huangyhg

differential settlement of spread footing
i am adding a column onto an existing spread footing. it is not the bearing capacity that i am worried about but the differential settlement due to additional load that i need to check. can someone enlighten me how i can check this? how much additional load on a spread footing (in terms of percentage) would aid in differential settlement? is there a chart with load to settlement that i could use? the footing is a 30'x30' with 2000 kips load on it. i am adding an additional 200 kips to it at 11.5' from centerline of footing. the bearing capacity is 4000 psf.
this would depend entirely on the layers of soil beneath the footing. there is no simple or universal answer. you might get better advice in the geotechnical engineering forum.
a geotechnical engineer should be by your side to read the response here .
i feel there is a great chance you are over concerned for 2 reasons:
1. if the 2000 kips is the true maximum load the footing has experienced, it produces a soil pressure of 2.22 ksf that is far less than the 4 ksf allowable. in another word, the footing hasn't reached the anticipated settlement permitted in the original criteria, which would not be exceeded even with the 10% added load. (the original assumption/anticipation could be: if all footings in this site being loaded to 4 ksf, a settlement of x" would occur with the max. differential settlement of y". since the actual pressure is less than the given allowable, thus there is margin left over for add'l load to push towards x", and not exceeding y")
2. the add'l load might produce little or no add'l settlement at all due to effect of consolidation, if the building has been there for a long while.
there may exist other influencing factors i have failed to point out, or your setup is very sensitive even to a slight increase in settlement, check all out with a geotech as recommended.

aftg= 30x30 = 900 sf
sftg= 30x302/6 = 4500 feet^3.
pressure before new column load = 2000/900 = 2.22 ksf
change in pressure = p/a +- m/s = 200/900 +- 200*11.5/4500
= 0.222 +- 0.511
max. pressure = 2.22 + 0.222 + 0.511 = 2.95 ksf < 4.0 ksf
min. pressure = 2.22 + 0.222 - 0.511 = 1.93 ksf >0 < 4.0 ksf
so where's the problem?

ba
there is not a problem with the bearing capacity, as the op stated, and assuming that the original load is symmetrical. but with this amount of eccentricity of the added load, there could still be a differential settlement (tilting) issue, which depends on the soil structure and the time periods involved. probably not a problem with a great big footing, but anytime i add 200 kips to a footing, i think reevaluation is appropriate. i worry more about settlements than bearing capacity.
hokie,
the op would have to talk it over with the geotech on the job. they may want to compensate for the eccentricity of loading. the eccentricity of the load is 200*11.5/(2000 + 200) = 1.05', so by attaching a strip 2' x 30' with epoxy and rebar on the proper side, the centroid is moved one foot over and concentricity (if that is a word) is once again restored.
personally, i don't think i would worry much about it but neither did the guy who designed the tower in pisa.
ba
the allowable bearing capacity generally allows for limiting the settlement.
i had a similar situation with a wall and a bridge footing. the bridge footing was on a flowable fill mix and the wall footing was not. the geotech report said max settlement 1/4" for both. the question was asked about why we didnt use flowable fill below the wall, and what the differential settlement would be. i said the wall footing is not close to max bearing pressure, and that at max bearing capacity it would all be settling 1/4" max. so i gave a answer of the ratio of my actual bearing capacity and the allowable times a 1/4". is this what is going on here, since the bearing capacity is not met, there is no differential settlement, just settlement overall?
kinda long winded, hope you can understand
no. it does not mean no differential settlement when given bearing capacity has not exceeded, but the quantity of such difference is in general within allowable/tollerance.
thanks a lot people for your input. i have decided to use meyerhof's formula to determine settlements based on the bearing pressures that i calculated to see how bad of differential settlements we are talking about. somehow, the geotechnical engineer has not got back to me yet on this. but if he does not, i will just calculate differential settlements based on meyerhof's equation and feel satisfied if they are within allowable. so, any comments on this? has anybody used this before this way?
i will leave it to the geotech. but wouldn't hurt if you want to try. watch out for overlapping if there are other footings are close by.

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