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]

Even if it isn't within the kern you don't need to do anything as long as it is withing the extent of the footing. The kern is a term that means the length in which the resultant load can act that maintains bearing on the entire footing. If you have a resultant load that is outside the kern, but within the extents of the footing, you will still have equilibrium without holding down the end of the footing, but you will have a triangular bearing pressure (that stops short of the end of the footing) instead of a trapezoidal bearing pressure that covers the entire footing.
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.

 

[csd72]

Ailmar,

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).

 

[Robbiee]

Csd72:
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.

 

[csd72]

Ailmar,

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?

 

[Robbiee]

Seismic and wind. When calculatig bending moment at the base of a shear wall, do you assume a full fixity?