Spread Footing Design

[cdi12]

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.

http://www.mooload.com/new/file.php?file=files/211106/1164137438/Spread+Footing.pdf

Thanks,

IV

 

[cdi12]

Here is an attachment instead of link.

 

[eric1037]

It appears that in Case A you are subtracting the weight of soil and in Case B you are adding the weight of soil.

Am I correct in that this is the only difference in the calcs.?

 

[eric1037]

The net allowable soil bearing pressure is defined as the pressure in excess of the existing overburden pressure at the foundation that can be safely transmitted to the bearing stratum by the shallow foundation. The gross allowable soil bearing pressure is defined as the net allowable bearing pressure plus the pressure from the existing overburden pressure.

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.

 

[DaveAtkins]

Case B is a more "exact" solution. Case A is a faster, more conservative solution.

Either solution is acceptable.

DaveAtkins

 

[cdi12]

DaveAtkins:
Case A is not conservative solution especially with bigger eccentricity. Even in this example case A has bigger qmax than case B.
IV

 

[haynewp]

No, that is why case A is the more conservative in this situation.

 

[DaveAtkins]

I stand by my original reply--and like haynewp
said, because Case A yields a higher bearing
pressure, it is the more conservative solution.

DaveAtkins

 

[LOKSTR]

cdi12,

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

 

[DaveAtkins]

LOKSTR,

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

 

[UcfSE]

Dave, I believe the term "net" refers to the fact that you don't have to account for the weight of the footing concrete in your soil bearing pressure calculations. Given that, there's no need to reduce your allowable bearing by the weight of the soil overburden on the footing or the weight of the footing.

 

[WillisV]

Generally when I receive an allowable bearing from a Geotech report I assume that it is the allowable bearing at the bottom of footing elevation - not reducing it by whatever soil burden is above (and assuming its close enough so say the concrete displaces about its weight in soil...150pcf vs 120pcf).

 

[DaveAtkins]

From "Foundation Engineering," by Peck, Hanson and Thornburn:
"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

 

[WillisV]

Agreed...don't think it really amounts to a hill of beans in everyday practice. It seems easier (IMHO) calculation-wise to ignore the soil in the dead load and just use the provided allowable bearing.

 

[zestructural]

in case A, why do you substract the weight of soil instead of ignore it? Is there a reason? Thank you.

 

[chichuck]

I think case B is a correct analysis.

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

 

[cdi12]

Thank you all for your inputs.
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

 

[chichuck]

cdi12,

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

 

[BigH]

The concept of net allowable bearing has been discussed on a number of threads in the past in some detail. WHen a load is placed on the footing, there are two aspects of concern - 1) the shear capacity of the soil to withstand the gross loading of the footing and 2) the settlement of the footing. In almost all cases in granular soils, settlement of the ground below the footing governs the allowable bearing pressures which the geotechnical engineer will provide to you - hence it is the net allowable bearing pressure - only that loading at the footing level in excess of the existing overburdne pressure at that level will 'cause' settlement (neglecting for practical purposes the slight expansion and then recompression as the overburden pressure is removed and then reapplied principally, as some have indicated, by the weight of the concrete. For clays, however, it may be the shear strength of the clay - or of settlement, too,tha will govern. Generally, only for firm to soft woils would the shear strength likely govern. In this case, the allowable bearing pressure is the gross pressure placed by the loading on the footing (the overburden pressure is taken into account in the computation of the bearing capacity of the footing by the (gamma)*D term. SEveral texts have explained this in some detail. When a soils engineer gives you the allowable bearing pressure - you have the need to know if it is the gross bearing pressure (based on shear) or the net allowable bearing pressure (based on settlement). I usually get around this by saying net allowable bearing pressure for settlement controlling situations and allowable bearing capacity b for those situations governed by shear.
- sorry for some mistypings, but the sketch drawing has affected my ability to see the last inch or so of the reply.

 

[Intermilan]

check this:

http://www.webcivil.com/rcspread.aspx

this is a web-based software for civil/structural engineering design (

http://www.WebCivil.com)