huangyhg

design of footings for uplift/eccentric loads
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.
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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.
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
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.
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
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
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
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.
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.
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.
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.

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