Footing Uplift, new question

[DWHA]

After reading the "Footing Uplift" thread, I have a new question on the subject. When examining a foundation for overturning, I thought that the eccentricity (M/d) was always to be in the center 1/3rd of the footing. Is this a code provision? or a rule of thumb? When checking this, what load combination should be used? 0.6d+1W? 1d+1w? Is this check normally done in addition to the overturning FS check as discussed in the thread "Footing Uplift"?

Another thing that sparked the question is that I am studying for the PE and in the 6 min solutions it has a question that calls for eccentricity in the center 1/3rd. But the solution shows a load combination of 1d+1L+1w, yep, sure enough they show to use live load for this check.

Thanks all.

 

[slickdeals]

Based on what the code requirements are, the 0.6D+W will produce the worst case eccentricity. There are times when this will be greater than L/6 (middle third). As long as the eccentricity e = M/P is less than L/2, the footing design should be ok.

You will have partial bearing when e > L/6. As long as this does not cause your maximum bearing pressure to exceed the allowable, your design should be okay.

You will need to check both D+W and 0.6D+W load combinations for Overturning, stability and allowable bearing pressures.

 

[AVak]

If the WL on the footing is positive (i.e. downward), then the DL+LL+WL case will likely control for the bearing pressure check. For stability, the case of 0.6DL+WL will almost always control.

As the previous poster pointed out, if the eccentricity lies outside of the middle third of the footing you will have partial bearing. Some engineers prefer to avoid this situation. This is where the rule of thumb that you mentioned originated. My personal opinion is that partial bearing is acceptable - especially if it is only under short term loading such as WL.

Adam Vakiener, P.E.

 

[BAretired]

Having the load within the central third of a rectangular footing guarantees that you will have compressive stress throughout the footing. It is not a code requirement in Canada. I do not believe it is in the USA but others can confirm that.

It may be a rule of thumb for some engineers, but not for others. If the load is outside the kern, you would simply consider a lesser footing dimension with a triangular pressure distribution below it.

If you are concerned about staying within allowable bearing pressure then you should include any loads which contribute to the soil pressure, including live load.

Your question is a misnomer as it is not about footing uplift.

BA

 

[delagina]

attached is a good footing spreadsheet

 

[Lion06]

There is no requirement that I know of to make sure the location of the resultant axial load is in the kern (i.e. that there is full bearing of the footing on the soil). As long as the location of the resultant is within the footprint of the footing (e<L/2) and the bearing pressure works out then all is good.

 

[DWHA]

Just to clarify. Check both 1D+1W & 0.6D+1W for e<L/2. Also check 0.9D+1W & 0.6D+1W for footing uplift with FS = 1.5 & 1.0 respectively?

 

[slickdeals]

I think you only need to check 0.6D+W for a FS=1.0.

 

[Lion06]

Slick is right. You don't need any "FS" on any of the combinations, you just need to make sure it's stable (that the resultant occurs within the footprint of the footing and that there is no net uplift at the footing/soil interface).

 

[sandman21]

For CA PE's 1806A.1 requires full bearing for retaining walls and site walls unless you use the exception.