Spread Footing Design
Spread Footing Design
(OP)
Here is a link for example of eccentrically loaded spread footing with two design approaches I have seen
and I am just wandering which one is correct.
htt p://www.mo oload.com/ new/file.p hp?file=fi les/211106 /116413743 8/Spread+F ooting.pdf
Thanks,
IV
and I am just wandering which one is correct.
htt
Thanks,
IV






RE: Spread Footing Design
RE: Spread Footing Design
Am I correct in that this is the only difference in the calcs.?
RE: Spread Footing Design
For the purpose of sizing the shallow foundation, for the net allowable soil bearing pressure, the weight of the concrete in the foundation can be neglected because the weight of the concrete in the shallow foundation is approximately the same as the weight of the soil that was excavated to construct the foundation.
This is not necessarily the case if you add fill to a site or if you construct the foundation at grade prior to filling.
RE: Spread Footing Design
Either solution is acceptable.
DaveAtkins
RE: Spread Footing Design
Case A is not conservative solution especially with bigger eccentricity. Even in this example case A has bigger qmax than case B.
IV
RE: Spread Footing Design
RE: Spread Footing Design
said, because Case A yields a higher bearing
pressure, it is the more conservative solution.
DaveAtkins
RE: Spread Footing Design
The exact solution is as follows:
ASSUMING SIZE OF PEDESTAL AS 2'x2'(no size was there on sheet)
Wt. of concrete below ground=W1=[(8*8*1.5)+(2x2x1)]*(0.15-0.10)
Wt. of concrete ABOVE ground=W2=(2x2x2)]*(0.15)
tOTAL DOWN LOAD=W==P+W1+W2
M=50+(5*4.5)
e=M/W
Then calculate max pressure based on this ecc. and check against "NET" allowable value which is 2 ksf.
LOKSTR
RE: Spread Footing Design
In checking net pressure, it is also acceptable
to subtract the soil overburden pressure and the
weight of the footing from the maximum pressure.
DaveAtkins
RE: Spread Footing Design
RE: Spread Footing Design
RE: Spread Footing Design
"The net ultimate bearing capacity qd is defined as the pressure that can be supported at the base of the footing in excess of that at the same level due to the surrounding surcharge."
From "Foundation Design," by Teng:
"net ultimate bearing pressure, psf, = pressure at bottom of footing in excess of the pressure at the same level due to the weight of the soil immediately surrounding the footing."
So, you CAN subtract the weight of the soil overburden from the calculated bearing pressure, IF you used the weight of the soil as part of your dead load.
DaveAtkins
RE: Spread Footing Design
RE: Spread Footing Design
RE: Spread Footing Design
In case A, you are not correctly calculating with all the loads on the footing: you have ignored the 1' of soil and you have taken only 50 pcf of concrete footing weight. This understates the total P and results in overstating the eccentricity. (the moment used is correct.) This results in an oversted value for total toe pressure.
The net bearing capacity quoted by DaveAtkins is correct (both sources are giants in the field). To correctly calculate the net bearing pressure, use Case B as listed in the original post.
zestructural, in case A, he is trying to use a "net" weight of the concrete, using (gamma conc - gamma soil) * concrete volume. This is not a correct way to calculate net bearing on a footing with overturning loads. It would however be correct for a footing with only axial loads.
regareds,
chichuck
RE: Spread Footing Design
Chichuck – Would you, please, explain why case A is not a correct way to calculate net soil bearing on a footing with overturning loads?
IV
RE: Spread Footing Design
It does not account for all the downward loads on the footing: it ignores the weight of the soil, and it does not use the enitre weight of the concrete footing, only part of that. So this gives an incorrect value for P. that makes the value e = M / P incorrect also.
Those two loads are there, resisting overturning, so to get the correct value for e, they must be included.
regards,
chichuck
RE: Spread Footing Design
RE: Spread Footing Design
this is a web-based software for civil/structural engineering design (www.WebCivil.com)
RE: Spread Footing Design
be able to take it one step further
and subtract the bearing pressure
due to the soil overburden in Case B
therefore reducing Qmax a little more.
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