Differential Settlement of spread footing
Differential Settlement of spread footing
(OP)
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.






RE: Differential Settlement of spread footing
RE: Differential Settlement of spread footing
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.
RE: Differential Settlement of spread footing
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
RE: Differential Settlement of spread footing
RE: Differential Settlement of spread footing
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
RE: Differential Settlement of spread footing
RE: Differential Settlement of spread footing
Kinda long winded, hope you can understand
RE: Differential Settlement of spread footing
RE: Differential Settlement of spread footing
RE: Differential Settlement of spread footing
RE: Differential Settlement of spread footing
BA
RE: Differential Settlement of spread footing
Estimating settlements are not an exact science - you can use the same basis and several different methods (see Som and Das 'Theory and Practice of Foundation Design'). For permissible differential settlements, Tomlinson and others have identified permissible rotations for various types of structures.