Impact of stepped footing on calculation of sliding

[KPFFIRVINE]

We have a DSA plancheck comment that insists on the fact that the overall sliding of the building is reduced at areas where we have stepped footings.
Our geotechnical engineers disagrees with the plan checker, but Mr plan checker is asking him to prove via existing publication that there is no sliding reduction due to the stepped footing.
Does anyone know of such publication? Have any of you run into this issue? Is the plan checker right? Is my geotechnical engineer right?
I can see a potential reduction in one direction (towards the hill), but not in the other (see attachment)...

http://files.engineering.com/getfil...-cfb1-4b20-a430-c74581b2d784&file=sliding.pdf

 

[hokie66]

I think the plan checker is right. The sliding resistance down the hill will be reduced by the footing steps. Towards the hill, the sliding resistance should not be as critical, as passive pressure will assist.

 

[weab]

I would think that you can prove one way or the other. Draw a freebody of the wall. If you have a stepped slope, I would think that you'd have active pressure along the "exposed" wall side at the step. Compare this value to the sliding of a sloped footing.

I agree with hokie. I have only done one sloped footing and it was sufficient. I haven't done the calcs, but I'd bet that the stepped footing is more stable.

 

[hokie66]

I should have said that the stepped footing has reduced sliding resistance compared with a horizontal footing. I think a sloped footing would have even less sliding resistance down the slope.

 

[KPFFIRVINE]

Thanks for your answers, but the sliding/friction resistance I am loosing is 10 times as much (CMU building, 65ft tall) as the passive resistance.
If the sliding is reduced, HOW MUCH is it reduced by?
I don't think it is right to completely ignore the friction in the stepped area (and sliding won't work anymore).
Do you guys know of any litterature that quantifies the reduction?
Why is there a reduction downhill? Isn't friction acting the opposite direction as the force, as I sketched out in the attachment?

 

[dgillette]

Not sure I'm understanding the question, and it's outside my usual area of practice. Is the sliding that's being discussed just sliding on a surface that's roughly parallel with the overall slope of the stepped footing, like slope stability with the weight of the building on the slide mass? What governs, sliding under the weight of the building (like slope stability) or an active wedge? Earthquake? How big are the steps (width and height)? I ask the last question to see what the average slope is if you were to draw a sliding plane that just misses the lower right-hand corner of each step.

Regards,
DRG

 

[KPFFIRVINE]

dgillette, we are talking about sliding due to earthquake loads. Our steps are 2(H) to 1 (V). As described in the attachment, the difference in elevation is 11ft total.
Thanks for your post.

 

[BigH]

I'm not sure, either, that I wholly understand why you think that the resistance to sliding would be different. You will have sliding on the base of the footing and the "length" of the horizontal footing is (is it not) the same as the length of the footing if not stepped? I attach a sketch I made up - didn't realize it was earthquake at the time but that basically only changes the location of the resultant earth pressure forces. The vertical loading would be the same - nearly so - as shown. The stepped footings mimic the way fills are placed against existing slopes. . . . I'd be interested to see how one thinks that the sliding resistance is actually reduced - calcs - concepts?

 

[dgillette]

What would happen if you looked at it as we do a pseudostatic slope stability problem? See attached markup of BigH's drawing. The blue lines show a potential sliding surface. You could apply some horiz acc to the mass of the building and the soil above the blue line, and find the FS against sliding. I think the checker has a point, because the stepped portion of the foundation would be sliding down an inclined plane, whereas a level foundation would be sliding on a horiz plane.

 

[FixedEarth]

I don't see how you have a sliding issue with the stepped footings. Assuming that your footing widths, the loadings and the coefficient of friction at the base are all the same along the wall length, how is sliding reduced? Aren't we designing per foot length of wall?

For seismic considerations, it is either sliding FS or bearing capacity or earth pressures. For sliding FS, you can reduce FS to 1.5/1.33 or to FS = 1.1 For B.C., you can increase allowable by 1/3 rd. For earth pressures, if walls are shorter than 12 ft, they don't apply & for taller walls you can use the inverted seismic triangle but then you can reduce all your F.S. to 1.1

I have to agree with your Geotechnical engineer.

 

[BigH]

dgillette - I understand your point to a degree and thought of it too. We are talking about sliding of the building, not the global stability. My sketch really isn't to scale, the "slope of the slide" as you have shown is very very flat. My stepped footing is in the order of 6H:1V or so - not too many slopes will fail at that shallow angle. If they do, then we would have to worry about a lot of highway embankments. The "global stability" as shown on your sketch should be checked (as one would for any retaining wall, too). But if the issue at hand is "sliding" and the OP can show the global stability is satisfactory under a seismic event, then why would you need to reduce the "sliding" component when you have adequate SF against sliding. (Don't things always look clearer with sketch!) Thanks, Dave.

 

[dgillette]

With 6:1 steps, I would expect no detectable reduction, but KPFFIRVINE's steps are 2h:1v.

If you replaced the foundation soils with strong rock cut in stairsteps, all the movement would be translational, with no decrease in sliding resistance. Since the foundation soil is not 'stronger than dirt' (BigH is old enough to remember those detergent ads), the reduction might be to account for shearing through the soil that allows the footing to move downhill as well as horizontally. It might be the checker's way of accounting for the global stability issue.

KPFFIRVINE - Ask the plan checker for a written explanation of why the reduction occurs and how much it should be. We have a divergence of opinion here, and we might do better if we knew where he's coming from (soil mechanics or outer space ).

 

[BigH]

Point taken but we don't know what kind of "ground in" dirt is under the structure.