Design of footings for uplift/eccentric loads 3

[Latvian83]

Hi,

Anyone has examples, hand calcs, procedures or anything for designing footings for uplift or for eccentrici loads. I'm stuck with this job which is to design shear wall footing for uplift and I don't know how to do it. Any help will be appreciated.

Thank you.

 

[Lion06]

The shearwall should have an OTM on it, not uplift.
Once you have the P and M, check what the eccentricity is (M/P). if e<L/6, then you have full bearing and you can do P/A +- M/S. If your e>L/6, then you have partial bearing and you will have to draw an FBD of the footing with the loads and find qmax like that.

 

[msquared48]

The shearwall can have uplift at the ends if the dead load is insufficient to resist the overturning. Look to either installing a grade beam to span the shearwall out to nearby footings, or adding more mass via a footing at the ends of the shearwall to counter the uplift. The former usually is the better and cheaper way.

Mike McCann
MMC Engineering

 

[Lion06]

Mike-

you are talking about just having a bearing pressure distribution in which the entire footing is not experiencing bearing pressures, correct?
There is nothing wrong with that under lateral loads, is there? I've designed plenty of shearwall footings like that as long as the max pressure doesn't exceed the allowable.

 

[msquared48]

I've done that too, but the footing can get very large to limit the bearing pressure in that situation. Most of the time, I find it more economical to use grade beams to take care of it. That's all I'm saying.

Mike McCann
MMC Engineering

 

[Latvian83]

Is there any book with examples describing this? All the e's i'm getting are > L/6 and i'm having a problem in finding the partial bearing on the fbd. I'm sorry, i'm an intern.

thanks

 

[msquared48]

We were all interns at one time. I have a one page sketch I can scan and post tomorrow with the equations you need. I have been using it for 30 years... Kinda raggety now.

Mike McCann
MMC Engineering

 

[Lion06]

Here is what I have done. I want to add that this assumes a rectangular ftg. If your ftg has some funky shape then the centerline of the ftg will will not be L/2.

 

[Latvian83]

Thank you very much for your help and for your sketch. The problem is, it's always a funky shape footing for the shear wall! . I'm attaching the sketch of the kind of footings I'm dealing with. The problem as I said before is with the FBD and the distrubution (The B). In this case, do you use the larger B, the smaller B or do you combine them together?

Best regards.

 

[Lion06]

Here is how I would do it. Your e is right, but the location of the centroid of your footing is not right. You have to think about the plan of the footing as the cross section of a beam. I am getting a max bearing pressure of 70.4 ksf. You can get some help by considering the axial load on the wall to contribute a counteracting moment because the centroid of the wall does not coincide with the centroid of the footing (most likely). This all assumes lateral load in one direction only, you should really consider the lateral load to act in the opposite direction as well, in which case the axial load from the wall will add to the OTM.

 

[Lion06]

My apologies, I just realized I used your M, where I should have used P. That changes the max bearing pressure to 8.53 ksf, NOT 70.4 ksf.

 

[msquared48]

Here is my sketch. Hope you can read it OK. Post if you can't and I will clarify.



Mike McCann
MMC Engineering

 

[Latvian83]

StructuralEIT: Thank you very much. You saved my day.

msquared48: Thanks for the sketch and it's very clear.

Another question, so when e> L/6, does this mean that there is an uplift? When can you determine if there is an uplift or not?

 

[jike]

There really is no uplift. I would describe it as there is not soil pressure across the full length of the footing (soil cannot take tension) when e > L/6.

The trick to figuring out the triangular soil pressure is that the applied load resultant (P at some eccentricity) has to equal the resultant of the soil pressure. The location of those have to be the same and of course the forces are in opposite directions.

 

[Lion06]

It's not really uplift, as jike mentions. It just means that there isn't full bearing of the footing on the soil.
not necessarily if e>L/6 if you have an odd shape. Check P/A - M/S, if that's negative, then you don't have full bearing and you have to follow the procedure outlined above. If there is full bearing, then you simply do P/A + M/S.

 

[Latvian83]

StructuralEIT: I'm attaching the same sketch for the same footing but this time for the moment in the other direction. I just want to make sure that I did get the idea right. If my calc is right, I'm getting a 17 KSF as a maximum bearing pressure, while the number I got from the geotechnical engineer report for the soil capacity was 16 KSF. What are the options do I have to make this thing work in this direction? Do I have to make the footing longer?

Thanks everyone for your help.

 

[Lion06]

I didn't double check your numbers, but I would hesitate to used B=22.75. The first reason is that it would actually be 14.5+22.75 = 37.25, but also, I don't know that I believe the assembly is stiff enough to have that section behave that way. The final reason is that it doesn't help much. I would try making the footing (the 14.5' portion) a foot or a foot and a half wider.. say 15.5' or 16 feet.
Are you really allowed a 16ksf bearing pressure? That is very high. Double check the geotech report, you may have a provision in there that allows you to bump up the allowable by 1/3 for transient wind/seismic forces. That would bump up your allowable for this to 21.33ksf and you likely wouldn't have to do anything with your footing.

 

[Robbiee]

StructuralEIT and Jike,
Sorry, but I don't understand how no bearing pressure and no uplift can happen at the same time.

 

[Lion06]

Uplift occurs if your resultant load is outside the extent of your footing. If the resultant load is within the extent of your footing, but outside the kern, then you can have equilibrium without needing a tensile force at one end of the footing. If the resultant load is outside the extent of the footing, then you must have a tensile force holding down one end of the footing (usually a rock anchor).

 

[Robbiee]

StructuralEIT,
Do you mean that no action needed to counter act uplift if the resultant is witing the kern because the footing is still in equilibrium state?