you have not necessarily stopped it from upwards translation. You check overturning stability to ensure that upwards translation is not possible. Even if the soil fails, it is not likely to sink out of sight, but it sure will uplift on the other side, if adequate moment resistance is not provided.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[WinelandV]

1503-44,

I think phamENG is just trying to get to what situation the OP is concerned about - global overturning/stability of the whole structure, or a local OT at the footing.

 

[phamENG]

Thanks, 1503-44. My question is directed to the OP. Trying to understand his nomenclature, not provide a detailed analysis in my 2 second sketch.

Edit: thanks, winelandv. That's precisely what I was asking.

 

[1503-44]

I get it. But IMO there is already enough confusion and he has got some kind of a problem in understanding why you check overturning anyway. Let's don't make it worse by leaving out loads in our pictures too.

It's more interesting to know if you agree with my explanation. That should explain why both must be checked.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[KootK]

I can't see a footing below grade be overturned, before that, soil mass mut be movilized and create a condition free to rotate, which involves that soil pressure reach it's capacity. But if soil pressure check is ok and below it's capacity, how could overturning happen?

I would describe the situation like this:

1) The footing being buried in the soil will likely supply some degree of resistance to overturning and uplift.

2) Without some form of numerical evaluation, the footing being buried in the soil is not a guarantee that overturning resistance is sufficient.

3) Your best tool for evaluating the situation will be a free body diagram like the one that I've shown below which considers a hypothetical, net uplift in the supported column.

In summary, [footing buried in soil <> overturning OK]

i

 

[LuisUgarte]

Thank you all, now I am more confident on this.

phamENG :

I was talking about B. I agree with your explanation, for academic purposes, I will try to study a 3D model a see what happen.

1503-43:

That is a useful point of view. If uplift begins on one side, moment resistance is compromised.

Kootk:

Agree with that. That is the way, lateral soil resistance could help, specially in the presence of pedestals and stiff soils.


Regards

 

[phamENG]

Got it. The 3d analysis comment was more about A, but you could still do it by releasing the fixity at the base of the column and see what happens. If it was designed as pinned, then it doesn't really matter.

 

[WinelandV]

I would be hesitant to depend on the lateral soil resistance for sliding and overturning resistance. I'm sure some of the geotechs that frequent these forums could weigh in better, but the long-story short is that in order to develop that passive pressure, your structure will need to be able to accommodate a not insignificant amount of lateral displacement.

 

[KootK]

I'm sure some of the geotechs that frequent these forums could weigh in better, but the long-story short is that in order to develop that passive pressure, your structure will need to be able to accommodate a not insignificant amount of lateral displacement.

A fairly common, compromise approach to help alleviate that concern is to use the active lateral soil pressure rather than the passive lateral pressure.

When net uplift is present in these situations, soil friction ceases to be available and some form of lateral soil resistance is really all that you've got unless you want to add grade beams, push on the slab on grade, or add so much mass to the footings that you're well in excess of the net uplift. For moment frames, I am a pretty big fan of grade beams connecting all of the participating columns in a line.

 

[le99]

For spread footing for building, I usually check the bearing, sliding, and uplift (per stability criteria). Overturning stability is to be checked on the entire structure, not the individual footing

 

[WinelandV]

I could maybe buy using the active pressure, but wouldn't want to depend on it myself.

When net uplift is present in these situations,

I can honestly say that I've never designed a footing that had net uplift. Just due to the ASD LC's, you should always have ~ 1.5 FS for uplift. Even using the LRFD combos (and applying the 0.9 to the weight of the concrete) should result in more concrete weight than the ultimate uplift load. HOWEVER, I'm willing to give you the benefit of the doubt - how is it that you're ending up with net uplift?

 

[KootK]

Overturning stability is to be checked on the entire structure, not the individual footing

I would agree with that when we're talking about both of these things simultaneously:

1) A moment frame with pinned based connections and;

2) Columns that land directly on footings with no intervening piers.

When either of those things is untrue, so is your blanket assertion.