Design of footings for uplift/eccentric loads
Design of footings for uplift/eccentric loads
(OP)
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.
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.






RE: Design of footings for uplift/eccentric loads
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.
RE: Design of footings for uplift/eccentric loads
Mike McCann
MMC Engineering
RE: Design of footings for uplift/eccentric loads
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.
RE: Design of footings for uplift/eccentric loads
Mike McCann
MMC Engineering
RE: Design of footings for uplift/eccentric loads
thanks
RE: Design of footings for uplift/eccentric loads
Mike McCann
MMC Engineering
RE: Design of footings for uplift/eccentric loads
RE: Design of footings for uplift/eccentric loads
Best regards.
RE: Design of footings for uplift/eccentric loads
RE: Design of footings for uplift/eccentric loads
RE: Design of footings for uplift/eccentric loads
Mike McCann
MMC Engineering
RE: Design of footings for uplift/eccentric loads
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?
RE: Design of footings for uplift/eccentric loads
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.
RE: Design of footings for uplift/eccentric loads
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.
RE: Design of footings for uplift/eccentric loads
Thanks everyone for your help.
RE: Design of footings for uplift/eccentric loads
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.
RE: Design of footings for uplift/eccentric loads
Sorry, but I don't understand how no bearing pressure and no uplift can happen at the same time.
RE: Design of footings for uplift/eccentric loads
RE: Design of footings for uplift/eccentric loads
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?
RE: Design of footings for uplift/eccentric loads
As you can imagine, it is not uncommon to have a footing where the resultant is in the footing, but outside the kern fail in bearing pressure. Even though it fails in bearing pressure, generic equilibrium can be obtained. Equilibrium is not possible when the resultant load is outside of the footing unless you hold the end down.
I hope that makes it a little more clear.
RE: Design of footings for uplift/eccentric loads
think of it like a concrete beam (but without the reinforcement). When you design a concrete beam you ignore the tensile capacity of the concrete, and you do the same for a footing.
Uplift is a term that is normally used to indicate that the whole footing would tend to lift up, which can never be the case for a retaining wall. Normally what happens at a retaining wall is referred to as overturning.
Your incorrect use of the term uplift is causing a bit of confusion even though I understand what you are getting at.
Since you stated that you have moment in both directions then there is one more point to note:
If the bearing under the whole footing is positive then it is simply a matter of P/A +- M/S (in both directions) otherwise the equation gets much messier and you either need a good foundation analysis program or relevant charts from an old geotech text (I use the latter).
RE: Design of footings for uplift/eccentric loads
Thank you. I agree that the term uplift is confusing when used for footings. Probably, partial uplift is more descriptive, which could happen to a footing due to overturning moment yet not necessarily needs to be counter acted. To say that there is no uplift or (partial uplift, I guess)when no counteraction is required, means that we don't have to worry about the rotation in the footing and basically assume it is fixed, which is not the case, correct?. Drift is dependant on what assumption you are makeing for the type of restraint at the bottom of the shear wall.
RE: Design of footings for uplift/eccentric loads
The actual amount of rotation is a different matter. It is not normally checked for a cantilever wall.
It is generally assumed that the rotation will be sufficient to allow use of active soil pressure coeficients(Ka). The use of at rest coeficients (Ko) is normally only for situations where there is a restraint against rotation such as a suspended concrete slab.
If you really need to know the rotation then you will need to assume a spring constant for the soil, but this is far above anything I would normally do for a retaining wall.
Now that you mention drift, does this mean that you are designing this for seismic loads or is it just subject to gravity loads?
RE: Design of footings for uplift/eccentric loads