Sliding of Foundation 1

[Surya77]

I am checking foundation for sliding. I have the base shear form the structure above.

Now my question do i have to consider lateral force due to foundation weight itself to add to the demand sliding force.

If that is case, I have hard time qualifying foundation for the sliding. Any thoughts

 

[canwesteng]

What lateral force is due to the foundation weight? If you mean lateral earth pressure due to unbalanced soil loads on each side of a grade beam or something similar, yes those should be considered.

 

[KootK]

I vote yes. The footing's acceleration will match that of the ground and inertial forces will develop as a result. In my mind this is similar to how seismic earth pressures develop behind retaining walls.

I've wondered about the need for a sliding check under seismic loads. If sliding actually occurred, that would theoretically limit the development of subsequent seismic forces, somewhat like a plastic hinge in a shear wall. On the other hand, if there will be large ground movements, I guess you wouldn't want your building sliding into your neighbour's building.

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.

 

[Surya77]

@canwesteng

no not the lateral earth pressure, the inertial forces of the foundation itself

 

[EngineeringEric]

What internal forces of a foundation? do you mean something like Cs*Weight? Or is it a retaining wall system?

 

[canwesteng]

I get it, inertial forces from ground acceleration. I would think yes, think of the ground accelerating the structure in one direction, then acceleration switches to the other direction. In that case I think the foundation mass contributes to the sliding. Is it only the mass of the foundation causing problems, or are you including the mass of soil on the footing as well?

 

[JStephen]

"On the other hand, if there will be large ground movements, I guess you wouldn't want your building sliding into your neighbour's building."

It is possible to run into situations where the design vertical acceleration is greater than 1g, which to me implies that the structure (and the earth below it) is being lobbed into the air. And I think it was pointed out along the way, that those accelerations tend to be at higher frequencies and involve small amplitudes. So your structure may be lobbed into the air, but it's lobbed a millimeter high, not 20' high. I suspect this would apply to the sliding issue as well.

Last year, I was at one of the ASCE seminars on seismic design of equipment and other non-building structures. One of the things discussed there was that it was possible to use time-history analysis from some past earthquakes and calculate estimated movement for something like this numerically. Obviously, there's a lot of room for variation in those kinds of results, but it serves to show whether the sliding involved would be a fraction of an inch or several feet or what.

 

[canwesteng]

Come to think of it, is it not possible to have spectral acceleration and a friction coefficient such that resisting sliding is impossible?

 

[Surya77]

ok after reading lot of reports from FEMA and talking to seniors. I realized that there is no specific criteria prescribed by coded. According to some seniors, ASCE was planning to come up with clear procedure. FEMA talks a lot about sliding due to flooding but not clear about seismic forces

For my case, I did consider inertial forces as I thought its more correct. I changed to configuration of foundation, made it look like channel (rotated 90 degrees with flanges down) with two keys at the bottom. It was cumbersome to do all the calcs, but I was able to make it work.

 

[jt12]

It's been a while since I've done a seismic design, but if my recollection is correct, it seems like I decided there likely would be at least a reduced seismic sliding force due to the foundation weight, but that ASCE 7-05 didn't seem to mandate that it be included.

 

[mathcadboy]

KootK has a good point there. If sliding did actually occur during seismic condition, wont you have like a slide bearing support on a building wherein it massively reduces the seismic forces experienced by the structure?

My opinion is no, dont consider it. Just use the seismic weight on everything above the grade line.

 

[KootK]

Let's undertake a mental experiment. Start with a greenfield site next to the San Andreas. Next, place a large precast block on the ground and apply a seismic event. Will the precast block exert a seismic shear on the supporting soil? You bet it will.

Now, how is a building foundation different in a way that would eliminate the seismic shear?

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.

 

[Surya77]

Kootk. Very true.

It tough to Ignore this. I have seen many existing designs where they ignored it. I can't convince myself to do that.

 

[mathcadboy]

Placing the precast rock on the ground and burying it are two different things. I haven't seen a footing design wherein the weight of a footing is added to the seismic weight for stability checks on a foundation.

 

[Surya77]

Let me discuss more this week with people. This looks like a grey area. I will let u guys know

 

[KootK]

Placing the precast rock on the ground and burying it are two different things.

If you understand why they are two different things with respect to seismic inertial loads, please elaborate.

Thanks for starting this thread Surya. And please do report back when you reach a consensus with your colleagues. I've never included foundation self weight as seismic weight and know of no instance of anyone else doing so either. You've really given me something to think about with this.

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.

 

[mathcadboy]

Yes I do believe lots of engineers have different opinions regarding this. I think it will boil down to how conservative you want to be. If it would make you sleep well at night and cost is not that of an issue, then just include it.

But I do still want to see the designs that you mentioned. Is it from a reliable engineering reference or just calculations made by colleagues?

 

[Surya77]

KootK,

I am happy that all u guys spent time on this. I will let you all know.

Mathcadboy
The rationale I heard differed from each engineer to other. Some said it's too conservative.

One of the better reason from a senior was, post seismic tests indicated not much sliding on foundations, but he said the codes are silent on this and left it to engineers. having said that he added he would consider that force Risk cat IV structures just to cover his neck

 

[KootK]

I guess the next question would be, if the foundation is to be considered a source of seismic weight, what should be the seismic load generated? I would argue that:

1) It should be calculated separately from the superstructure.
2) It should be based on the elastic response spectrum (R=1).
3) It should be based on a period of zero.

Because the seismic response spectrum drops off so sharply to the left of the constant acceleration transition period, this may lead to fairly minor seismic forces. Consider two possibilities:

1) short building with short period. The acceleration of the footing would be much less than the superstructure and forces wouldn't add up to much.

2) tall building with long period. Accelerations might be comparable but foundation weight would get lost in the noise of all the contributing seismic weight of the many floors above.

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.

 

[JStephen]

If you assume an elastic subgrade (or some variation thereof), you wouldn't be limited to zero period for a foundation.
For overturning moment, the foundation weight won't have a lot of effect due to being down low, the original question related to shear, where it would be more critical.
I believe all the seismic loading is tabulated for surface level, so at some depth down, you ought to get a little lower loading, also.
The seismic design forces aren't just derived from fundamental principles, they're all codified by committees, etc., so if you get into a situation not covered by the codes, you don't necessarily have a "right" way to do it, either.
If you consider what's happening in the foundation, you can also consider soil-structure interaction, which can lower the design loads.
In a few minutes googling around and looking through the bookshelf, I couldn't find anything really relating to the topic one way or the other.